Southern Hemisphere westerlies as a driver of the early deglacial atmospheric CO rise.

The early part of the last deglaciation is characterised by a ~40 ppm atmospheric CO rise occurring in two abrupt phases. The underlying mechanisms driving these increases remain a subject of intense debate. Here, we successfully reproduce changes in CO, δC and ΔC as recorded by paleo-records during Heinrich stadial 1 (HS1). We show that HS1 CO increase can be explained by enhanced Southern Ocean upwelling of carbon-rich Pacific deep and intermediate waters, resulting from intensified Southern Ocean convection and Southern Hemisphere (SH) westerlies. While enhanced Antarctic Bottom Water formation leads to a millennial CO outgassing, intensified SH westerlies induce a multi-decadal atmospheric CO rise. A strengthening of SH westerlies in a global eddy-permitting ocean model further supports a multi-decadal CO outgassing from the Southern Ocean. Our results highlight the crucial role of SH westerlies in the global climate and carbon cycle system with important implications for future climate projections.