The response of the Indonesian Seas to the monsoonal winds is investigated using a regional ocean model with an emphasis on understanding the mechanisms behind the observed seasonal sea surface temperature (SST) variability. Understanding the seasonal SST variability in the Indonesian Seas is crucial for understanding tropical climate variability since atmospheric deep convection is located directly above these seas. Our model results suggest that the monsoonal winds play a dominant role in creating the spatial SST variability and are responsible for inducing about half of the basin-averaged SST variability. The remaining half is found to be caused by conditions in surface heating, which is brought about by radiation and atmospheric temperature and humidity. The northwesterly wind during Austral summer is roughly uniform over the basin so wind mixing occurs roughly uniformly and no significant spatial SST variability is forced. The southeasterly wind during Austral winter, on the other hand, has a significant spatial variation with a maximum wind speed located in the middle of the basin. This spatial variability causes Ekman upwelling and downwelling in the north and south of the Indonesian Seas, respectively, and establishes a cold SST region in the northeast. The presence of the shallow shelf region in the east is also found critical to the formation of this cold SST event. Without the shelf, lateral advection of warm water from the west will trap the cold SST region to the eastern boundary.
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