Ocean Response to CINDY/DYNAMO MJOs in Air-Sea-Coupled COAMPS

Abstract

The response of the ocean to three Madden-Julian Oscillation (MJO) events during the fall of 2011 is simulated by the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) in a fully coupled mode with high resolution in the atmosphere and ocean. The model simulates the cooler sea surface temperature (SST) and disappearance of the diurnal cycle in SST during the active phase of the MJO and it compares well with the observed SST from Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) buoys. The most striking direct 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-1. These jets are found in the model as well as the RAMA buoy observations. In the model, the Yoshida jet is superimposed on the seasonal Wyrtki jet that has a subsurface local maximum between 50 and 150 m. A shear layer separates the 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 propagating equatorial Kelvin wave response. After the third MJO event, the Yoshida jet spans most of the equatorial Indian Ocean. Upon reaching the Indonesian coast, the associated equatorial Kelvin wave reflects and generates additional westward propagating equatorial Rossby waves. The volume transport associated with these waves causes the westward advection of low-salinity north and south of the equator, impacting the tropical ocean circulation.