Modulation of Indonesian Throughflow transport by the Madden-Julian Oscillation across ENSO and IOD phases in the Makassar Strait

Abstract The Indonesian Throughflow (ITF) is the principal low-latitude ocean gateway linking the Pacific and Indian Oceans, and its variability contributes to inter-basin redistribution of heat and freshwater. Although seasonal and interannual variability of the ITF is well documented, the extent to which intraseasonal forcing interacts with the background climate conditions remains insufficiently quantified. Here we investigate the modulation of Makassar Strait ITF variability by the Madden-Julian Oscillation (MJO) under concurrent El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) conditions. Daily 1/12° GLORYS12 reanalysis fields for 2004–2020 are evaluated against INSTANT mooring observations and used to construct strong MJO phase composites of circulation and transport. The results show clear phase-dependent intraseasonal variability. Surface currents generally strengthen during suppressed MJO phases and weaken during active phases, with seasonal dependence in the magnitude and depth of the response. The largest transport variability occurs within the thermocline, with values at 100 m reaching approximately 4.7-5.0 Sv during JJA phases 1–2, while transport during other seasons and phases is generally lower. ENSO and IOD modify the MJO response in different ways. El Niño can enhance early-phase surface currents while reducing late-phase transport, whereas La Niña maintains a stronger background throughflow without proportionally increasing MJO anomalies. Positive IOD events introduce additional seasonal and phase-dependent modulation, particularly during JJA-SON. These results indicate that Makassar Strait transport variability reflects state-dependent interaction among monsoonal forcing, interannual climate modes, and intraseasonal MJO variability rather than simple linear superposition.