Oceanic restratification processes associated with tropical cyclone intensification

Tropical Cyclones (TC) are strongly coupled systems as the underlying warm ocean serves as an energy source for the TC while the strong cyclonic winds modify the ocean state. Good predictions of the TC development are dependant on our knowledge of the ocean heat content which may favor or inhibit the TC. Understanding how the ocean stratification evolves at the same time the TC does is thus crucial to improve TC forecasts.

The 2018-2019 cyclonic season of the South Western Indian Ocean was active and saw the development of nine intense TCs. These cyclones went through regions with different oceanic properties in terms of stratification and heat content. The aim of this study is to understand how such ocean properties affect TC evolution.

To this end, we conducted several idealized simulations of TC using the same atmospheric state but with different oceanic profiles (temperature, salinity) derived from 5-month MERCATOR analysis data (from November 2018 to March 2019). The experiments were conducted using a state of the art coupled modelling system with CROCO (for the ocean) and Meso-NH (for the atmosphere) models with a grid spacing of 4 km.

The TC lifecycle (i.e intensity, structure) as well as the ocean response (i.e. sea surface cooling, advection and mixing processes) are investigated with a particular emphasis on the heat budget analysis. We found that a rapid TC intensification phase occurred due to the warm oceanic surface layers (the first 40 meters) and a strong decaying phase occurred due to the cooler underlying ocean. Moreover we highlight the chronology of the cooling processes in the oceanic mixed layer and the importance of the advection processes within it, which are then relayed by vertical mixing.