The role of tropical waves in the genesis of Tropical Cyclone Seroja in the Maritime Continent

Tropical cyclone Seroja was one of the first tropical cyclones to significantly impact Indonesian land, and the strongest one in such close proximity to Timor Island. In April 2021 Seroja brought historic flooding to near-equatorial regions of Indonesia and East Timor, as well as impacting Western Australia. Here we show that the unusual near-equatorial cyclogenesis in close proximity to a land mass was due to “perfect storm” conditions associated with multiple wave interactions. Specifically, this was associated with enhanced equatorial convection on the leading edge of a Madden–Julian Oscillation (MJO) event. Within the MJO, the interaction between a convectively coupled equatorial Rossby wave and two convectively coupled Kelvin waves span up the initial vortex and accelerated cyclogenesis. On average, such favorable atmospheric conditions can occur once per year. These results indicate the potential for increased predictability of tropical cyclones over the Maritime Continent

Dynamics of the Atlantic Marine Intertropical Convergence Zone

International audienceA generalized ω-equation is used to identify the contributions from different processes that force upward motions in the Atlantic Marine ITCZ (AMI) from a numerical mesoscale simulation of June 2010. This ω-equation separates the diabatic heating contributions, which lie at the core of the Weak Temperature Gradient (WTG) framework, from the dynamical terms. Three layers of atmosphere are found with different balance. In the Marine Atmospheric Boundary-Layer (MABL), the upward motions in the AMI are induced by the frontogenesis and buoyancy components, which are regulated by the ageostrophic adjustment due to the presence of thermal-wind imbalance. The balance of these three processes well captures the variability of the vertical velocity and the associated precipitation, meaning that boundary-layer processes play a central role in the AMI dynamics. In the layer [600-2,000 m], a zone of strong vertical wind-shear just above the MABL, the upward motions are induced by the ageostrophic adjustment and radiative components, which are counteracted by evaporation of convective precipitation. Above 2,000 m the ascending motions are driven by the deep convection heating, as expected by the WTG framework, and more surprisingly by the ageostrophic adjustment term within the Tropical Easterly Jet. Thanks to the use of the ω-equation, these results extend the current WTG framework to the boundary layer, where it is not expected to hold. In the free troposphere, the WTG framework only accounts for half of the AMI ascent, the other half being forced by the dynamical terms

Tropical Waves Are Key Drivers of Extreme Precipitation Events in the Central Sahel

International audienceExtreme precipitation events (EPE) are often associated with severe floods and significant damages in Central Sahel. To better understand their formation and improve their forecasts, we investigate the sub‐seasonal drivers of EPEs. A composite analysis reveals that moist, cyclonic and upper‐level divergence anomalies are found on average as a result of several tropical waves. The equatorial Rossby wave (ER) dominates at large scale providing a moist and convectively‐active anomaly over the northern Sahel together with a smaller‐scale African Easterly Wave (AEW). The Madden‐Julian Oscillation provides upper‐level divergence anomalies and a Kelvin wave increases convection during the EPE. Statistics show the prevalence of AEW and emphasize ER as a key driver of EPE. The co‐occurrences of several tropical waves, especially those involving AEW, ER, and Kelvin waves, increase the probability of EPE. Monitoring these tropical waves combinations could improve EPEs forecasts

Sahelian Precipitation Change Induced by SST Increase: The Contrasting Roles of Regional and Larger‐Scale Drivers

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Extreme precipitating events in satellite and rain-gauge products over the Sahel

International audienceOver the recent decades, extreme precipitation events (EPEs) have become more frequent over the Sahel. Their properties, however, have so far received little attention. In this study the spatial distribution, intensity, seasonality, and interannual variability of EPEs are examined, using both a reference dataset based on a high-density rain gauge network over Burkina Faso and 24 precipitation gridded datasets. The gridded datasets are evaluated in depth over Burkina Faso while their commonalities are used to document the EPE properties over the Sahel. EPEs are defined as the occurrence of daily accumulated precipitation exceeding the all-day 99th percentile over a 1° × 1° pixel. Over Burkina Faso, this percentile ranges between 21 and 33 mm day−1 . The reference dataset show that EPEs occur in phase with the West African monsoon annual cycle, more frequently during the monsoon core season and during wet years. These results are consistent among the gridded datasets over Burkina Faso but also over the wider Sahel. The gridded datasets exhibit a wide diversity of skills when compared to the Burkinabe reference. The Global Precipitation Climatology Centre Full Data Daily version 1 (GPCC-FDDv1) and the Global Satellite Mapping of Precipitation Gauge Reanalysis version 6.0 (GSMaP-gauge-RNL v6.0) are the only products that properly reproduce all of the EPE features examined in this work. The datasets using a combination of microwave and infrared measurements are prone to overestimate the EPE intensity, while infrared-only products generally underestimate it. Their calibrated versions perform better than their uncalibrated (near-real-time) versions. This study finally emphasizes that the lack of rain gauge data availability over the whole Sahel strongly impedes our ability to gain insights in EPE properties

Development of Tropical Cyclones from African Easterly Waves : comparative study of three African Easterly Wave events during the CADDIWA campaign

During the CADDIWA (Cloud-Atmospheric Dynamics-Dust Interaction in West Africa) airborne campaign that took place in September 2021, three tropical disturbances initiated from African Easterly Waves were sampled. Two of them reached the Tropical Storm state and were named Rose and Peter by the National Hurricane Center. The last one, later named Pierre-Henri, failed to develop. After a validation of reanalysis data for september 2021 against the data collected during the campaign, a comparative study of the three events will be conducted using ERA5, satellite and campaign data, with a focus on energy budgets. Several processes that may be catalysts or inhibiters for tropical cyclogenesis (AEW-Monsoon -, AEW-AEW -, dust-dynamics interaction ...) will especially be discussed

Évolution de notre compréhension du système de mousson ouest-africain

International audienceSince the pioneering GATE international field experiment in 1974, little progress had been made on the documentation and understanding of the West African Monsoon system. Ten years after the AMMA project was initiated, how has our knowledge of this monsoon evolved? Although the main actors of this atmosphere ocean-continent coupled system were already known, AMMA has allowed us to study thoroughly the details of the processes at play and their coupling. Starting from a static view of the system, a more dynamical view is now emerging, involving the whole Earth system : the whole African continent, mid-latitudes, the Indian monsoon and the entire tropical belt. This has opened new avenues of investigation into the intraseasonal variability of the West African monsoon.Depuis l'expérience internationale pionnière GATE en 1974, notre documentation et notre compréhension du système de mousson africaine avaient évolué relativement lentement. Comment notre vision de cette mousson ressort-elle après les 10 ans du projet AMMA ? Si les principaux acteurs de ce système couplé atmosphère-océan-continent étaient déjà bien connus, AMMA a permis d'approfondir le détail des processus en jeu et de leur couplage. D'une vision statique de ce système émerge petit à petit une vision plus dynamique où intervient désor-mais l'ensemble du globe : l'Afrique en entier, les latitudes tempérées, la mous-son indienne et même toute la ceinture tropicale. Une voie est ainsi ouverte pour comprendre sa variabilité intra-saisonnière. Abstract Progress on understanding of the West African Monsoon syste

The role of tropical waves in the genesis of Tropical Cyclone Seroja in the Maritime Continent

International audienceTropical cyclone Seroja was one of the first tropical cyclones to significantly impact Indonesian land, and the strongest one in such close proximity to Timor Island. In April 2021 Seroja brought historic flooding to near-equatorial regions of Indonesia and East Timor, as well as impacting Western Australia. Here we show that the unusual near-equatorial cyclogenesis in close proximity to a land mass was due to "perfect storm" conditions associated with multiple wave interactions. Specifically, this was associated with enhanced equatorial convection on the leading edge of a Madden-Julian Oscillation (MJO) event. Within the MJO, the interaction between a convectively coupled equatorial Rossby wave and two convectively coupled Kelvin waves span up the initial vortex and accelerated cyclogenesis. On average, such favorable atmospheric conditions can occur once per year. These results indicate the potential for increased predictability of tropical cyclones over the Maritime Continent

Equatorial waves triggering extreme rainfall and floods in southwest Sulawesi, Indonesia

On the basis of detailed analysis of a case study and long-term climatology, it is shown that equatorial waves and their interactions serve as precursors for extreme rain and flood events in the central Maritime Continent region of southwest Sulawesi, Indonesia.Meteorological conditions on 22 January 2019 leading to heavy rainfall and devastating flooding in this area are studied. It is shown that a convectively coupled Kelvin wave (CCKW) and a convectively coupled equatorial Rossby wave (CCERW) embedded within the larger-scale envelope of the Madden-Julian oscillation (MJO) enhanced convective phase, contributed to the onset of a mesoscale convective system that developed over the Java Sea. Low-level convergence from the CCKW forced mesoscale convective organization and orographic ascent of moist air over the slopes of southwest Sulawesi. Climatological analysis shows that 92% of December-February floods and 76% of extreme rain events in this region were immediately preceded by positive low-level westerly wind anomalies. It is estimated that both CCKWs and CCERWspropagating over Sulawesi double the chance of floods and extreme rain event development, while the probability of such hazardous events occurring during their combined activity is 8 times greater than on a random day. While the MJO is a key component shaping tropical atmospheric variability, it is shown that its usefulness as a single factor for extreme weather-driven hazard prediction is limited