A dynamical ocean feedback mechanism for the Madden-Julian Oscillation

Composite analysis is applied to study the dynamical ocean response to Madden-Julian (MJ) events, measured by anomalies in sea surface height from the merged TOPEX/Poseidon-European Remote Sensing satellite altimetry dataset. In each of the tropical ocean basins, significant equatorial waves are forced, which are shown to modulate the sea surface temperature (SST) by 0.2-0.3 degC in the absence of strong surface heat fluxes. In the Indian Ocean there is a clear dynamical response which may play a significant role in generating later MJ events. Surface westerly winds, associated with the active phase of the Madden-Julian Oscillation (MJO), force an eastward-propagating oceanic downwelling equatorial Kelvin wave, which, on reaching the eastern boundary at Sumatra, forces reflected downwelling equatorial Rossby waves and coastal Kelvin waves. The coastal Kelvin waves propagate southwards towards northern Australia and northwards into the Bay of Bengal, and will be important for local physical, chemical and biological processes. The equatorial Rossby waves propagate westward across the Indian Ocean, arriving in the western Indian Ocean approximately 80-100 days after the initial Kelvin wave was generated. The arrival of these waves generates positive SST anomalies which leads to convection and may trigger the next-but-one MJ event, or amplify the low-frequency tail of the MJO. This constitutes a coupled feedback mechanism from the ocean dynamics onto the MJO, somewhat similar to the delayed oscillator mechanism for the El Nino Southern Oscillation.