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

Phase-preferential impacts of the Western-Pacific Subtropical High (WPSH) and Australian High (AH) on the Indian Ocean Dipole (IOD) is investigated in this study. We show that 14 out of 15 IOD events since 1990 have co-occurred with anomalous WPSH and AH during boreal summer and variations in the meridional sea-level pressure (SLP) gradient spanning the Maritime Continent (MC), induced by the WPSH and AH, play a key role in forming IOD. A strong and westward-extended summer WPSH can weaken the meridional gradient over the flanks of MC through increasing SLP over the South China Sea and thus suppress monsoonal cross-equatorial southerlies that further reduce both surface evaporation off Sumatra and coastal ocean upwelling, leading to positive sea surface temperature (SST) anomalies in the southeastern tropical Indian Ocean (TIO) and then the negative IOD. However, when the AH is strong, the subsidence and SLP is enhanced over the southeastern TIO and the strengthened inter-hemispheric pressure gradient intensifies the southeasterlies off Sumatra which enhance evaporation and ocean upwelling, leading to an SST cooling in the southeastern TIO and then the positive IOD. Conversely, weak AH-induced northwesterlies promote SST warming in this region, facilitating the negative IOD growth. The IOD phases can be further strengthened through exciting the positive air-sea feedbacks over the TIO. These results highlight the phase-preferential role of the two subtropical highs in modulating IOD, with a strong WPSH favoring the negative phase and a strong (weak) AH favoring the positive (negative) one.