The Role of the Indonesian Throughflow in Independent Indian Ocean Dipole Events: Mixed Layer Temperature Variation in the Tropical Eastern Indian Ocean

The Indian Ocean Dipole (IOD) is a major climate mode affecting the tropical Indian Ocean and remote regions, but the role of oceanic processes—particularly the Indonesian Throughflow (ITF)—in IOD development, especially for events independent of El Niño-Southern Oscillation (ENSO), remains poorly understood. Here we investigate the impact of ITF transport variations on the development phase of IOD using a reanalysis dataset over 1950-2018. We find that ITF anomalies are more pronounced during independent IOD than during ENSO‑related IOD. For independent positive IOD (pIOD), ITF transport significantly strengthens from March to May (MAM) and precedes the peak IOD by about three months; for independent negative IOD (nIOD), a significant weakening occurs from May to June (MJ) with a one‑month lead. After removing ENSO signals, a significant positive correlation emerges between ITF anomalies and the Dipole Mode Index (DMI), with maximum correlation when ITF leads by 1-3 months. Regression analysis shows that ITF contributes about 40-50% of the DMI amplitude during independent IOD. The physical mechanism is revealed by the heat budget: enhanced ITF transport drives anomalous upwelling (10°S-equator) that brings cold subsurface water upward, cooling the mixed layer off Sumatra; the vertical advection term dominates the cooling tendency (>90%). Our findings demonstrate that the ITF acts as an independent oceanic forcing for IOD development, particularly in the absence of ENSO, and provide a quantitative basis for incorporating ITF monitoring into IOD early warning systems.