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This paper evaluates the intraseasonal variability of sea surface temperature (SST) along the Sumatra-Java southern coast using available satellite-derived oceanic and atmospheric data combined with output from a numerical model. The result reveals that the intraseasonal variability of SST is greater during boreal summer-fall (June-October) than during boreal winter-spring (November-May). Composite analysis shows a correlation between positive/negative intraseasonal SST variabilities and coastal downwelling/upwelling, as well as onshore/offshore Ekman transport during summer-fall. During this period, with the significantly increasing role of oceanic advection, oceanic processes are evidently enhanced and dominate the intraseasonal variability of SST. Meanwhile, the contribution of atmospheric processes drops by 67%. During winter-spring, the intraseasonal SST is primarily contributed by atmospheric processes but has a nonsignificant relationship with sea level anomalies. Intraseasonal SST anomalies vary out of phase with surface wind anomalies. The result also shows a relatively small contribution by vertical processes throughout the year, with the maximum in April and the minimum during August-September. Further analysis reveals that the alternating dominance of atmospheric and oceanic processes on intraseasonal variability of SST is responsible for the seasonality along the Sumatra-Java southern coast. Moreover, the result indicates that the seasonality in intraseasonal SST is different in the eastern Indonesian Seas, which tends to be relatively strong in boreal winter. Distinct dominance of atmospheric and oceanic processes in intraseasonal SST is the main reason for these differences in seasonal variation characteristics.

Plain Language Summary Previous studies indicated energetic intraseasonal variability of sea surface temperature (SST) along the Sumatra-Java southern coast. Using available satellite-derived oceanic and atmospheric data combined with output from a numerical model, we found that the intraseasonal variability of SST is greater during June-October than during November-May; that is, seasonality occurs in the intraseasonal variability. Then, we focused on the potential factors responsible for this seasonality. Our additional analysis reveals that oceanic processes, especially horizontal advection, contribute most to intraseasonal variabilities during summer-fall. During winter-spring, atmospheric processes, especially shortwave and latent heat flux, contribute most to intraseasonal variabilities. The alternating dominance of atmospheric and oceanic processes on intraseasonal SST induces seasonality along the Sumatra-Java southern coast. By revealing the seasonality and ascertaining the main physical processes of the intraseasonal variability of SST, we believe this work can also confer some insight into the ongoing study on regional oceanography and climate change.