Combined Effects of Tidal Mixing in Narrow Straits and the Ekman Transport on the Sea Surface Temperature Cooling in the Southern Indonesian Seas

Although the SST cooling events around the Lesser Sunda Islands (LSIs), which contribute to the seasonal variation of SST in the southern Indonesian Seas, are thought to be controlled by tidal mixing, the distribution of tidal mixing was highly idealized in previous studies. This study reinvestigates the mechanisms of such SST cooling events using MITgcm, which incorporates the realistic distribution of tidal mixing. The observed SST variability is well reproduced in the numerical experiment; SST is largely reduced along the northern (or southern) coast of the LSIs during austral summer (or winter). Particle tracking suggests that the narrow tidal straits between the LSIs, where well-mixed waters are produced by vigorous tidal mixing, play a key role in controlling the SST cooling. During austral summer, the cold SST signal of well-mixed waters propagates both northward and southward from the narrow straits as baroclinic Kelvin waves. On the northern side of the LSIs, in particular, coastal upwelling caused by the northwest monsoon raises the background thermocline, which affects the propagation of baroclinic Kelvin waves such that the accompanying cold SST signal becomes much enhanced compared with that on the southern side of the LSIs. During austral winter, in contrast, the Ekman transport enhances southward through-strait flow so that baroclinic Kelvin waves can propagate only southward affecting the SST on the southern side of the LSIs. This demonstrates the importance of the combined effects of tidal mixing in narrow straits and the Ekman transport on the SST cooling in the southern Indonesian Seas.