Impact of Southern Hemisphere westerlies on deglacial and recent changes in oceanic carbon and atmospheric CO2

Abstract:

During the early part of the last deglaciation, atmospheric CO₂ rose in two abrupt phases of 23 ppm. The underlying mechanisms driving the CO₂ increase remain a subject of intense debate. By performing transient simulations of the deglaciation with a 3-dimensional carbon isotope enabled model, and performing an extensive comparisons with paleo-proxy records, we show that the atmospheric CO₂ rise can be explained by enhanced Southern Ocean ventilation resulting from intensified southern hemisphere (SH) westerlies and reduced buoyancy forcing.

The SH westerlies have intensified and shifted polewards over the last decades and are projected to further increase over the coming century. Since the Southern Ocean is today's largest sink of carbon, we further explore the impact of recent changes in SH westerlies on the ocean carbon storage using a high-resolution ocean-sea-ice-carbon cycle model. Similar to past changes, the recent strengthening of the SH westerlies led to a loss of natural carbon. However, it also induced an increase in anthropogenic carbon uptake, thus resulting in a stagnation of Southern Ocean carbon uptake since ~1980.

Future changes in Southern Ocean carbon uptake will thus result from a fine balance between natural carbon release and anthropogenic carbon uptake, which will itself depend on changes in SH westerlies and Southern Ocean stratification.