Vendredi 6 avril - Air-sea coupling negative feedback on South Indian Ocean tropical cyclones in a regional coupled model
de 11:00 à 12:00
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Guillaume Samson (Mercator)
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Guillaume SAMSON (Mercator)
Titre: Air-sea coupling negative feedback on South Indian Ocean tropical cyclones in a regional coupled model
Sea surface enthalpy fluxes play a crucial role for tropical cyclones (TCs) by providing the necessary humidity to their development. But the sea surface temperature (SST) cooling driven by intense winds under the storm core reduces these fluxes and hence acts as a negative air-sea feedback on TC intensification. Until now, how this air-sea coupling impacts on TCs characteristics has only been quantified through case study analyses in the Atlantic and North Pacific basins.
In this study, we assess the influence of ocean-atmosphere coupling on Southern tropical Indian Ocean tropical cyclones characteristics from 20-years long experiments of a ¼° regional coupled model. The model realistically simulates the climatological mean state, the TCs characteristics and their related surface cooling, but overestimates the number of TCs by roughly 1 TC per month. A twin atmospheric experiment forced with SST from the coupled experiment is further performed to isolate the effect of air-sea interactions from other environmental factors influencing TCs.
The spatial distribution of cyclogenesis is not affected by air-sea coupling because other large-scale atmospheric fields associated with tropical cyclones such as vertical wind shear are not sensitive to ocean-atmosphere coupling in our model. However, the forced simulation displays considerably more cyclogenesis than the coupled simulation (+30%) and more intense tropical cyclones (+10% of TCs > 30 m/s). Mean intensification rate of TCs > 30 m/s is decreased by 20% in the coupled simulation in relation with a 15% reduction of inner-core surface fluxes due to the cold SST anomalies. Our analysis further reveals that pre-storm SSTs must on average be at least 0.5 to 1°C higher in the coupled run to allow a TC to reach an equivalent intensity compared to the forced run. The robustness of these results is confirmed by another set of 20-years long experiments that uses a different convection parameterization.