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. Observational datasets from HALOE (1991–2004) and SABER (2002–2020), together with simulations from WACCM6, are analyzed using composite and regression methods. Results reveal asymmetric atmospheric responses between positive and negative MIOD events. Positive MIOD events are associated with substantial stratospheric and mesospheric temperature perturbations and with vertically and meridionally structured zonal-wind anomalies, including height-dependent sign reversals at high latitudes. In contrast, negative events generally yield weaker and less statistically robust signals. The underlying processes involve enhanced planetary-wave propagation from the Indian Ocean sector, which deposits momentum and modifies zonal winds and the residual meridional circulation, accompanied by thermal-wind adjustments. The altered circulation redistributes ozone in the midlatitude stratosphere, while changes in stratospheric zonal winds regulate gravity-wave filtering and contribute to additional variability in the mesosphere. These findings highlight the importance of incorporating Indian Ocean variability into stratospheric and climate modeling to improve understanding of atmosphere–ocean coupling mechanisms and their implications for polar climate change.