Séminaire

Given the devastating socioeconomics impact of tropical cyclones (TCs), understanding the amplitude of natural variability is of paramount importance for society. Historical records of TCs are too short to be able to capture such changes and TC paleo-sedimentary archives, spanning several millennia are still very few and patchy.
 
 Here we use a fully coupled ocean-atmosphere model to simulate the climate during the African Humid Period (AHP, ~10,000 – 5,000 years BP) and investigate potential changes in TC activities compared to pre-industrial (PI) climate. We prescribe the mid-Holocene (MH, 6,000 years BP) orbital forcing as well as more extensive vegetation and reduced airborne dust concentrations relative to PI due to wetter climatic condition in the Sahara. Using a downscaling technique for simulating large numbers of TCs, we analyze a set of sensitivity experiments in which prescribed vegetation and dust concentrations are changed in turn.
 
 The changes in solar insolation during the MH (MH_PMIP simulation) lead to a westward shift in the Western Pacific tropical storm activity, with an increase in TCs in the South China Sea. In the MH_PMIP simulation, the TC activity is also increased in the Southern Hemisphere. While stronger boreal summer insolation during the MH lead to a small increase in the tropical Atlantic cyclones, the Sahara greening and especially the reduced dust largely enhance the storm activity in that region (Fig. 1d), in particular in the Caribbean Sea. The greening of the Sahara and reduced dust alter the thermodynamic state, leading to more favorable conditions for tropical storm development. In particular, the strengthening of the West African Monsoon induced by the Sahara greening triggers a change in the atmospheric circulation that embraces the entire boreal tropical regions with a net reduction in wind shear during summer. The dust reduction further increases the warming of the ocean surface up to over a 1°C in the North Atlantic and South China Sea compared to the simulation using orbital forcing alone. Finally, our study highlights the importance of regional changes in land cover and dust concentrations in affecting the potential intensity and the genesis of TCs as well as how such changes may be able to feed back on a future warming climate.

marco.gaetani@latmos.ipsl.fr