Intraseasonal Variability of the Indonesian Throughflow

Abstract:

The Indonesian Throughflow (ITF), a primary ocean current within the Maritime Continent (MC), is a major part of the global thermohaline circulation. This work investigates the intraseasonal variability (20-90 days) of the ITF, which is primarily forced by the Madden-Julian Oscillation (MJO). Previous studies show that during the MJO active phase, Kelvin waves generated in the central equatorial Indian Ocean propagate along the coast of Java and Sumatra islands, affecting the ITF transport at exit passages in the Indonesian Seas and Makassar strait. However, the variation of the ITF transport through these straits over the MJO life cycle, especially during the suppressed phase, has not been quantified. A primary purpose of this study is to quantify the MJO-induced ITF transport through major straits in the Indonesian Seas using a 1/12° global HYbrid Coordinate Ocean Model (HYCOM) reanalysis and satellite altimeter data. MJO composites of ITF transport through major straits are constructed. A prominent reduction of ITF transport through major straits is found during the MJO active phase, and a transport enhancement comparable to the reduction is evident during the suppressed phase. As a result, the net effect of the MJO on the ITF transport is very small due to the cancellation of the enhancement and reduction. The magnitude of the MJO-associated ITF transport variation through all ITF exit passages is about 6 Sv, which is about 50% of the total transport. While the propagation of coastal Kelvin waves during the MJO active phase and their impact on ITF transport is clearly evident in the composite, such Kelvin wave influence on ITF transport is not clearly detected during the suppressed phase. This suggests that anomalous winds over the MC area are mostly responsible for the ITF variation during the suppressed phase in most MJO events. Yet, during some MJO events, remotely-forced upwelling Kelvin waves and their significant impact on the ITF transport are evident.