Preferential Impacts of the Western-Pacific Subtropical High and Australian High on the Indian Ocean Dipole Phases

The influence of the western Pacific subtropical high (WPSH) and Australian high (AH) on the Indian Ocean Dipole (IOD) phase is investigated in this study. We refer to this influence using the term phase-preferential impacts. The results show that 14 out of 15 IOD events since 1990 have co-occurred with an anomalous WPSH and AH during boreal summer. These circulation anomalies induce variations in the meridional sea-level pressure (SLP) gradient spanning the Maritime Continent (MC), which play a key role in IOD formation. A strong and westward-extended summer WPSH can weaken the meridional SLP gradient over the flanks of MC by increasing SLP over the South China Sea, thereby suppressing monsoonal cross-equatorial southerlies. This reduction in winds decreases both the surface evaporation off Sumatra and coastal ocean upwelling, leading to positive sea surface temperature (SST) anomalies in the southeastern tropical Indian Ocean (TIO) and a subsequent negative IOD. In contrast, when the AH is strong, the subsidence and SLP over the southeastern TIO increase, strengthening the inter-hemispheric pressure gradient and the southeasterlies off Sumatra. These winds enhance evaporation and ocean upwelling, leading to SST cooling in the southeastern TIO, thereby setting the stage for a positive IOD. Conversely, weak AH-induced northwesterlies promote SST warming in this region, facilitating the evolution of a negative IOD. The IOD phases are further strengthened through the excitation of positive air-sea feedback over the TIO. These results highlight the phase-preferential role of the two subtropical highs in modulating the IOD, with a strong WPSH favoring the negative phase and a strong (weak) AH favoring the positive (negative) phase.