The aim of the session is to promote discussions between scientists focusing on the physics and dynamics of tropical phenomena. This session is thus open to contributions on all aspects of tropical meteorology between the convective and planetary scale, such as:
- Tropical cyclones,
- Convective organisation,
- Diurnal variations,
- Local circulations (i.e. island, see-breeze, etc.),
- Monsoon depressions,
- Equatorial waves and other synoptic waves (African easterly waves, etc.),
- The Madden-Julian oscillation,
- etc.
We especially encourage contributions of observational analyses and modelling studies of tropical cyclones and other synoptic-scale tropical disturbances including the physics and dynamics of their formation, structure, and intensity, and mechanisms of variability of these disturbances on intraseasonal to interannual and climate time scales.
Findings from recent field campaigns are also encouraged.
While African easterly waves (AEWs) are a common precursor to tropical cyclogenesis in the Atlantic basin, the majority of AEWs weaken and fail to develop upon departing Africa. Despite this, prior research has found that AEWs that undergo genesis in the Caribbean are on average drier and weaker when first departing Africa than AEWs that develop in the open Atlantic, closer to Africa. In this study, we investigate the initial structure and evolution of Caribbean developing AEWs and how they differ from non-developing and open Atlantic developing AEWs.
Caribbean developing AEWs and non-developing AEWs that reached the Caribbean were identified from 1996 to 2024 using the AEW tracking algorithm developed by Downs et al. (2025). Using ECMWF Reanalysis v5 (ERA5) data, we found that Caribbean developing AEWs had statistically significant low-level northerly wind anomalies and mid-level easterly wind anomalies when first departing Africa compared to non-developing cases. While open Atlantic developing AEWs have been shown to be significantly moister with stronger low- to mid-level relative vorticity and anomalously warm upper-level temperatures compared to non-developing cases, these favorable anomalies were not statistically significant in Caribbean AEWs until around 40°W. Although not initially as favorable for genesis, Caribbean developing AEWs were, on average, able to avoid the initial significant weakening observed in non-developing cases over the eastern Atlantic and were therefore better able to take advantage of a more favorable downstream environment and strengthen before eventually undergoing genesis in the Caribbean.
How to cite: Wilson, A., Majumdar, S., Downs, W., Zawislak, J., and Dunion, J.: Initial Structure and Downstream Evolution of Caribbean Developing African Easterly Waves, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18087, https://doi.org/10.5194/egusphere-egu26-18087, 2026.
The diurnal cycle is one of the most fundamental characteristics of tropical cyclones (TCs), influencing a wide range of processes including cloud coverage, rainfall, and the timing of intensity changes. Recent studies have identified outward-propagating features known as diurnal pulses (DPs). These pulses appear as rings of localized cooling in brightness temperature that propagate radially outward from the storm center over several hours. DPs occur in the majority of TCs and have been observed across all storm intensities, from tropical storms to major hurricanes. Furthermore, DPs have been linked to changes in TC structure and intensity.
Because DPs have mainly been observed via brightness temperature anomalies in the cirrus canopy, their vertical structure remains poorly understood. While prior work indicates that they may extend vertically, the complete depth of these pulses throughout the storm has yet to be determined. Here, we show that DPs are not confined to the upper troposphere but instead represent column-depth features, producing coherent anomalies across multiple atmospheric variables.
Despite the growing body of literature on DPs, their propagation mechanism remains an open question. Several hypotheses have been proposed, with particular emphasis on inertial–gravity waves. Support for this interpretation comes mainly from observed propagation speeds, which are consistent with theoretical inertial–gravity wave speeds in the TC environment, as well as from the strong latitude dependence of the pulses. Here, we investigate this hypothesis using classical dry gravity wave theory, finding propagation angles and inter-variable phase relationships that are consistent with theoretical expectations.
Beyond their propagation, the timing of DP initiation has been a central focus. Originally, DPs were described using a “diurnal clock” framework (Dunion et al., 2014), with initiation typically occurring between 00:00 and 04:00 local solar time (LST) and outward propagation to a radius of approximately 200 km by 04:00–08:00 LST. However, accumulating observational evidence suggests that this timing is not universal. Accordingly, we examine the preferred nighttime initiation of DPs and investigate the physical mechanisms that may underlie this tendency.
How to cite: Schmitt, K., Ruppert, J., Sakaeda, N., and Vogel, R.: Simulated diurnal pulses in developing tropical cyclones, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11320, https://doi.org/10.5194/egusphere-egu26-11320, 2026.
Evidence from idealized and real-case modeling suggests that the longwave (LW) component of the cloud-radiative feedback (CRF) is fundamental to tropical cyclogenesis (TCG), accelerating a ~5-day process by multiple days in idealized studies. However, existing CRF mechanism-denial frameworks - such as spatial homogenization of radiative heating or utilizing “cloud-transparent” radiative schemes - preclude counterfactual analysis on how TCG efficiency is affected by specific radial and vertical structures of CRF, which is determined by the distribution of different cloud types in the TC.
To address whether an “optimal LW CRF pattern” that maximizes TCG efficiency exists, we developed an ML-informed WRF modeling framework that enables counterfactual experiments by adding an external 3D, ML-discovered, heat forcing to the total heating tendency returned by the RRTMG longwave radiation scheme. Physics-informed inverted LASSO regressions, trained on a WRF ensemble on Typhoon Haiyan (2013), isolate an “optimal LW perturbation” in the form of inner-core mid-level heating. which is strikingly different from a longwave perturbation that maximizes near the cloud top obtained with simple data analysis (2-day azimuthal mean).
We conduct a series of pattern-perturbation experiments to validate this data-driven proposed “optimal CRF pattern”: the ML-discovered mid-level perturbation accelerates the intensification of Haiyan more efficiently than an empirical 2-day mean upper-level LW perturbation. Changes in vertical velocity precede changes in precipitation characteristics in the perturbation experiments, establishing a causal chain from CRF to intensity change. The mid-level-heating runs exhibit higher inner-core stratiform fractions, more intense convective bursts, stronger mid-level vorticity, and lower mean sea-level pressure. These results demonstrate that ML can serve as an objective hypothesis generator, reducing the scientific search space and facilitating efficient data-driven discovery of the structural drivers to TCG.
How to cite: Tam, F. I.-H., Ruppert, J., and Beucler, T.: Mid-level longwave heating accelerates tropical cyclogenesis: insights from ML-informed WRF simulations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16691, https://doi.org/10.5194/egusphere-egu26-16691, 2026.
Hurricanes are important sources of convectively generated gravity waves (GWs) that play a critical role in modifying the thermodynamics in the upper troposphere-lower stratosphere (UTLS) region. There are several widely used theories that describe the GW convective generation, but their validation using model simulations or satellite measurements is not sufficient due to the lack of high resolution measurements and the difficulties in isolating the convection force. The GNSS - radio occultation (RO) measurements therefore stand out since they are capable of determining the hurricane thermal structures and identify the small-vertical-scale GWs, because of their high-vertical-resolution (<0.1km) and high accuracy (<0.5K) temperature retrievals.
This study seeks to characterize the GWs generated by hurricanes by investigating three intense hurricanes of similar intensity, to determine their common GW features that can be extended and compared to GWs induced by other hurricanes. We approach this goal by analyzing the GW properties using multiple RO temperature retrievals along each hurricane’s track by illustrating the consistency of the tropopause height and convection strength within the analyzed periods. We then evaluate the correspondence between the RO-determined GW properties and the hurricane environment revealed by the ERA5 model-level data. This comparison clearly demonstrates that the RO profiles could vertically penetrate the hurricane structure close to the eye with a small horizontal drift, allowing us to identify the link between the wave characteristics and their sources of generation. Our results show a good agreement with the conceptual GW theories, from which we identify three distinct GW wavelength bands that correspond to different generation mechanisms with strong consistency among the hurricanes studied. The Least Squares Wavelet Analysis (LSWA) also uniquely demonstrates the wind filtering effects that modify the GW wavelength via the dispersion relation. Our study suggests a common GW pattern exhibited by multiplication of the selected profiles from each studied hurricanes that might be applicable to other hurricanes of similar intensity.
How to cite: Wang, Y. (., Pagiatakis, S., and Vergados, P.: Common Gravity Wave Patterns from Multi-Hurricane Analysis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8329, https://doi.org/10.5194/egusphere-egu26-8329, 2026.
Diabatic heating and microphysical processes remain major sources of uncertainty in tropical cyclone simulations. This study evaluates the sensitivity of cyclone structure and intensity to microphysical assumptions using a dynamically calibrated WRF-ARW framework applied to Extremely Severe Cyclonic Storm (ESCS) Fani (2019). Track and timing errors were first reduced via grid nudging in the outer domain, coupled with the Multi-Scale Kain–Fritsch (MSKF) cumulus scheme, YSU boundary layer physics, RRTMG radiation, and an ocean mixed-layer model. This configuration improved landfall timing accuracy from ~11 to ~2 hours and reduced spatial error to ~60–70 km. Initial results indicate that ice-inclusive physics enhance vortex strength and structural realism compared to warm-rain schemes, albeit with a reduced translation speed. Building on this setup, we compare several double-moment microphysics schemes that prognose both hydrometeor mass and number concentration. Simulated radar reflectivity fields are generated using a physically consistent forward operator, which incorporates hydrometeor-specific reflectivity (dBZ) retrievals and liquid-equivalent scattering assumptions. These fields are evaluated against Doppler Weather Radar (for eyewall and convective structure), GPM DPR (for vertical hydrometeor and melting-layer profiles), and GPM IMERG (for surface rainfall distribution). A phase-locked evaluation strategy enables a structural comparison across schemes despite differences in landfall timing. The results highlight how microphysics choices modulate convective organisation and precipitation features in high-resolution simulations over the Bay of Bengal, offering guidance for improving microphysical representations in cyclone forecasting models.
How to cite: Khan, S. and Roy, A.: Sensitivity Of Tropical Cyclone Structure To Double-Moment Microphysics In A Dynamically Calibrated WRF Framework , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1085, https://doi.org/10.5194/egusphere-egu26-1085, 2026.
Accurately simulating tropical cyclones (TCs) in global climate models remains a key challenge due to not only multiscale interactions that govern storm genesis, intensity, and structure, but computational constraints that limit model grid spacing. Here we evaluate TC representation in a development version of the Community Atmosphere Model, version 7 (CAM7), which introduces several major updates, including higher vertical resolution, a revised Zhang-McFarlane deep convection scheme, and a new prognostic formulation for turbulent momentum fluxes in the boundary-layer scheme CLUBB. Using a suite of globally-uniform and variable-resolution simulations at 0.25deg grid spacing, we assess both large-scale statistics (global and basinwise TC frequency) and storm-scale characteristics (inflow angle, inflow depth, and wind structure).
CAM7 with prognostic momentum fluxes produces improved spatial patterns of TC activity and more realistic intensity metrics compared to prior CAM generations. However, default configurations overproduce storms by roughly a factor of two. By increasing the parameterized CAPE consumption by deep convection during TC genesis, we achieve a ~40% reduction in global TC counts and improved agreement with observed basin distributions. At the storm scale, we reduce boundary-layer diffusivity, leading to stronger tangential winds, larger inflow angles, and shallower inflow layers, consistent with idealized f-plane sensitivity experiments and more in line with both observations and large-eddy simulations.
These results demonstrate that targeted parameter tuning in deep convection and boundary-layer turbulence schemes can substantially improve both the frequency and structure of simulated TCs in CAM7, advancing its capability for high-resolution climate and weather prediction applications.
How to cite: Stephens, B., Zarzycki, C., Bacmeister, J., Larson, V., Nardi, K., Thayer-Calder, K., and Hannay, C.: Tropical Cyclones in CAM7: Assessing the Impact of Prognostic Momentum Fluxes and Convective Parameterization at Global and Storm Scales, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1339, https://doi.org/10.5194/egusphere-egu26-1339, 2026.
Tropical cyclone (TC) intensification is strongly influenced by the oceanic thermal structure, shaped by both temperature and salinity stratification. However, the role of salinity stratification remains debated. We conducted idealized experiments using a fully coupled atmosphere–ocean model to systematically assess its impact on TC evolution. Generally, vertical advection dominates the thermal response. An intact barrier layer (BL) enhances vertical advection by intensifying vertical velocity gradients, whereas partial erosion suppresses it but inversion layer compensation emerges. Once the BL is fully eroded, the inversion layer vanishes, and the influence of salinity stratification on TC intensity is substantially diminished. These processes are modulated by TC translation speed. At the fastest translation speed (6 m s⁻¹), strong stratification maintains an intact BL that confines vertical velocity to the upper ocean. This enhances the contribution of the velocity gradient to vertical advection, allowing the TC to reach maximum intensity. Under moderate and weak stratification, partial erosion of the BL weakens vertical advection, leading to reduced TC intensity. At a moderate translation speed (3 m s⁻¹), BL erosion becomes more pronounced, weaker salinity stratification exerts less suppression on vertical advection and mixing, amplifies thermal compensation from the inversion layer, and favors TC intensification. For slow-moving TCs (1 m s⁻¹), the BL and inversion layer are fully eroded, salinity stratification plays a negligible role in modulating TC intensity. Overall, these findings highlight the non-negligible role of salinity stratification in regulating TC intensity and provide physical insights for improving intensity forecasts.
How to cite: li, L., li, Y., and Tang, Y.: The modulation of Tropical Cyclone Intensity by Subsurface Salinity Stratification: An Idealized Study Using Coupled General Circulation Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7742, https://doi.org/10.5194/egusphere-egu26-7742, 2026.
Deep convection associated with tropical cyclones (TCs) can reach the tropopause, which can induce mixing between the troposphere and the stratosphere. Such exchanges have been documented in both numerical simulations and observational studies, which indicate that stratospheric subsidence into the eye of an intensifying storm contributes to the formation of an upper-level warm core. Despite these findings, the influence of this upper-level warming on TC intensity is still poorly understood. In our study, we demonstrate using idealized simulations that the upper-level warming originates from subsiding stratospheric air outside of the storm eye, rather than from subsidence in the eye alone. We show that overshooting convection penetrating into the stratosphere is responsible for the induced subsidence, with both processes intensifying with higher sea surface temperatures (SSTs).
How to cite: Charinti, G. A., Davin, A., Polesello, A., Muller, C., and Pasquero, C.: Upper-level warming and its effect on tropical cyclone intensity, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2526, https://doi.org/10.5194/egusphere-egu26-2526, 2026.
Understanding how changing conditions influence tropical cyclone (TC) intensity is of great importance. This study applies a stochastic model (IRIS) to attribute the causes of the increased North Atlantic hurricane intensity from 1979 to 2024. In the model, the increased potential intensity and southward track shifts towards higher potential intensity comparably contribute to an increasing trend of 0.08 m/s per year in the lifetime maximum intensity. However, the simulated trends were not sensitive to the epochal changes in relative intensity to date. The model also predicts a southward shift in landfall (-0.10 °/yr), which is hard to detect. Our findings emphasize an increasing recent TC risk, particularly at low latitudes.
How to cite: Li, M. and Toumi, R.: Attributing causes of increased intensity of North Atlantic hurricanes using a stochastic model (IRIS), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4300, https://doi.org/10.5194/egusphere-egu26-4300, 2026.
This study investigates the climatological environments and large-scale forcing mechanisms that promote anomalous northward trajectories of African Easterly Waves (AEWs) over the central-eastern Atlantic. AEWs are identified through a tracking algorithm based on 700 hPa relative vorticity using the ERA5 reanalysis dataset from 1940 to 2024, and a subset of waves with anomalous trajectories is selected (aAEWs). The synoptic-scale atmospheric and oceanic environments associated with these aAEWs are characterized and compared against a 30-year climatology to identify the key dynamical and thermodynamical factors favouring their northward propagation. The results reveal that aAEWs follow a particular large-scale configuration. This setup is characterized by a significantly strengthened Azores High, displaced poleward from its climatological position, in conjunction with an enhanced mid-level trough over the northeastern Atlantic. A pronounced cooling near the tropopause and anomalously warm sea surface temperatures within the wave’s intensification zone are also identified. Furthermore, substantial modifications in low level moisture transport and wind shear along the West African coast are identified as acritical factor in steering the aAEWs from their common westward trajectories. These results have important climatic implications, as these anomalous environments promote the northward migration of AEWs and significantly increases the likelihood of tropical cyclogenesis in the northeastern Atlantic, a region that is climatologically weakly active.
How to cite: Rodríguez Acosta, E. J., Gómez Plasencia, P., González Alemán, J. J., Calvo Sancho, C., Bolgiani, P., Díaz Fernández, J., Luna, M. Y., Montoro Mendoza, A., Martín, M. L., and Gomara, I.: Synoptic-Scale Environments of African Easterly Waves with Anomalous Northward Trajectories , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6850, https://doi.org/10.5194/egusphere-egu26-6850, 2026.
African Easterly Waves (AEWs) are the dominant synoptic-scale tropical disturbance in the boreal summer Atlantic. However, direct three-dimensional observations of these waves and their modulation of the Intertropical Convergence Zone (ITCZ) remain limited.
Observations from the Eastern Atlantic leg of ORCESTRA are used to characterize the mean vertical and horizontal structure of seven robust AEWs in a wave-relative framework. The observations reveal coherent vertical wind and moisture structures, with upstream–downstream asymmetries relevant for both synoptic-scale organization and deep convection. These results motivate a focused investigation of how AEWs influence the structure and organization of the ITCZ.
We test the hypothesis that AEWs play a central role in constraining ITCZ structure during boreal summer. Based on the observed AEW structure, we hypothesize that wave-modulated moisture distributions and gradients influence the organization of deep convection within the ITCZ. Large-eddy simulations conducted over the campaign period are compared with the observations to assess the representation of AEWs and to further explore AEW–ITCZ interactions.
How to cite: Rowe, D.: The Relationship Between African Easterly Waves and the Atlantic ITCZ: Structural Insights from ORCESTRA , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-10760, https://doi.org/10.5194/egusphere-egu26-10760, 2026.
The organization of clouds in the Intertropical Convergence Zone (ITCZ) varies markedly from day to day. To investigate how different mesoscale cloud patterns relate to the mean properties of the ITCZ, e.g. precipitation, we have identified and classified recurrent patterns. Focusing on the Atlantic ITCZ, we define five mesoscale cloud patterns: Line, Double Line, Broad, Cluster, and Speckles. We investigate the patterns using two different methods, human labeling and automated identification based on profile-fitting. The human labelers classified a total of ~6,600 images for the seasons July, August and September and December, January and February. The profile-fitting is based on typical signals seen in the human classified labels but automates the detection of the patterns. Both methods show that the preferred location of the most cloudy pattern (Broad) and the least cloudy pattern (Speckles) is seasonally dependent. Additionally, they hint at a connection between the pattern distribution and the regions of highest and lowest precipitation. While the human labeling results are restricted to the afternoon peak in convection, the automated detection method is applied to additional seasons, regions and even times of day.
How to cite: Hayo, L. and Windmiller, J.: Beyond the Mean: the Mesoscale Cloud Patterns of the Atlantic ITCZ, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11016, https://doi.org/10.5194/egusphere-egu26-11016, 2026.
The governing mechanisms of mesoscale convective organization and precipitation across the inter-tropical convergence zone (ITCZ) are an open area of research, due in part to the lack of detailed observations over the tropical oceans. The Process Investigation of Clouds and Convective Organization over the atLantic Ocean (PICCOLO), a sub-campaign of Organized Convection and EarthCARE Studies over the Tropical Atlantic (ORCESTRA), deployed the Colorado State University Sea-Going Polarimetric (Sea-Pol) radar on the German R/V Meteor during August and September of 2024, to bridge this gap. This is the first ship-stabilized polarimetric radar deployment in this region to our knowledge. In this study we use 3D 120 km range Sea-Pol radar retrievals to analyze the spatial structure, rate, and microphysical characteristics of precipitation across the Atlantic ITCZ. We perform calculations of the height of the 10 dBZ echo and find three convection groups based on a trimodal division of frequency: shallow (1- 4 km), congestus (5- 7 km), and deep (8 km+). Results show echoes in the congestus group contributing the most to the total rain accumulation across the campaign. These congestus echoes are frequently obscured on satellite brightness temperatures by higher clouds. Deep convective echoes were found to be more infrequent but have higher rain rates per fractional area. Higher populations of deep and congestus clouds are often found in proximity, while the remaining cloud population of shallow convection is found to be more distinct spatially. The interdependence of these convective populations in the context of the ITCZ and African Easterly Wave passages, will be discussed.
How to cite: Colón-Burgos, D., Bell, M., Klocke, D., Wing, A., and Ruppert, J.: Characteristics of Precipitation across the Atlantic Inter-Tropical Convergence Zone from Shipborne Sea-Pol Radar Observations during ORCESTRA, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14787, https://doi.org/10.5194/egusphere-egu26-14787, 2026.
The diurnal cycle of cold cloud cover is underestimated within Earth system models (ESMs) with the greatest underestimation in the afternoon. To better understand the diurnal cycle of tropical oceanic cloud cover, the diurnal cycle of deep convective system (DCS) initiation and the subsequent contributions to cloud cover resulting from systems initiating at earlier times is analyzed using newly developed DCSs Lagrangian tracking methodologies. Satellite infrared-based Tracking Of Organized Convection Algorithm through 3D segmentatioN (TOOCAN) DCSs are matched to Global Precipitation Measurement (GPM) mission precipitation and diabatic heating products. Matched data are then binned by their hour of initiation (in local solar time) to evaluate morphological characteristics and contributions to rain and cloud cover diurnal cycles. Analysis reveals an unexpectedly large peak in daytime DCS initiation that produce subsequent afternoon cloud cover, thus suggesting that the discrepancy between ESMs and observations is likely due, in part, to ESM misrepresentation of initiation or maintenance of daytime-initiated DCSs. Results also show that daytime DCSs produce less precipitation, but relatively more cloud shield compared to DCS that initiate overnight. As a framework to understand these diurnal variations in cold cloud production, we will apply a semi-empirical source-sink cold cloud area growth model that includes a convective area source term and latent heating source term. Vertical latent heating profiles from GPM, DCS morphology from TOOCAN, and atmospheric lapse rates and density from ERA5 are fit to the semi-empirical model to estimate cloud growth and decay timescales. Observation-estimated timescales and the source term variations will be evaluated to understand key drivers in the differences in DCS cold cloud production across the diurnal cycle.
How to cite: Wessinger, S., Rapp, A., Elsaesser, G., Roca, R., and Fiolleau, T.: Exploring Drivers of Unexpected Diurnal Variations in Tropical Oceanic Cold Cloud Production, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15893, https://doi.org/10.5194/egusphere-egu26-15893, 2026.
The tropical climate variability is characterized by various oscillations across a range of timescales. Oscillations that imprint the tropical mean state are generally attributed to slow processes, such as the seasonal cycle or interannual variability. Here, we identify a pronounced tropics-wide intraseasonal oscillation (TWISO) in satellite observations and reanalyses. This oscillation, with a period of 30 to 60 days, is evident across multiple variables and involves interactions between convection, radiation, surface fluxes, and large-scale circulation. It is primarily manifested as convective perturbations in the tropical Indo-Pacific warm pool accompanied by oscillations in the large-scale tropical overturning circulation. Here, we examine the relationship between TWISO, the Madden-Julian Oscillation (MJO), and the instability of radiative-convective equilibrium. Certain phases of TWISO coincide with specific phases of the MJO, suggesting a potential connection between the two. However, although the MJO can amplify the oscillation amplitude of TWISO, it is not essential for TWISO to occur. Finally, due to its broad manifestation across the tropics, TWISO potentially exerts widespread influence on tropical weather and climate at regional scales.
How to cite: Bao, J., Bony, S., Takasuka, D., and Muller, C.: Tropics-wide intraseasonal oscillations, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5320, https://doi.org/10.5194/egusphere-egu26-5320, 2026.
The Madden–Julian oscillation (MJO) is a planetary-scale tropical weather disturbance marked by eastward propagating cumulus cloud clusters over the Indo-Pacific region, causing severe weather and climate events worldwide. The mechanism and predictability of MJO propagation remain elusive, partly because relevant multi-scale processes are poorly understood. Here, we reveal chaotic MJO propagation arising from cross-scale nonlinear interactions, based on 4,000-member ensemble simulations of two MJO events in November and December, 2018, using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) at 14-km horizontal resolutions. Against conventional linear MJO theories, multiple regimes with distinct timings of MJO propagation emerge under a single atmosphere-ocean background in December. The emergence of regime bifurcation depends critically on the equatorial asymmetry of climatological sea surface temperature, mainly regulated by the seasonal march. Selection of the bifurcated regimes is probabilistic, influenced by whether tropical-extratropical interplay promotes moistening associated with westward-propagating tropical waves over the western Pacific. Specifically, this regime distinction is rooted in differences in MJO-related upper-tropospheric westerly strengths over the western Pacific when MJO convection is located in the Indian Ocean, affecting the degree of the extratropical Rossby-wave refraction that can interefere with the tropical waves. These results contribute to a more complete MJO conceptual model and help foresee when coherent MJO propagation emerges.
How to cite: Takasuka, D., Suematsu, T., Miura, H., and Nakano, M.: Deterministic chaotic behavior in the propagation of the Madden-Julian oscillation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18118, https://doi.org/10.5194/egusphere-egu26-18118, 2026.
Extreme humid heat threatens human health by limiting the body’s ability to cool through sweating. Its impacts are greatest in the tropics and subtropics, where high population density coincides with hot and humid conditions that are projected to intensify under climate change. While temperature extremes in the mid-latitudes have been widely studied, the drivers and predictability of tropical humid heat remain poorly understood.
We identify historical humid heat extremes in reanalysis across tropical and subtropical land. We use logistic regression to holistically examine the relationships between hot-humid days and the major modes of tropical variability.
We find that ENSO exerts a dominant influence on humid heat extremes across much of the tropics at interannual timescales, acting through combined effects on temperature and humidity. The Indian Ocean Dipole and the Atlantic modes further modulate the extremes.
On shorter timescales, the MJO is the dominant driver of humid heat variability in the regions surrounding the Indian Ocean. Humid heat peaks according to a fine, regionally varying, balance between increased humidity and longwave warming in the active MJO phases and increased shortwave warming in the suppressed MJO phases.
Over central Africa, north-west South America and parts of the Maritime Continent, Kelvin waves dominate over the MJO. The divergent and easterly phases of Kelvin waves increase humid heat predominantly through temperature increases, driven adiabatically through subsidence and diabatically through shortwave warming. Rapid transitions between the convergence and divergence Kelvin wave phases tend to constrain the duration of humid heat extremes, typically to no more than three consecutive days. Rossby and WMRG waves only dominate over smaller regions of the sub-tropics.
This study has significantly advanced understanding of the drivers of tropical humid heat and highlights pathways for improved prediction. Our findings have important implications for model evaluation, seasonal outlooks, and the design of early warning systems.
How to cite: Birch, C., Jackson, L., Hidayat, A., Chagnaud, G., Marsham, J., Taylor, C., Schwendike, J., Sanchez, C., and Matthews, A.: A holistic view of tropical modes of variability as drivers of humid heat , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3333, https://doi.org/10.5194/egusphere-egu26-3333, 2026.
The Madden–Julian oscillation (MJO) is the dominant intraseasonal wave phenomenon influencing extreme weather and climate worldwide. Realistic simulations and accurate predictions of MJO genesis are the cornerstones for successfully monitoring, forecasting, and managing meteorological disasters 3–4 weeks in advance. Nevertheless, the genesis processes and emerging precursor signals of an eastward-propagating MJO event remain largely uncertain. The year 2023 has witnessed the sequential genesis of a record-breaking Madden–Julian oscillation (MJO) and an unprecedented coastal El Niño in March–April, thus offering another opportunity to understand the dynamics of MJO–El Niño interactions. Here, we show that the March 2023 MJO is quite unusual as it starts from the South China Sea due to the dry intrusion of extratropical cold northerly winds and moist preconditioning effects of equatorial Rossby waves, propagates eastward fast as a double Kelvin wave system, and expands over the entire tropical Pacific largely as a Kelvin wave response to its strong suppressed convection over the Maritime Continent. Because of these unusual features, the MJO exerts widespread westerly wind forcing to the ocean surface, with two maxima over the western and far eastern tropical Pacific. Due mainly to the depressed local Ekman upwelling under MJO westerly, the upper ocean gets warmer than normal near the coast of South America, thereby helping trigger the 2023 coastal El Niño. Using an El Niño ensemble forecasting system, we quantify that the MJO westerly over the far eastern Pacific explains approximately 30% of coastal warming signals off Peru. Although only marginally increasing the end-of-year Niño-3.4 index, the March MJO can induce small-scale oceanic westward-propagating disturbances, which significantly decrease the intermember spread of the forecasted basin-scale 2023/24 El Niño. These results highlight the pivotal importance of tropical–extratropical interactions in initiating those MJOs from outside the Indian Ocean and also point out the potential roles of MJOs in dynamical El Niño evolution and prediction.
How to cite: Wei, Y.: Initiation of the Record-breaking March 2023 MJO Event: Implications for El Niño Onset and Prediction, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15555, https://doi.org/10.5194/egusphere-egu26-15555, 2026.
Rainfall variability critically affects rainfed agriculture and water resources across the Southern Africa subcontinent, where communities are highly vulnerable to shifts in rainy season dynamics. This study provides a comprehensive assessment of the rainy season’s modality, onset, cessation, duration, and trend over the Southern African subcontinent using multiple and recent gridded rainfall datasets together with an extensive collection of in-situ observations. The analysis uses the rainy season definition methodology based on Liebmann & Marengo (2001) that can be applied to a range of climates ranging from semi-arid to humid. The method is applied to daily satellite, satellite-gauge-calibrated, gauge-only, and reanalysis datasets and a large collection of daily station data from about 1980 to 2020. The approach is complemented by Fast Fourier Transform (FFT) analysis to enhance the robustness of seasonal signal detection rainy season modality.
Our results reveal a clear north-to-south rainfall gradient, with wetter equatorial regions and drier southwestern areas. This gradient is less pronounced southeastwards. Rainfall modality varies spatially, with bimodal regimes dominating the equatorial zone linked to the north-south movement of the rain belt (aka. Intertropical Convergence Zone (ITCZ)), while interior and southeastern zones exhibit unimodal summer rainfall peaks. The southwestern tip of South Africa displays a distinctive winter rainfall peak, mostly driven by extratropical low pressure systems. Transitional zones with complex orography as well as coastal zones show larger dataset disagreement, bringing challenges in capturing rainfall seasonality.
Trend analysis over recent decades indicates a trend towards delayed onsets (~2-3 days/year), earlier cessations (-3 to <~-4 days/year) and shortened durations of the rainy season (-2 to ~-4 days/year ) in regions such as Angola, Namibia and western South Africa. Cessation trends show higher spatial variability than onset trends. These changes are more pronounced in gridded datasets but also appear in station records, reinforcing confidence in the observed tendencies. The findings align with future climate projections under high-emission scenarios, highlighting risks for water availability, agricultural planning, and food security. The study emphasizes the need for improved observational coverage and integration of onset and cessation monitoring into early warning and climate adaptation systems.
How to cite: Fink, A. H., Peirera, C. A., Soares, P. M., and Ramos, A. M.: Rainy Season Climatology and Trends in Southern Africa Using Multiple Rainfall Datasets, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12860, https://doi.org/10.5194/egusphere-egu26-12860, 2026.
The Maritime Continent is a complex system of islands—often with significant topography, deep oceans, and shallow seas—located in the heart of the Indo-Pacific warm pool. The region is characterized by very high average daily precipitation, strongly modulated by a pronounced diurnal cycle. Precipitation variability in the Maritime Continent, influenced by subseasonal, seasonal, and interannual modes, has long been of interest to the research community, as it can easily lead to extreme precipitation events. However, the observational network over the region is sparse, and coherent datasets capable of assessing the physical properties of the atmosphere are limited, particularly on diurnal timescales.
An example of this complex interaction can be found in Sumatra, where the diurnal evolution of convection and precipitation is characterized by two modes. Convective clouds begin developing before noon along the western, upwind slopes of the Barisan Mountains; they grow and move inland during the afternoon, advected by the mean flow in the lower to middle troposphere. However, there is also propagation in the opposite direction: offshore, upwind-moving squall lines that produce an offshore precipitation maximum throughout the evening and night. This local variability is strongly modulated by large-scale circulation variability, which in turn affects precipitation over the island. Several physical mechanisms have been proposed to explain the offshore progression of precipitating cloud systems on diurnal timescales, but these have been based primarily on high-resolution numerical modeling. Due to the lack of observational data these mechanisms remain poorly constrained.
The aim of the Barisan–Anai Meteorological Network (BAM-Net) is to fill this observational gap by providing consistent, long-term near-surface meteorological data (pressure, temperature, humidity, horizontal winds, and rainfall), as well as cloud cover and column-integrated water vapor. To date, the dataset spans over one full year of observations collected across five stations along the Anai Valley, between the Indian Ocean coast and the first mountain pass across the Barisan Mountains at 1000 m ASL. This dataset provides a unique opportunity to continuously monitor the diurnal cycle of near-surface atmospheric properties and to assess its variability associated with seasonal, intraseasonal, synoptic, and mesoscale circulations.
In this submission, BAM-Net observations are used to study variability in the diurnal cycle from day-to-day up to seasonal timescales across the five locations, focusing on November 2025 period, when unprecedented extreme precipitation event span across west and north Sumatra and southern part of Malay peninsula, associated with development of a rare near-equatorial Tropical Cyclone Senyar in Malakka Strait. This high impact event caused over 1000 fatalities, vast devastation of civil infrastructure and personal property. BAM-Net stations in West Sumatra show precipitation accumulation exceeding 1000 mm of rain in 10 days, and indicate even higher rainfall amounts in the north and north-east part of the island. BAM-Net observations provide unique insight into event’s dynamics in the up-wind slope region, including spatio-temporal variability across region and within it. This type of observations can be used in process studies of extreme events as well as high resolution model validation.
How to cite: Baranowski, D., Marzuki, M., and Baldysz, Z.: Extreme precipitation event in November 2025 in Sumatra observed with high resolution in-situ observations from BAM-Net, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17740, https://doi.org/10.5194/egusphere-egu26-17740, 2026.
This study is motivated by the observation of a unique intraseasonal power in the upper tropospheric equatorial meridional winds over the Western Hemisphere during boreal winter. The presence of intraseasonal power at both westward and eastward wavenumbers is unusual and intriguing, as the dominant tropical modes of intraseasonal variability typically exhibit little amplitude in equatorial meridional winds and are instead characterized by strong zonal wind perturbations. Furthermore, the intraseasonal disturbances are confined to the upper troposphere. The spatial structure and dynamical characteristics of these disturbances are consistent with those of mixed Rossby-gravity waves (MRGWs), indicating the presence of intraseasonal MRGWs in the atmosphere. A systematic relationship between the location and amplitude of the intraseasonal MRGWs and the upper-tropospheric westerlies suggests that background circulation is fundamental to their existence. This hypothesis is investigated through a set of diagnostic analyses guided by the dispersion relation of a linear shallow water model incorporating a homogeneous background flow. The results not only explain the emergence of intraseasonal MRGWs but also the overall distribution of meridional wind power throughout the troposphere. The strength and direction of the background flow govern the observed spatial and temporal characteristics of MRGWs via Doppler shifting of intrinsic MRGWs. Consequently, the spectral power distribution of equatorial meridional wind perturbations across pressure levels represents a composite of MRGWs that have been Doppler-shifted by a range of background flow regimes.
How to cite: Mehak, M. and Ettammal, S.: Can Background Circulation Facilitate Intraseasonal Mixed Rossby-Gravity Waves over theCentral-Eastern Pacific?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-691, https://doi.org/10.5194/egusphere-egu26-691, 2026.
The dominant northwestward propagation of Monsoon Depression (MD) has been well established by the existing theoretical framework that is analogous to Beta Drift theory; however, rare northeastward-moving cases remain unexplored. We investigate six northeastward-moving systems (NEMS) that occur over the Bay of Bengal and the Indian subcontinent, while comparing them with northwestward-moving systems (NWMS) to identify their distinctive structures and the mechanisms driving atypical propagation.
Structural analysis reveals that NEMS possess a substantially deeper relative vorticity core at the mid and upper troposphere, along with higher rainfall to the east of the depression center. Vorticity equation diagnosis reveals that horizontal vorticity advection, specifically the asymmetric advection of symmetric vorticity (AASV) term, dominates the vorticity tendency and exhibits a persistent dipole structure for both NEMS and NWMS, although towards different directions at different pressure levels. This highlights a multi-level steering, particularly prevalent in NEMS cases, which is effective in understanding track variabilities.
Further analysis reveals distinctive negative geopotential anomalies (centered at ~37°N) at the upper troposphere extending from the extratropics into the subtropics for NEMS and eventually interacting with these depressions to modulate their trajectories. These anomalies are significantly stronger and quasi-stationary, resulting in large-scale impacts on overall track directions. The previous theory fails due to the assumption of a single pressure level primarily impacting depression propagation. This work establishes that understanding and predicting monsoon depression tracks requires explicit representation of multi-level steering and deep vortex structures.
How to cite: Boruah, S. J., Nanjundiah, R., and Chakraborty, A.: Beyond Beta Drift: Multi-level Steering and Deep Vortex Structures in Anomalous Monsoon Depression Propagation , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-697, https://doi.org/10.5194/egusphere-egu26-697, 2026.
Synoptic-scale vortices play a central role in regulating atmospheric energy and the hydrological cycle, but the dynamical identity of Indian Summer Monsoon Low-Pressure Systems (ISM-LPSs), critical for delivering 60−80% of India's seasonal rainfall, remains fundamentally unresolved. This persistent failure stems from reductive attempts to interpret these crucial systems within a binary tropical or baroclinic framework, which is inadequate for the unique monsoon environment. To address this gap, we present a systematic, comparative dynamical analysis of the large-scale environment (LSE) and storm-centered structure of synoptic vortices across the Northern Hemisphere. Using high-resolution reanalysis, we reveal that ISM-LPSs are associated with strong large-scale ascent forcing despite negligible near-surface baroclinicity, a unique combination that distinguishes them sharply from both canonical tropical and mid-latitude cyclones. Storm-centred composite analyses further reveal vertically coherent circulations that lack the pronounced frontal asymmetry characteristic of baroclinic systems. Together, these results indicate that ISM-LPSs occupy a distinct dynamical regime defined by monsoon-specific large-scale conditions, exhibiting systematic similarities to and departures from canonical tropical and baroclinic storms. By moving beyond dualistic classification, this study provides a clearer dynamical context for interpreting ISM-LPS genesis and evolution, with implications for their representation in weather and climate models.
How to cite: J. Puri, C. and Sukumaran, S.: Barotropic or Baroclinic: The Hybrid Genesis of Indian Summer Monsoon Low-Pressure Systems, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-704, https://doi.org/10.5194/egusphere-egu26-704, 2026.
Changes in Monsoon Storm Under Extreme Sea Surface Warming
Rising global temperatures have become a serious concern in the 21st century. As a result, the intensity and frequency of extreme weather events have increased worldwide. This study examines anomalies in Sea Surface Temperature (SST) in the Bay of Bengal (BoB), identified as Marine Heat Waves (MHWs), and their influence on the synoptic-scale atmospheric vortex, using satellite-derived SST observations and atmospheric reanalysis data spanning 1982-2022.
The regions that frequently experience marine heatwaves (MHWs) closely correspond with areas where the Monsoon Low-Pressure Systems (LPS) — common atmospheric vortices during the boreal summer monsoon — originate, typically occurring during the latter half of the MHW life cycle. While widespread studies have examined MHWs and monsoon systems independently, the coupled interactions between these phenomena in the Bay of Bengal remain poorly understood, despite the region's vulnerability to the impacts of extreme weather.
The presence of MHWs during the genesis phase of Monsoon Depressions (MDs), the intense monsoon LPS, appears to intensify as it modifies the pressure gradient, wind, and rainfall distributions. MDs forming under MHW conditions tend to be more intense, faster-moving, and associated with stronger winds and enhanced precipitation. Furthermore, an increase in Extreme Rainfall Events (EREs) is observed in these MHW-influenced MDs, with most EREs concentrated in the southwest quadrant of the systems. Underlying environmental conditions that could modulate this variability were analyzed using empirical indices to quantify the major mechanisms at play. The analysis reveals absolute vorticity and relative humidity as the two dominant factors contributing to MD intensification through MHWs. These analyses provide insights into how MHWs modify the background state of the atmosphere and ocean.
How to cite: Changamsseril Raj, S. and S Nair, V.: Changes in Monsoon Storm Under Extreme Sea Surface Warming , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1003, https://doi.org/10.5194/egusphere-egu26-1003, 2026.
We study the large-scale westward-propagating quasi-biweekly oscillation (QBWO) in the
global tropics. During the boreal summer, these waves exhibit significant activity over
Southeast Asia and the Western Pacific Ocean. Although comparatively weaker, this os-
cillation is also found across much of the northern tropics throughout this season. The
structure of the QBWO shows a strong resemblance to equatorial Rossby waves, but a
few fundamental features vary across different regions and result in different growth and
propagation mechanisms. Due to their cyclonic-anticyclonic gyres, once formed, these
waves can trigger or suppress extreme events, such as tropical cyclones/depressions. In
fact, these large-scale systems can also influence heatwaves/regional temperature changes,
intense/suppressed rainfall events, and changes in humidity.
Composites from multiple decades of data reveal significant differences between circula-
tion and convection structures in various tropical regions. Convective coupling modifies
the theoretically predicted structure of the equatorial Rossby waves [1] in relatively moist
regions, such as the Western Pacific, Bay of Bengal, and the Arabian Sea. Specifically,
in the very moist regions over the Bay of Bengal and the Arabian Sea, convection is
collocated with circulation, instead of the expected quadrature lag in these variables [2].
A vorticity budget indicates that while meridional advection of planetary vorticity is the
primary controller of the tendency in both moist and dry regions, other terms are essential
in approximating the evolution of the vorticity anomaly. Planetary stretching hinders the
propagation, while horizontal advection by the zonal wind supports it in the dry regions.
In moist regions, while stretching appears to aid growth, it is required in combination
with horizontal vorticity advection to match the vorticity tendency. The moisture budget
illustrates that in relatively dry regions, the zonal mean advection of perturbed moisture
in regions with strong easterlies contributes to the evolution of moisture. On the other
hand, in moist regions, horizontal advection of the background moisture by the anoma-
lous winds and a combination of vertical advection, evaporation, and precipitation are
crucial for approximating the moisture tendency. These results help us develop a better
understanding of the QBWO and lead the way for simplified theoretical models of this
intraseasonal tropical mode of variability.
[1] T. Matsuno, Journal of the Meteorological Society of Japan. Ser. II, 44(1):25–43, (1966).
[2] Y. Nakamura and Y.N. Takayabu, Journal of the Atmospheric Sciences, 79(1):247–262,
(2022).
How to cite: Biswas, S., Sukhatme, J., and Gayen, B.: Quasi-biweekly Oscillations During the Boreal Summer, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-1117, https://doi.org/10.5194/egusphere-egu26-1117, 2026.
The Maritime Continent (MC) has long been hypothesized to hinder the eastward propagation of the Madden–Julian Oscillation (MJO) through its strong diurnal cycle of convection. To evaluate this mechanism, a regional model is used to simulate a boreal spring 2013 MJO event that weakened and stalled over the MC. Two experiments are performed: a control with realistic diurnal insolation (CTL) and a no-diurnal-cycle experiment (NO_DC). MJO propagation is objectively identified using a large-scale precipitation tracking (LPT) method, which distinguishes propagating and non-propagating behavior better than the conventional RMM index. In NO_DC, suppressed diurnal heating reduces land precipitation, leading to more continuous eastward propagation. Moist static energy budget analysis shows that MJO maintenance in NO_DC arises from enhanced longwave heating and reduced advection, while persistent propagation is linked to increased advection and reduced longwave and latent heat flux damping. These responses vary regionally across the MC, highlighting the complex role of diurnal processes in modulating MJO propagation.
How to cite: Zhou, X., Ray, P., Dudhia, J., Hagos, S., Johnson, N., Nikolopoulos, E., and Barrett, B.: Influence of the Diurnal Insolation Cycle on MJO Propagation Across the Maritime Continent, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-3251, https://doi.org/10.5194/egusphere-egu26-3251, 2026.
The initiation and subsequent intensification of tropical disturbances (TDs) remain challenging for numerical weather prediction. This study examines a systematic over-intensification tendency in the Korean Integrated Model (KIM) for a June 2025 case near the Mariana Islands and explores plausible mechanical and thermodynamic contributors. Using high-resolution (8 km) WRF simulations, we find evidence that the tendency is linked to an interaction between island-induced low-level convergence and the energy-based initiation logic of the operational KSAS convection scheme. In this case, topography appears to provide an important physical trigger for TD initiation by modifying low-level flow and enhancing convergence. While topography is closely associated with initiation, the convection scheme influences subsequent vertical development and the resulting intensity. Specifically, the operational KSAS-KIM configuration tends to respond early to lower-tropospheric energy maxima, which can favor rapid growth. In contrast, applying the NTDK (New Triggering Design for KIM; KSAS-EXP) approach—which uses buoyancy and condensate thresholds—reduces unrealistic intensification by limiting deep convection to more physically plausible conditions. These results suggest that as global models move toward finer resolutions, carefully balancing topographic forcing and convection-initiation mechanisms may help improve forecast realism.
How to cite: Kim, H.-R. and Kim, B.-M.: Numerical Simulation of Rapid Tropical Disturbance Development: Sensitivity to Topography and Convection Triggering, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4404, https://doi.org/10.5194/egusphere-egu26-4404, 2026.
Bridging the gap between medium-range weather forecasting and seasonal outlooks, the Central Weather Administration (CWA) has implemented a multi-model ensemble framework to enhance tropical cyclone (TC) monitoring on sub-seasonal timescales (weeks 1–4). This operational platform synthesizes objective TC detection from leading global systems, including the 46-day ECMWF ensemble, the 32-day NCEP ensemble, and the CWA’s Global Ensemble Prediction System (GEPS), etc. We also developed a region-specific Probabilistic Formation Index, which serves as an operational Forecast Confidence Level (FCL) for the TC Threat Potential Forecast product.
The FCL is developed by using a deep learning architecture utilizing a Long Short-Term Memory (LSTM) model. The model is specifically designed to extract signals from key sub-seasonal drivers, such as the Western North Pacific Monsoon Index (WNPMI), sea surface temperature (SST), and intraseasonal oscillations including the Madden-Julian Oscillation (MJO) and Boreal Summer Intraseasonal Oscillation (BSISO). A specialized loss function was implemented during the training phase to address the inherent data imbalance of TC formation events.
Systematic evaluations across the 1–4 week horizon demonstrate substantial forecast skill, particularly within the first two weeks. Notably, the correlation between dynamical model performance and the AI-derived FCL reveals the latter's efficacy as a proxy for forecast reliability in real-time operations. The practical value of this integrated approach is exemplified by the successful subseasonal prediction of Super Typhoon Ragasa (2025). This case study highlights the system's ability to provide early TC formation signals and reliable track outlooks, offering critical leadtime for disaster risk reduction. Complementing these efforts, probabilistic TC rainfall outlook products specifically designed for S2S timescales have been developed to provide valuable reference for water resources management and disaster mitigation. More details will be presented at the meeting.
How to cite: Tsai, P.-E., Lin, J.-Y., Tsai, H.-C., and Lo, T.-T.: Week 1–4 Tropical Cyclone Forecasting in the Western North Pacific: Verification of Super Typhoon Ragasa (2025) and Application of a Deep Learning-based Probabilistic Index, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4657, https://doi.org/10.5194/egusphere-egu26-4657, 2026.
This study investigates the characteristics of east Pacific (EPAC) easterly waves (EWs) in ten ensemble members of the Community Earth System Model version 2 Large Ensemble (CESM2-LENS) under the current climate, as well as their projected changes by the end of the 21st century under the Shared Socioeconomic Pathway 3–7.0 (SSP370) warming scenario. Under the current climate, the CESM2 ensemble mean produces realistic summer climatological mean precipitation, horizontal winds, and EW-filtered vorticity variability over the EPAC, although the amplitude of EW filtered precipitation variability is underestimated relative to observations. CESM2 also realistically simulates the distribution of EW tracks identified using 700-hPa curvature vorticity in the Caribbean and EPAC, along with the horizontal structure, amplitude, and propagation characteristics of EW disturbances. However, CESM2 EWs exhibit a more top-heavy vertical velocity profile and an eastward-tilted vertical structure relative to ERA5, suggesting some biases in vertical dynamical processes.
In the future warming scenario at the end of the 21st century, the mean precipitation over the EPAC strengthens and shifts southward, accompanied by intensified low-level mean easterly winds in the Papagayo jet region, although the increase in EW-filtered precipitation variability in this region is modest. EW track density and anomalous vorticity amplitude decrease along the Central American coast, while increasing within the southwestern ITCZ region. Changes in static stability with warming and weaker upper-level vertical velocity per unit precipitation explain the weakened midlevel vorticity in EWs, indicating that CESM2-simulated EPAC EWs have moisture mode characteristics. The weakened dynamical signal of the EWs may limit convective activity, thus resulting in only modest increases in EW precipitation amplitude.
How to cite: Maloney, E. and Zhou, Y.: East Pacific easterly waves in the CESM2 large ensemble: Present-day characteristics and projected future changes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5901, https://doi.org/10.5194/egusphere-egu26-5901, 2026.
The sudden turn of tropical cyclones (TCs) can rapidly alter the affected disaster-prone regions and associated rainfall distributions, posing severe threats to coastal areas and creating major challenges for operational forecasting. However, most of these events occur over the open ocean, where the scarcity of in situ observations limits our understanding of how precipitation and cloud microphysical processes evolve during the sudden turning. In this study, we analyzed the precipitation evolution and associated microphysical characteristics during the sudden turn of Super Typhoon Vongfong (2014) using the latest GPM satellite observations. The main findings are as follows: (1) During the sudden-turning period, the precipitation coverage expanded significantly. Strong convective precipitation was distributed from the inner eyewall to the outer eyewall and spiral rainbands and weakened in intensity, whereas stratiform precipitation broadened in coverage and intensified. (2) The increase in stratiform precipitation was attributed primarily to increased cloud water content, which strengthened collision–coalescence processes, promoted the formation of larger and more numerous raindrops, and consequently increased precipitation efficiency and intensity. (3) The weakening of convective precipitation was related to the reduction in eyewall updrafts, which suppressed ice-phase processes and limited the development of deep convection.
How to cite: Ye, G., Zhang, W., Leung, J., Wang, F., Zhang, B., and Dong, W.: Precipitation Microphysics Evolution of Typhoon During the Sharp Turn: A Case Study of Vongfong (2014), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6190, https://doi.org/10.5194/egusphere-egu26-6190, 2026.
Providing reliable information on tropical cyclone (TC) track forecast uncertainty is essential for effective disaster preparedness. Conventionally, the radius of the Cone of Uncertainty (CoU) is derived from historical distance errors, often resulting in a static, climatological value that fails to account for the characteristics of individual storms. While traditional approaches attempt to categorize scenarios based on factors like translation speed, they are often limited by sample sparsity and struggle to objectively incorporate complex environmental influences.
To address these limitations, this study proposes an autoregressive encoder-decoder Long Short-Term Memory (LSTM) framework to generate situation-dependent CoU estimates. We utilize a multi-source dataset comprising official forecasts from the Central Weather Administration (CWA) and global models (ECMWF and NCEP) from the past five years. By employing an autoregressive architecture, the model can also iteratively generate a large ensemble of potential track realizations to characterize the forecast error distribution while preserving serial correlations across lead times.
In this presentation, we compare traditional methods with the proposed LSTM approach to highlight the advantages of situation-dependent estimation. Our results also show that the LSTM-based CoU provides a robust representation of observed tracks, covering approximately 68% and 95% of observations within one and two standard deviations, respectively. Furthermore, integrating global numerical model information significantly reduces the uncertainty radius while maintaining reliable coverage. Overall, this work demonstrates how deep learning can offer context-aware uncertainty quantification, serving as a promising advancement for TC forecasting.
How to cite: Liu, Y.-S., Lin, F.-Y., Yang, Y.-T., and Tsai, H.-C.: Situational Estimation of Tropical Cyclone Track Forecast Uncertainty Using an Autoregressive Encoder-Decoder LSTM Framework, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6200, https://doi.org/10.5194/egusphere-egu26-6200, 2026.
This study investigates Typhoon Ma-on (2022) genesis in a high-resolution simulation. Results show that multiple peaks of convection occur in the inner core (within 100 km) of Ma-on, accompanied by periodic evolution of convective and stratiform clouds. A new concept, key period, is defined as a period that starts with increasing vertical motion associated with convective bursts, followed by the development of stratiform clouds and dissipation of convective clouds. To identify potential indicators of TC genesis, the key period of genesis is compared with an earlier key period.
Diagnosis of the vorticity equation reveals that convective clouds make most contributions to vorticity growth through vertical advection and stretching. The results further indicate that deeper convection is not necessarily more conducive to genesis; rather, persistent convection with its maximum upward motion at lower to middle levels more effectively drives lower-level spin-up. Additionally, diagnosis of water vapor equation shows that, convective-dominated inner-core clouds enhance the secondary circulation through diabatic heating, thereby ensuring the radial inflow of moisture. In contrast, when stratiform clouds occupy large areas in the inner core, lower-level divergence becomes dominant, which may cause moisture outflow and therefore insufficient moisture supply.
These confirm the crucial role of persistent convective-dominated inner-core clouds during the key period approaching TC genesis. That requires a strengthened mid-level vortex, to which temperature responds to maintain thermal wind balance, forming a cold anomaly near the disturbance center, below the mid-level vortex. Consequently, convective instability increases in the boundary layer, favoring more sustained convective clouds and new convective bursts. That maintains convective-dominated inner-core clouds and ultimately promotes TC genesis.
How to cite: Wang, Z. and Chen, G.: Persistent Convective-Dominated Inner-Core Clouds: A Key Driver of Tropical Cyclone Genesis, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6609, https://doi.org/10.5194/egusphere-egu26-6609, 2026.
Typhoon rainfall is one of the primary meteorological hazards in Taiwan, with its spatial distribution strongly modulated by storm tracks and complex topography. In this study, landfalling typhoons affecting Taiwan from 1950 to 2024 are analyzed using gridded precipitation data. Hierarchical clustering is first applied to typhoon tracks to derive interpretable track types, with cluster numbers objectively determined from within-cluster variance. For each track type, a data-driven pattern extraction framework is subsequently applied to the corresponding rainfall fields, enabling the identification of dominant spatial features and representative canonical rainfall patterns. We further focus on typhoon events associated with pronounced rainfall impacts, systematically examining the correspondence between their rainfall characteristics and the identified canonical patterns, and quantitatively assessing the relationship between track clusters and high-risk rainfall spatial features. In addition, detailed grid-point analyses are conducted for extreme rainfall cases, including Typhoons Gaemi (2024) and Danas (2025), to evaluate whether their rainfall spatial distributions conform to the identified canonical patterns. By quantifying the linkage between typhoon track clusters and rainfall spatial patterns, this study provides a physically grounded reference framework for subseasonal-to-seasonal typhoon rainfall prediction and scenario-oriented analysis.
How to cite: Huang, W.-H., Chen, S.-T., Tsai, H.-C., and Chen, S.-Y.: Spatiotemporal Patterns of Typhoon Rainfall in Taiwan Associated with Track Clusters, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-7496, https://doi.org/10.5194/egusphere-egu26-7496, 2026.
In Earth System Model (ESM) simulations, estimates of Frozen Water Path (FWP), i.e., the column-integrated mass of precipitating and suspended ice particles, exhibit large uncertainties. In tropical regions with prevailing deep convection, FWPs are further characterised by a strong diurnal variability in simulations and observations. However, evaluating the simulated diurnal cycles has been difficult due to a lack of long-term satellite observations. Here, we use the novel machine-learned Chalmers Cloud Ice Climatology (CCIC), based on merged satellite datasets, to explore potential deviations of the captured diurnal cycle of FWP, both in ERA5 and km-scale models of the DYAMOND project. Moreover, we crosslink the diurnal cycle of FWP with the ones of precipitation and high-cloud cover to gain a broader view of the diurnal cycle of deep convection. We find a general agreement on the phase of the diurnal cycle of FWP in CCIC and DYAMOND km-scale models. In contrast, ERA5 shows shifted FWP diurnal cycles over all evaluated tropical regions. Both DYAMOND models and ERA5 underestimate the diurnal amplitude of FWP and overestimate the diurnal amplitude of precipitation. Diurnal cycles of the observed variables are characterised by pronounced land-ocean contrasts. Tropical land areas show a year-round afternoon peak of precipitation, which is closely followed by a peak of FWP, while high cloud cover peaks are delayed towards evening or midnight, depending on the season. Tropical oceans have a broad peak in high cloud cover in the evening hours. This is followed by a build-up of both precipitation and FWP over the night towards an early morning peak. These findings indicate that different processes drive the observed diurnal cycles of tropical deep convection over land and ocean, in line with previous research. This complicates the task of properly capturing the diurnal cycles of deep convection in reanalysis products, km-scale or in highly parameterised ESMs. Here, novel satellite products like CCIC with a high temporal resolution will help to identify and assess biases of the modelled diurnal cycles of deep convection in the tropics and to better understand the underlying drivers of deep convection.
How to cite: Leko, L., Behrens, G., Müller, N., Amell, A., Lauer, A., and Eriksson, P.: Diurnal Cycles of Tropical Convective Processes in Satellite-Observed, Reanalysed and Simulated Frozen Water Paths, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11111, https://doi.org/10.5194/egusphere-egu26-11111, 2026.
Accurately reproducing historical extreme tropical cyclone (TC) seasons is essential for understanding the physical mechanisms governing TC intensity and for improving future projections. However, previous studies have primarily relied on observational analyses, and the capability of climate models to reproduce extreme TC intensity—especially during the pre-satellite era—remains poorly explored. As an example, the year 1959 represents one of the most extreme TC seasons over the western North Pacific (WNP), during which five Category-5 TCs occurred between August and October, accounting for 15.6% of all TC records, the highest on record.
Based on TC best-track data and reanalysis products, we show that both anomalous TC genesis locations and frequent rapid intensification (RI) events contributed to the exceptionally high basin-mean lifetime maximum intensity (LMI) in 1959. More TCs formed over the open WNP basin around 150°E, where storms tend to achieve higher LMI, while the number of RI events far exceeded the climatological mean. These features are closely linked to large-scale circulation anomalies, including an enhanced monsoon trough, a weakened subtropical high, and the eastward shift of tropical upper-tropospheric trough (TUTT). Together, these circulation changes modulated TC genesis positions and enhanced RI occurrences, ultimately leading to a higher basin-mean TC LMI.
To further investigate the role of sea surface temperature anomalies (SSTAs) and assess the reproducibility of this extreme season, we conducted a set of three-month ensemble simulations using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) at 28-km horizontal resolution, with 10 ensemble members for each experiment. Four experiments were performed: a climatological SST experiment (CLIM), a realistic 1959 SST experiment (REAL), and two sensitivity experiments representing subtropical Central Pacific warming (CPW) and Indian Ocean warming (IOW), respectively. The REAL experiment successfully reproduces the enhanced TC intensity in 1959, along with the associated large-scale circulation anomalies, demonstrating the capability of NICAM to simulate historical extreme TC seasons. Sensitivity experiments reveal that CPW plays a dominant role in driving the extreme TC activity. The positive SSTA in the central Pacific induces a Matsuno–Gill–type response, generating anomalous low-level cyclonic circulation and upper-level anticyclonic circulation over the WNP. This response strengthens the monsoon trough and weakens the subtropical high, thereby shifting TC genesis locations, increasing RI frequency, and finally enhancing basin-mean LMI. In contrast, the IOW experiment shows a much weaker impact on both large-scale circulation and TC intensity.
These results highlight the critical importance of subtropical central Pacific SST forcing in shaping historical extreme TC seasons and demonstrate the value of high-resolution climate models in advancing our understanding of TC intensity variability.
How to cite: Chen, X. and Satoh, M.: Reproducing the Extreme 1959 Tropical Cyclone Season over the Western North Pacific Using a High-Resolution Model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15690, https://doi.org/10.5194/egusphere-egu26-15690, 2026.
Tropical cyclone (TC) intensification is strongly regulated by air–sea interactions and the thermal–salinity structure of the upper ocean. While salinity stratification influences vertical mixing and mixed-layer stability, its role in modulating rapid intensification (RI) remains insufficiently quantified in fully coupled modeling systems. Here we investigate multiscale air–sea coupling during Super Typhoon DOKSURI using a high-resolution Unified Wave Interface–Coupled Model (UWIN-CM). The UWIN-CM couples the Weather Research and Forecasting (WRF) Model, the University of Miami Wave Model (UMWM), and the Hybrid Coordinate Ocean Model (HYCOM) in a single framework that explicitly resolves momentum, heat, and freshwater exchanges among the atmosphere, surface waves, and ocean. Model simulations show that salinity-stratified barrier layers and subsurface warm layers suppress vertical entrainment beneath the storm core, thereby limiting storm-induced sea surface cooling and preserving near-surface ocean thermal energy during the intensification phase. The reduced surface cooling sustains stronger air–sea enthalpy fluxes and maintains elevated boundary-layer moist static energy, reinforcing the thermodynamic support for continued RI. Comparative experiments further demonstrate that salinity-controlled modulation of upper-ocean mixing governs the surface thermal response, whereas the associated enhancement of sea surface temperature and latent heat flux acts mainly as a positive feedback that reinforces storm convection.
How to cite: Mo, H. and Su, H.: Influence of Ocean Salinity on Tropical Cyclone Intensification in the Western North Pacific, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15948, https://doi.org/10.5194/egusphere-egu26-15948, 2026.
How to cite: Yan, L.: Composite analysis of the rainfall distribution caused by strong and weak landfalling tropical cyclones over the China Mainland, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21966, https://doi.org/10.5194/egusphere-egu26-21966, 2026.
Reliable seasonal forecasts of tropical cyclone (TC) activity are fundamental in helping stakeholders make informed decisions and mitigate economic and societal losses. While several public and private institutions issue seasonal forecasts of tropical storms for traditionally investigated basins, like the North Atlantic and the Western North Pacific, only a few provide global coverage, limiting confidence for other densely inhabited regions. Here, we evaluate the retrospective seasonal forecasts of TC activity across five basins (North Atlantic, Eastern and Western North Pacific, South Indian and South Pacific) over the period 1993-2016, using the Euro-Mediterranean Center on Climate Change Seasonal Prediction System 3.5 (CMCC-SPS3.5), a coupled general circulation model used for operational seasonal forecasts. CMCC-SPS3.5 skillfully captures key features of TC climatology (i.e., spatial distribution and seasonal cycle) and predicts with statistically significant skill their interannual variability, both in terms of numbers of tropical cyclones (NTC) and pressure-based accumulated cyclone energy (PACE). The model shows asymmetric performance, with TC activity overestimated in the Southern Hemisphere and underestimated in the Northern Hemisphere compared to the observations. Using a probabilistic clustering approach, we show that the model has statistically significant skill in year-to-year variability for specific track patterns across each basin. Using the North Atlantic basin as a case study, we show that the ENSO-TC teleconnection is stronger in CMCC-SPS3.5 compared to the observations, with implications for cyclone predictability. Moreover, our findings suggest that the basin-wide predictability is the result of the cumulative skill of individual clusters, providing insights for developing track-based forecasts. This study also demonstrates the readiness of CMCC-SPS3.5 for operational global TC seasonal forecasting.
How to cite: Giuliani, G., Cavicchia, L., Pascale, S., Sanna, A., Vidale, P. L., and Scoccimarro, E.: Regional and sub-basin seasonal tropical cyclone activity in the CMCC-SPS3.5 model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13618, https://doi.org/10.5194/egusphere-egu26-13618, 2026.
Accurate representation of the diurnal cycle of convection remains a persistent challenge in numerical weather and climate models. Previous studies have highlighted the importance of the phase relationship between convective available potential energy (CAPE) and precipitation in improving the simulation of rainfall timing and intensity. This study investigates the spatial and temporal characteristics of this phase relationship during the Indian summer monsoon, emphasizing the occurrence, intensity, and underlying causes of lead (days when CAPE maxima leads precipitation maxima) and lag (days when CAPE maxima lags precipitation maxima) days using 22 years of half-hourly IMERG precipitation data alongside temperature and humidity profiles from ERA5. Spatial maps reveal that precipitation exhibits greater variability in its diurnal phase than CAPE, with CAPE maxima generally preceding rainfall peaks except over the Bay of Bengal (BB), the Himalayan foothills, and parts of the Arabian Sea and Pakistan. Over Central India (CI), CAPE leads precipitation by about 3.5 hours, whereas over BB, it lags by approximately 9.5 hours. CAPE over CI shows a bimodal structure driven by both temperature and humidity variations, while over BB it displays a single, humidity-controlled peak. Across the monsoon season, about 72% of days are lead days and 25% are lag days. Despite their lower frequency, lag days often produce comparable or greater rainfall intensity, contributing 10–30% of the total seasonal precipitation, compared to 60–80% from lead days. The diurnal phase shift between CAPE and precipitation is primarily governed by changes in precipitation timing rather than CAPE evolution. Enhanced early-morning convection on lag days is linked to strong negative surface pressure anomalies and associated mid-tropospheric moistening, highlighting a distinct thermodynamic control on rainfall phase variability over the monsoon region.
How to cite: Chutia, T., Chakraborty, A., and Bhat, G. S.: Diurnal Relation between CAPE and Precipitation over Indian Region, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-612, https://doi.org/10.5194/egusphere-egu26-612, 2026.
Tropical cyclone (TC) Freddy traversed the South Indian Ocean (SIO) in 2023, setting a record for longevity and ranking secondly among historical TCs for both accumulated cyclone energy and potential destructiveness. Freddy’s long lifespan benefited from an anomalously strong Mascarene High, which steered Freddy westward along its northern flank, thereby preventing it encountering cold water areas. In addition to its long lifespan, we found Freddy experienced a record-breaking six rapid intensification (RI) events and analyzed the atmosphere-ocean conditions driving Freddy’s multiple RI events. The results indicated that during the six RI events, Freddy experienced high sea surface temperatures, weak vertical wind shear, high potential intensity, and high relative humidity, with all four environmental factors more favorable than historical averages. Notably, three of these RI events occurred within the Mozambique Channel, which also prolonged its coastal activity (21 d). This phase of TC Freddy generated extreme rainfall with maximum cumulative precipitation of 877 mm, compounded by secondary disasters, which caused substantial economic losses and fatalities in coastal areas. Additionally, it was found that the frequency of RI events and the number of intense TCs reaching Category 4 or above in the SIO showed statistically significant upward trends during 1980–2023, indicating a growing threat to Africa, which is likely to face more intense TCs in a warming climate. With rising sea levels and increasing threat from TCs, greater focus on coastal disaster prevention and mitigation strategies is needed to address the escalating risks associated with TCs in the SIO in the context of global warming.
How to cite: Wang, Q. and Guan, S.: Record-breaking lifespan, rapid intensification, and long-lasting coastal activity of Tropical Cyclone Freddy (2023) in the South Indian Ocean, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4204, https://doi.org/10.5194/egusphere-egu26-4204, 2026.
Tropical cyclones (TCs) are natural hazards that greatly affect ecosystems and human society especially in the Western North Pacific (WNP), which experiences the most TC activity. Although the threat of intense TCs is increasing in the WNP overall, there has been an observed decrease in TC activity around Taiwan in the recent decade. This study analyzes TC activity over the 1977-2024 period with frequency, duration, lifetime maximum intensity, and accumulated cyclone energy (ACE) as TC activity metrics. Environmental variables from reanalysis datasets were used to determine sea surface temperature (SST), mid-level relative humidity (RHMD), and vertical wind shear (VWS) along the TC track to examine their relationship with TC activity metrics. Decadal signals of TC metrics and environmental variables were calculated using a nine-point running mean of the annual time series. Decadal signals for frequency, duration, lifetime maximum intensity and ACE show significant trends (p<0.001) for TCs in the WNP basin. Meanwhile the frequency, duration, intensity, and ACE of TCs that hit Taiwan show a decreasing trend in the recent decade. Analysis of environmental variables for WNP TCs reveal decadal signals of SST, RHMD, and VWS with significant trends (p<0.001) that are favorable for TC development. Meanwhile for TCs that hit Taiwan, these encounter increasingly unfavorable conditions especially in the recent decade. Examination of tracks during the period of low TC activity in Taiwan (2017-2022) also shows contrast in the paths of strong and weak TCs. Weak TCs moved towards Taiwan and the southern coast of mainland China while strong TCs recurved and avoided Taiwan, moving northwards towards Japan and the eastern coast of China. The results highlight regional variability in TC activity which is important to consider for accurate forecasting and effective adaptation under the changing climate especially for countries vulnerable and frequently impacted by TCs.
How to cite: Ampil, L. J. and Liou, Y.-A.: Decadal Analysis of Tropical Cyclone Activity in the Western North Pacific, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4917, https://doi.org/10.5194/egusphere-egu26-4917, 2026.
The complex behavior of the Madden–Julian Oscillation (MJO), a key source for global subseasonal-to-seasonal predictability, has often been attributed to stochastic forcing by unresolved processes. Here, we demonstrate that its erratic evolution is fundamentally deterministic. Our data-driven model reveals a spectral dichotomy in low-dimensional MJO dynamics: predictable, quasi-periodic oscillations coexist with and are perturbed by deterministic chaotic forcing. The latter governs the emergent complexity of the system. Contrasting true deterministic forcing against stochastic surrogates shows that the system with deterministic forcing preserves bounded amplitude, while the stochastic processes induce unboundedness. Furthermore, we quantify how deterministic forcing yields greater complexity and unpredictability in the MJO’s evolution than stochastic surrogates. Specifically, deterministic mechanisms induce chaos through the mixing of periodic orbits within the spectral dichotomy, whereas stochastic forcing can only generate quasi-periodic behavior via resonant interaction with these orbits. These results reveal the deterministic origins of MJO complexity and offer new pathways for improving its prediction and understanding its predictability.
How to cite: Chen, G.: Deterministic Nonlinearity Over Stochastic Noise: Resolving MJO's Complexity and Predictability Drivers, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-5114, https://doi.org/10.5194/egusphere-egu26-5114, 2026.
Under future climate change scenarios, warmer ocean conditions are expected to substantially modify the behavior of tropical cyclones, particularly in regions where these systems are currently uncommon. In this work, tropical storm Delta (2005), which developed in the northeastern Atlantic basin, is used as a case study to explore how elevated sea surface temperatures influence cyclone intensity, internal convection, and the characteristics of its extratropical transition. Using high-resolution HARMONIE-AROME simulations, a reference experiment of the storm with boundary and initial conditions from ERA5 is compared with a warmer scenario in which sea surface temperatures are increased. The simulations reveal that enhanced surface heat fluxes strongly reinforce convection in Delta’s eyewall in a warmer scenario, promoting more vigorous and sustained updrafts, driving a marked deepening of the cyclone during its tropical stage. This intensification allows Delta to reach hurricane intensity. Later, the transition to an extratropical system begins earlier, extends over a longer period, and evolves into a more severe system. These changes translate into substantially stronger impacts over the Canary Islands (Spain), particularly through extreme wind gusts during the post-tropical stage. The findings underline the potential for anthropogenic climate change to increase the severity of storms with tropical features affecting western Europe, with important implications for future risk assessment in the region.
How to cite: Gómez-Plasencia, P., Rodríguez-Acosta, E. J., González-Alemán, J. J., Calvo-Sancho, C., Díaz-Fernández, J., Montoro-Mendoza, A., Bolgiani, P., Martín, M. L., Gómara, Í., and Morata, A.: Analysis of convective activity in Tropical Storm Delta in a warmer climate, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6739, https://doi.org/10.5194/egusphere-egu26-6739, 2026.
The period between 1 June and 30 November has been established by the National Oceanic and Atmospheric Administration (NOAA) as the operational hurricane season in the North Atlantic (NA), reflecting the environmental conditions typically conducive to tropical cyclogenesis and tropical cyclone (TC) intensification. However, historical records indicate that cyclonic activity may occasionally begin prior to the official season start. This study investigates 12 TCs at the tropical storm (TS) stage that occurred in April or May (defined here as the preseason) in the NA from 1980 to 2023 (NOAA’s HURDAT2). The analysis of this particular stage is motivated by the fact that it represents the highest intensity reached by preseason TCs during the study period. The research focuses on associated atmospheric and oceanic conditions derived from ERA5 reanalysis data available in the Copernicus Climate Data Store, including sea surface temperature (SST), 600-hPa relative humidity (RH) and vertical wind shear (VWS) between 200 and 850 hPa. Anomalies associated with preseason TSs were identified, characterized by unusually high SST (up to 1°C above the climatological mean) and mid-tropospheric RH (up to 40% above average), accompanied by significantly reduced VWS (up to 10 m/s below average). Additionally, positive seasonal trends in both mean and percentile values of SST (up to 0.4°C per decade) and RH (up to 3% per decade), along with decreasing VWS (up to −2 m/s per decade), were observed in regions where preseason TSs typically occurred. The waters northeast of the Florida Peninsula emerged as a particularly sensitive area, as half of the analysed preseason TSs occurred there. Furthermore, the Florida Peninsula and its surrounding region exhibited statistically significant trends in the examined variables, all of which are associated with the occurrence of preseason TSs. Results indicate that the environmental window for the occurrence of TCs in the NA may continue to expand, potentially increasing the likelihood of such events. These results may contribute to improving long-range preseason TC outlooks and to identifying regions potentially vulnerable to an extension of the hurricane season. Observed springtime trends may be connected with climate change (rising SST and RH) or reflect complex circulation changes (decreasing VWS in certain areas), highlighting the need for further research, particularly through climate projections assessing the persistence of these trends and the physical mechanisms underlying the observed anomalies.
How to cite: Szczapiński, A.: Atmospheric and Oceanic Conditions Associated with Preseason Tropical Storms in the North Atlantic (1980–2023), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9538, https://doi.org/10.5194/egusphere-egu26-9538, 2026.
This study attempts to understand how convectively coupled equatorial waves (CCEWs) can modulate extreme rainfall events along the west coast of India during different seasons. The waves are filtered from Outgoing Longwave Radiation (OLR), and their impact on extreme precipitation events is explored. The results show that CCEWs significantly amplify rainfall extremes over the west coast, with Rossby waves having the highest impact, followed by Mixed Rossby-Gravity (MRG) and Kelvin waves. The amplification in rainfall is largely driven by wave-induced enhancement in moisture convergence and the formation of large deep convective cloud systems. The CCEWs are also observed increasing likelihood of extreme events, with Rossby and MRG waves being the main contributors. These results advance our understanding of processes that can trigger extreme rainfall along the west coast and emphasise the potential to improve their forecasts by using the filtered equatorial wave activity.
How to cite: Koovekallu, P., Kottayil, A., Xavier, P., and Podglajen, A.: The Role of Convectively Coupled Equatorial Waves on the Intensity of Extreme Precipitation Events Over the West Coast of India, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16193, https://doi.org/10.5194/egusphere-egu26-16193, 2026.
The increasing impact of tropical cyclone (TC) rainfall underscores the need to understand regional variations in its spatial distribution. Using high-resolution satellite precipitation data from 1998-2023, this study investigates changes in the spatial inhomogeneity of TC rainfall across China. Results show that TC rainfall inhomogeneity decreases significantly by about 65%, indicating a reduced rainfall in historically high-TC-rainfall areas and an increase in low-TC-rainfall areas. Regionally, this decline is mainly governed by the intra‐regional component in East China, where opposing trends between high- and low-rainfall areas have substantially reduced spatial disparities. In contrast, North China shows a marked increase in TC rainfall, accompanied by increased intra‐regional but decreased inter‐regional inhomogeneity, largely offsetting each other. Other regions exhibit minor or insignificant contributions. These spatial reorganizations of TC rainfall in East and North China are closely linked to the northward migration of TC activity in recent decades, driven by reduced landfalling TCs over Taiwan and Fujian and increased inland penetration of TCs into East and North China. The findings reveal a restructuring of TC rainfall patterns and emphasize the growing regional contrasts in TC‐related hydroclimatic impacts, providing new insights for disaster risk assessments and regional climate adaptation strategies in China.
How to cite: Tu, S., Zeng, S., Zhong, Q., and Xu, J.: Recent changes in tropical cyclone rainfall over China, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20305, https://doi.org/10.5194/egusphere-egu26-20305, 2026.
Tropical cyclones (TCs) are among the most damaging weather systems, making it essential to understand how their characteristics evolve over time. While long-term variations in TC track and intensity have been widely examined, long-term trends in their lifespans remain poorly quantified. Using observational data from 1982 to 2024, we show that the annual mean duration of TCs has decreased significantly at a rate of -14.6±5.3 and -7.1±5.8 hours per decade, corresponding to total reductions of approximately 63 and 31 hours, in the Eastern and Western Pacific, respectively. Over the same period, TCs exhibit faster intensification on average prior to reaching their lifetime maximum intensity, followed by more rapid weakening afterward. These changes likely reflect the combined influence of evolving large-scale environmental conditions and modifications in TC internal convective processes. The shorter TC lifespan over the open ocean before entering coastal zones poses greater challenges for weather forecasting and disaster preparedness.
How to cite: Zhu, K., Su, H., and Zhai, C.: A significant drop in tropical cyclones’ lifespan in the Pacific over the past 40 years, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20410, https://doi.org/10.5194/egusphere-egu26-20410, 2026.
Tropical cyclone (TC) translation is governed by large-scale environmental steering and by the interaction between the vortex circulation and the planetary vorticity gradient (β-effect). This interaction generates asymmetric secondary gyres that induce a systematic poleward and westward drift (β-drift), which can persist even under weak steering and substantially modify TC trajectories. Despite its recognized dynamical importance, β-drift remains poorly quantified over the Indian Ocean and is rarely treated explicitly in operational track prediction systems. In this research, we present a basin-wide assessment of β-effect–induced TC drift over the Indian Ocean during pre-monsoon and post-monsoon seasons. Zonal and meridional winds and relative vorticity from ERA5 reanalysis at 850–200 hPa are collocated with IBTrACS best-track data to compute vertically averaged environmental steering velocities. Residual translation vectors, obtained by removing the steering component from observed TC motion, are interpreted as β-drift within a barotropic dynamical framework. The analysis reveals a statistically significant increase in β-drift magnitude with latitude, consistent with planetary vorticity gradient control. A non-linear regression model applied to multi-storm residual motion identifies dominant predictors of β-drift and yields an empirical parameterization of β-effect–induced translation for Indian Ocean cyclones. The results demonstrate that β-drift contributes substantially to TC motion variability, particularly under weak-steering regimes, and represents a systematic source of track forecast error. Incorporating this parameterization into forecast systems offers a pathway to improve operational TC track predictability over the Indian Ocean.
Keywords: Indian Ocean, Tropical Cyclones, β-Drift, Track Predictability, Weak Steering Flow, ERA5 Reanalysis
How to cite: Anoop, A. and c., S.: Quantifying β-Effect–Induced Drift of Tropical Cyclones over the Indian Ocean and Its Implications for Track Forecasting , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-20987, https://doi.org/10.5194/egusphere-egu26-20987, 2026.
Tropical Cyclones Ashobaa and Komen over the North Indian Ocean in 2015 represent a rare and remarkable event that formed during the monsoon season, making their occurrences uncommon. Ashobaa developed over the Arabian Sea during the transition phase, just prior to monsoon onset, while Komen formed over the Bay of Bengal during the active monsoon phase. Notably, Komen was the first system during the monsoon month of July to intensify into a cyclonic storm during the active monsoon period in the past 25 years.
The current study investigates the ocean-atmospheric conditions that facilitated the genesis and intensification of these systems into cyclonic storms. The associated dynamic and thermodynamic characteristics during their formation and evolution are analyzed to elucidate their interaction with the monsoon circulation. Our analysis reveals that warm sea surface temperature anomalies and weak surface winds over the northern Arabian Sea provided conducive conditions for the genesis of cyclonic storm Ashobaa, despite forming during the monsoon onset phase. The intensification of Ashobaa was aided by low vertical wind shear, high tropical cyclone heat potential, and enhanced moisture availability. These favorable conditions enabled the monsoon vortex to intensify unusually into cyclone Ashobaa. On the other hand, oceanic influence was comparatively weaker during the active monsoon phase. The evolution of cyclonic storm Komen was primarily driven by high low-level relative vorticity, enhanced moisture convergence, and a gradual increase in surface wind energy over the Bay of Bengal. Although both cyclones developed under low to moderate wind shear, their genesis and intensification processes were different. The 2015 monsoon variability over the North Indian Ocean was modulated by Komen through enhanced atmospheric forcing and monsoon–land interactions, whereas Ashobaa was largely driven by the oceanic parameters. The vital role of the ocean surface and the subsurface in the genesis and the intensification highlights the importance of incorporating accurate ocean initial conditions (surface and sub-surface) in the operational cyclone forecasting framework.
How to cite: Sarkar, Dr. I., Biswas, Dr. H. R., Pal, Dr. J., Sana, B., and Chaudhuri, Dr. S.: Elucidation of Indian Summer Monsoon: Impact of “ASHOBAA” and “KOMEN”, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21161, https://doi.org/10.5194/egusphere-egu26-21161, 2026.
Posters virtual: Mon, 4 May, 14:00–18:00 | vPoster spot 5
EGU26-4071 | Posters virtual | VPS2
African Easterly Waves as Drivers of Saharan Dust Transport and PM2.5 Extremes in the Intra-Americas RegionMon, 04 May, 14:33–14:36 (CEST) vPoster spot 5
African Easterly Waves (AEWs) are a dominant synoptic-scale feature of the tropical atmosphere, widely recognized for their role as precursors of tropical cyclones and for modulating summertime rainfall over the Atlantic basin and adjacent regions. However, their potential influence on the transport of Saharan dust across the Atlantic and its impacts on air quality has received comparatively less attention. In this study, we investigate the role of AEWs in modulating Saharan dust transport and its relationship with high PM2.5 concentration episodes over the Yucatán Peninsula. Using reanalysis data, we document a pronounced seasonal cycle in dust transport, with maximum concentrations during boreal summer (June–August), coinciding with the peak activity of AEWs. Spectral analysis reveals a significant contribution at periods of 4–9 days, consistent with the characteristic timescales of AEWs. To quantify their impact on air quality, intense dust events associated with AEWs were identified based on anomalies exceeding one standard deviation and compared with episodes of poor air quality driven by particulate matter. Our results indicate that AEWs account for approximately 26–31% of PM2.5 pollution episodes linked to dust over the Yucatán Peninsula, with event durations ranging from 1 to 8 days. These findings highlight the important role of AEWs in shaping the synoptic-scale variability of aerosol transport and surface air quality in the Yucatán Peninsula and southern Mexico, underscoring their relevance beyond tropical cyclogenesis and precipitation, particularly during the boreal summer.
How to cite: Jaramillo-Moreno, A. and Vázquez-Jiménez, C. S.: African Easterly Waves as Drivers of Saharan Dust Transport and PM2.5 Extremes in the Intra-Americas Region, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4071, https://doi.org/10.5194/egusphere-egu26-4071, 2026.
EGU26-4128 | ECS | Posters virtual | VPS2
Intraseasonal Modulation of the Chocó Low-Level Jet by the Madden–Julian OscillationMon, 04 May, 14:36–14:39 (CEST) vPoster spot 5
The Madden–Julian Oscillation (MJO) is the dominant mode of intraseasonal variability (30–90 days) in the tropical atmosphere, characterized by eastward-propagating convective and circulation anomalies that strongly modulate tropical and regional climate. Through its large-scale dynamical perturbations, the MJO influences moisture transport, low-level circulation, and precipitation over regions far removed from its primary convective center. However, its role in regulating low-level moisture fluxes over northwestern South America has received comparatively limited attention. In this study, we investigate the influence of the MJO on moisture transport toward Colombia, with particular emphasis on its modulation of the Chocó Low-Level Jet. Using MERRA-2 reanalysis, we characterize intraseasonal variability in low-level moisture advection and wind fields associated with different MJO phases defined by the Real-time Multivariate MJO (RMM) index. The analysis examines changes in low-level moisture transport, wind intensity, and large-scale convergence associated with the zonal displacement of the MJO convective envelope. Results show that the strength of the Chocó Jet depends strongly on the longitudinal position of the MJO convective center. Certain MJO phases enhance moisture transport from the eastern Pacific toward Colombia, favoring orographic ascent along the Andes and organized convection over the Colombian Pacific region, while other phases are associated with weaker moisture fluxes and reduced convergence. These findings highlight the role of the MJO in regulating intraseasonal moisture transport and low-level circulation over northwestern South America.
How to cite: Toro-Arenas, J., Jaramillo-Moreno, A., Carvajal-Serna, L. F., and Mesa-Sanchez, Ó. J.: Intraseasonal Modulation of the Chocó Low-Level Jet by the Madden–Julian Oscillation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4128, https://doi.org/10.5194/egusphere-egu26-4128, 2026.
