On the Dynamics of Indian Ocean Teleconnections into the Southern Hemisphere during Austral Winter

Stationary Rossby waves, forced by the Indian Ocean dipole (IOD), have an important role in Southern Hemisphere (SH) weather and climate, including promoting Australian drought and driving Antarctic sea ice variations. However, the dynamics of these teleconnections are not fully understood. During winter, the subtropical jet (STJ) should prohibit continuous propagation of a stationary Rossby wave into the SH extratropics due to the negative meridional gradient of absolute vorticity (b^*) on its poleward flank. The mechanisms that enable this teleconnection are investigated using observational and reanalysis datasets, a hierarchy of atmospheric model experiments and Rossby wave diagnostics. We conduct 90-member simulations using the Community Atmosphere Model, version 5, with an imposed local diabatic heating anomaly over the eastern Indian Ocean. We find an initial zonal propagation along the STJ waveguide, but after about 10 days, a poleward-arcing wave train appears in the extratropics that has the characteristics of the observed IOD teleconnection. Our results suggest that the Rossby wave can overcome the negative b^* barrier by (i) propagating directly poleward in the midtroposphere and thus avoiding this evanescent region in the upper troposphere, (ii) partly propagating directly through this barrier, and (iii) propagating around this barrier farther upstream to the west. A transient eddy feedback, previously postulated to be the key mechanism to allow the stationary Rossby wave to appear on the poleward side of the negative b^* region, reinforces the response but is not a requisite, which we confirm through comparison with a simplified linear model.