Evolution of Southern Hemisphere Westerly asymmetry since the Early Miocene

The Southern Hemisphere Westerlies (SHW) are a crucial component of Earth's climate system because they regulate moisture, temperature, and ocean circulation throughout the Southern Hemisphere. They also affect the Northern Hemisphere through atmospheric and oceanic teleconnections. Yet, their variations are poorly constrained for past warm climate intervals that could serve as analogs for possible future climate evolution. Here we present geochemical and magnetic data from South Pacific Ocean sediments to reconstruct dust provenance since the Early Miocene. We find that the eolian dust source shifted from Central South America to Australia at ~8.4 million years ago. Our climate simulations also suggest that a strong middle Miocene subtropical jet facilitated dust transport from Central South America to the South Pacific Ocean. If the middle Miocene is used as a possible analog for future warming, we suggest that the SHW tends to be more zonally asymmetric in warm climate states and that SHW asymmetry may be important for future climate projections.