Intraseasonal Variability of the Indonesian Throughflow
Tamasiunas, Mariana Carolina Nieva
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Oceanic, atmospheric and land processes over the Maritime Continent (MC) play an important role in the Earth’s climate system. In addition, a primary ocean current within the MC, the Indonesian Throughflow (ITF), is a major part of the global thermohaline circulation. This work investigates the intraseasonal variability (20-90 days) of the Indonesian Throughflow, which is primarily forced by the Madden-Julian Oscillation (MJO) using a 1/12° global HYbrid Coordinate Ocean Model (HYCOM) reanalysis and satellite altimeter data. To quantify the ITF transport variations over the MJO life cycle, composites of the ITF transport through the major straits in the Indonesian Seas are constructed. A prominent transport reduction during the MJO active phase is found in all major straits. Also, a transport enhancement during the suppressed phase, that is comparable to the reduction, is evident. As a result, the net effect of the MJO on the ITF transport is very small because of the cancellation of the enhancement and reduction. Upper ocean variability associated with the MJO in the Indonesian Seas is then compared with major atmospheric variables. During both active and suppressed phases of the MJO, surface winds over the MC are consistent with the spatial pattern of MJO-induced upper ocean currents. The role of remote ocean response in the ITF variations is further investigated. During the active phase, sea surface height and along-shore current anomalies generated in the central Indian Ocean by the MJO forcing propagate along the coast of Java and Sumatra as a coastal Kelvin wave, which largely influences the ITF transport at exit passages and Makassar strait. During the suppressed phase, the propagation of Kelvin wave is not clearly detected in the composite, suggesting that local winds over the MC are mostly responsible for the ITF variation. The effect of Kelvin waves on the ITF variability is further examined for MJO events observed during DYNAMO field campaign. The propagation of Kelvin waves from the equatorial Indian Ocean to the ITF exit passages is evident during both active and suppressed phases, suggesting that upwelling Kelvin waves contribute to the ITF transport through the exit passages for some MJO events.