Rainfall-Salinity Coupling and its Impacts on Air-Sea Interaction in the MJO over the Tropical Pacific

Abstract:

The large-scale precipitation associated with the Madden-Julian Oscillation (MJO) is a significant fresh water source that can affect salinity and upper ocean thermodynamic and dynamic processes over the tropical Pacific. Deep convection and precipitation play a significant role in setting the upper ocean stratification in the tropics, especially over the warm pool. Barrier layers formation inhibit the mixing of surface properties deeper into the ocean. In this situation, it is more difficult for wind driven upper ocean mixing to cool SST by entrainment, which has implications for air-sea fluxes. The upper ocean salinity can be viewed as an aggregate effect of precipitation versus evaporation. In this study, a regional atmosphere-ocean coupled model (UWIN-CM in WRF-HYCOM mode) is used to explore the effect of the MJO on upper ocean stratification, air-sea fluxes over the western Pacific warm pool, and how the ocean feeds back into the MJO for the MJO events of February - April 2018. Salinity is input directly from the atmosphere model as a surface (e.g. top 1 meter) salinity source (sink) for evaporation (precipitation) without relaxation to climatology. This allows the upper ocean to respond to and feed back to the atmosphere in a manner that is more physically tied to the MJO events, compared with the method of salinity relaxation to climatology. To understand the impact of salinity stratification on the air-sea fluxes, a sensitivity experiment is conducted with salinity relaxed to climatology instead of input from precipitation. It is found that the direct precipitation-salinity coupling amplifies the dynamic and thermodynamic coupling between the MJO and the upper ocean. The comparison of these experiments show that the barrier layer is responsible for mixed layer changes that, in return, control SST changes with subsequent multi-scale interactions that extend to the eastern Pacific.