Interannual Variability of the Indonesian Throughflow: Importance of Salinity Effect

The Indonesian Throughflow (ITF) region possesses strong mixing and heavy freshwater input, but the role of salinity in the Indonesian Seas is largely unexplored. In this study, we calculate the ITF transports using various observations and assimilations and verify them using direct ITF transport estimates. We find that the halosteric component of ITF transport contributes (36±7)% of the total ITF variability, in contrast to (63±6)% by the thermosteric component, implying that the salinity variability in the Indonesian Seas is of remarkable importance in determining the interannual variability of ITF. Correlation analysis indicates that the interannual variability of ITF is mainly controlled by the El Niño-Southern Oscillation (ENSO) instead of the Indian Ocean Dipole. Salinity variability and propagation associated with precipitation, as well as Rossby waves associated with wind forcing and Ekman pumping, are examined to determine what processes cause the interannual variability of ITF. Result shows that, under the ENSO cycle, precipitation fluctuates over the Indonesian Seas due to the longitudinal shift of the Walker Circulation, modulates the salinity through strong mixing, and results in the variability of ITF transport. This result signals the importance of precipitation and salinity variability in controlling the interannual variability of ITF transport. The role of wind forcing and oceanic waves is revisited on the basis of this newly calculated ITF transport series. Result supports that ENSO-related wind forcing modulates the ITF transport via Rossby waves through the wave guide in the Indonesian Seas, which is in agreement with previous studies.