The combined effect of tidal mixing in narrow straits and the Ekman transport on the variability of SST in the southern Indonesian Seas

The Indonesian archipelago is characterized by the warmest sea surface temperatures (SSTs) in the world’s oceans and is a key region of the world climate system. Kida and Wijffels (2012) reported that SST cooling around the Lesser Sunda Islands (LSIs) located at the southern Indonesia plays a crucial role in the seasonal variation of SST over a large area in the Indonesian Seas. Although they suggested that both the monsoonal winds and tidal mixing might significantly impact such SST cooling, their numerical model settings such as the spatial distribution of vertical diffusivity (tidal mixing) were too much idealized.

In this study, to investigate the mechanisms of SST cooling around the LSIs, we carry out a numerical simulation (CTRL experiment) using a regional ocean model where the realistic distribution of vertical diffusivity estimated from a high resolution three-dimensional baroclinic tide model is incorporated. The simulated results show that significant mixing in the narrow tidal straits between the LSIs locally creates well-mixed waters, which spread out northward (southward) during Austral summer (winter) and finally causes significant SST cooling over a large area of southern Indonesian Seas. We therefore carry out numerical experiments where the localized tidal mixing in the narrow straits is switched off and compare the results with that obtained from the CTRL experiment. It is shown that well-mixed waters created in the narrow straits are continuously advected by the northward (southward) flow driven by the western (eastern) monsoonal wind during Austral summer (winter) until the Indonesian Throughflow spreads them out over a large area of southern Indonesian Seas, demonstrating the importance of the combined effect of tidal mixing in the narrow straits and the Ekman transport.