Impact of Ocean Diurnal Warm Layers on the Marine Atmospheric Boundary Layer Processes

In the current context of global change, numerical models are key tools to explore the characteristics of the Earth climate and anticipate its evolution. The appropriate representation of air-sea exchanges is still a major challenge for numerical weather forecasting and climate models (CM), since these exchanges contribute to the global Earth's energetics. The COmprehensive Coupling approach for the Ocean and the Atmosphere project (COCOA) aims at revisiting the overall representation of air-sea interactions in CMs by considering physical aspects among others. The tropical regions, and in particular the Indian Ocean area impacted by the Madden-Julian Oscillation (MJO) are preferential study areas where latent heat transfers largely condition the atmospheric water content and where important model biases are observed.

During suppressed MJO phase corresponding to light wind conditions, the solar radiation absorbed at the ocean surface can lead to the formation of ocean diurnal warm layers (DWL) that gives in turn a daily cycle of the sea surface temperature (SST). The response of the atmospheric boundary layer (ABL) to DWL is thought to be an important aspect of the convective initiation of the MJO. In this study, the mean and turbulent structures of the ABL is investigated using Large-Eddy Simulations (LES). The simulations are evaluated using in situ data collected at the air-sea interface and within the ABL during the Cindy-Dynamo field campaign. Using a conditional sampling analysis, the turbulent field appears to be organized into coherent structures which control the diurnal evolution of convection activity and modulate the transfers at the surface and trough the ABL. The impact of SST diurnal cycle on ABL fluxes is particularly analyzed as well as the ABL top evolution.

This study will be completed with a coupled ocean-atmosphere LES model allowing to simulate air-sea interactions. This approach will be useful to take into account subgrid processes and the impact of the gustiness effect on the air-sea turbulent fluxes in large scale models.