Impact of the Indian Ocean sea surface temperature on the Southern Hemisphere middle atmosphere

Abstract. An index representing the midlatitude Indian Ocean Dipole (MIOD) is derived from the second empirical orthogonal function (EOF) mode of austral winter (JJA) sea surface temperature (SST) anomalies to examine its impact on the Southern Hemisphere middle and upper atmosphere. ERA5 reanalysis datasets, together with satellite observations and WACCM6 simulations, are used to analyze the associated atmospheric responses to MIOD. The results show pronounced but asymmetric impacts between positive and negative events, primarily driven by differences in wavenumber-1 planetary wave activity. Positive MIOD events enhance planetary-wave propagation from the Indian Ocean sector, leading to momentum deposition, variations in temperature, zonal winds, as well as a strengthening of the residual meridional circulation. These dynamical anomalies warm the midlatitudes and modify the vortex's vertical–meridional structure. Changes in stratospheric winds further regulate gravity-wave filtering, providing a pathway for SST signals to extend upward into the mesosphere. In contrast, negative events generally produce weaker and less statistically robust signals. These results identify Indian Ocean SST variability as an additional driver of large-scale atmospheric variability and reveal a previously underappreciated pathway through which the Indian Ocean can influence the middle and upper atmosphere at interannual timescales.