Substantial SST decrease in the Banda Sea associated with the Madden-Julian Oscillation during the winter of 2015

A large portion of the Indonesian Throughflow (ITF) flows through the Banda Sea and thus the circulation within the Banda Sea plays an important role in the ITF transport. Previous studies show that the sea surface temperature (SST) of the Banda Sea exhibits strong seasonal and intraseasonal variations. In this study, we report a substantial 2^oC decrease of the basin-averaged SST identified from the satellite observation during December 2015. It occurs during the same period when a strong MJO event passing through the Maritime Continent (MC) region. The 2^oC decrease of SST is comparable to the mean seasonal SST variation (~3^oC), but much larger than the intraseasonal SST variation in the boreal winter season (0.6^oC-0.7^oC). In addition to the SST variation, satellite data also reveal that surface currents reverse its direction from westward to eastward in the central Banda Sea associated with the MJO event. The HYCOM reanalysis data are validated for this case based on the comparison with satellite observations, and they are used to further investigate the processes that cause the SST decrease. It is found that the large SST cooling is due to the combined effects of enhanced surface heat fluxes and strong upwelling of cold salty waters in the southwest Banda Sea. The cold upwelled waters are advected from the southwest to central basin by the reversed upper ocean currents associated with the MJO. While the upwelling is generally weak during boreal winter in the Banda Sea, the strong upwelling around the Ombai Strait during the MJO event largely contributes to the extreme SST cooling. We speculate the unusual SST cooling caused by strong upwelling links to the 2015 strong ENSO event.