Influence of the Southern Indian Ocean Dipole on Interannual Variability in the Intensity of the South Asian Subtropical Westerly Jet in Boreal Winter

Abstract In boreal winter, orographic blocking by the Iranian Plateau and Tibetan Plateau splits the upper-tropospheric westerly jet and gives rise to the South Asian subtropical westerly jet (SAJ) along the southern flank of the two plateaus. As a prominent component of the wintertime circulation over subtropical Eurasia, the SAJ exerts important influences on regional weather and climate. Using ERA5 reanalysis, the HadISST sea surface temperature dataset, and ECHAM6 numerical experiments, this study investigates the interannual variability of the SAJ and its underlying physical mechanisms. The leading mode of the SAJ is characterized by a coherent strengthening or weakening of the jet across its longitudinal extent, accounts for 42.2% of the total variance, and exhibits a significant quasi-4-yr periodicity. The Southern Indian Ocean Dipole (SIOD) is identified as a key driver of SAJ intensity variability. During the positive phase of the SIOD, the equatorward extension of warm SST anomalies over the southeastern Indian Ocean favors a Rossby wave response. Meanwhile, the dipolar SST forcing directly modifies atmospheric vertical motion and produces contrasting regional Hadley cell anomalies over the western and eastern Indian Ocean. The associated upper-tropospheric southerly and northerly anomalies favor the development of an anomalous anticyclone south of the Iranian Plateau and Tibetan Plateau, whose northern flank strengthens the westerlies within the SAJ corridor. In addition, SIOD-related tropospheric warming enhances the meridional temperature gradient within the SAJ region, further strengthening the upper-tropospheric westerlies through thermal wind balance. ECHAM6 simulations confirm the key physical processes inferred from statistical diagnostics.