Ocean-Atmosphere Interaction during the 2011 DYNAMO MJOs in COAMPS Simulations

During the Dynamics of the Madden-Julian Oscillation (DYNAMO) experiment in the fall of 2011, three MJO events were observed. The onset of the events was in mid-October (MJO1), mid-November (MJO2) and early December (MJO2). Hindcast simulations of these events were performed using the Navy's state-of-the-art Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) using analysis boundary conditions and data assimilation of the atmospheric observations. A unique 0.5 m resolution in the upper 10 m of the model ocean allows for a better prediction of the diurnal cycle in the mixed layer to obtain realistic surface fluxes. An analysis of the surface fluxes for the DYNAMO period and atmospheric conditions during the initialization of the November MJO atmospheric conditions from the COAMPS hindcast is presented. During the suppressed phase of the MJOs net surface heat flux were above 100 W/m2 over large areas in the western Indian Ocean and along the equatorial region, where daily averaged SSTs were above 29oC east of 60oN. Before the onset of the MJO events, SSTs were increasing and its diurnal amplitude is about 1.5 oC to 2 oC. During the active MJO phase, heat flux into the ocean was negative due to reduced downward solar radiation and wind-induced evaporation, rapidly cooling the upper ocean, deepening the mixed layer and strongly dampening the diurnal SST amplitude. The most striking direct oceanic response to the westerly zonal wind stress associated with the onset of the MJO is a rapidly accelerating Yoshida jet in the ocean mixed layer with equatorial zonal currents exceeding 1 m/s. A shear layer separates a subsurface seasonal jet and the surface jet forced by the MJO. The sea surface elevation response and upper ocean heat content show wind-generated westward propagating Rossby waves symmetric around the equator and an associated eastward equatorial propagating Kelvin wave response. The volume transport associated with these waves causes westward advection of low-salinity north and south of the equator, impacting the tropical ocean circulation.