Comparison of two extreme SST cooling events in the Banda Sea associated with the 2015 and 2007 boreal winter Madden-Julian Oscillations

Abstract:

Banda Sea is a large marginal sea in the Maritime Continent (MC). The variation of sea surface temperature (SST) and therefore air-sea interaction in the Banda Sea could play an important role in MJO propagation over the MC region. Two substantial SST cooling events in the Banda Sea associated with boreal winter MJOs are identified from satellite observations. One event took place in winter 2015 during strong El Niño, and the other occurred in winter 2007 during strong La Niña. Both events produce basin averaged SST cooling exceeding 2^oC, much larger than the typical cooling associated with most boreal winter MJOs. The HYCOM reanalysis data are validated for these cases and used to further investigate processes responsible for the large SST decrease. A one-dimensional ocean model, initialized with the HYCOM reanalysis and driven by hourly fluxes derived from satellite data and ERA5, is able to realistically simulate the basin averaged SST cooling during both events. The heat budget analysis reveals that while MJO-induced surface heat flux anomaly primarily controls the SST cooling during the 2007 event, vertical mixing largely contributes to the SST cooling in the 2015 event. The latter was because the subsurface waters in 2015 were much colder due to the elevated thermocline depth associated with the strong El Niño condition, which allows for a substantial SST cooling via vertical mixing under the MJO conditions. The importance of the upper ocean temperature structure of the Banda Sea for the enhanced SST cooling through vertical mixing in the winter of 2015 is further confirmed by the additional sensitivity simulations, in which initial conditions for the two events with nearly the same SSTs are replaced by each other. Results from the HYCOM reanalysis suggest that the coastal upwelling around islands in the southern basin and the advection of heat by MJO-induced surface currents contribute to changes in SST distribution within the Banda Sea.