Southern Ocean wind response to North Atlantic cooling and the rise in atmospheric CO2: Modeling perspective and paleoceanographic implications

Abstract

A recent study found enhanced upwelling rates in the Southern Ocean during the last glacial termination that coincided with the deglacial warming in Antarctica and the rise in atmospheric CO2. They hypothesized that the intensification of Southern Hemisphere midlatitude westerlies, the presumed cause of the increased wind‐driven upwelling, was triggered by an initial cooling within the glacial North Atlantic whose influence was then communicated to the southern midlatitudes through an atmospheric teleconnection. In this study, we explore the viability of the above hypothesis using a modeling strategy, focusing on the atmospheric teleconnection. In simulations where North Atlantic cooling was applied, the model Intertropical Convergence Zone shifted southward, and westerlies and wind stress over Southern Ocean increased by as much as 25%. While the perennial westerly anomalies occur over the entire Southern Ocean, they are strongest over the South Pacific during the austral winter. When the wind stress anomalies were applied to an Earth system model incorporating interactive marine biogeochemistry, atmospheric CO2 rises between 20 and 60 ppm, depending on the biological response. We thus confirm the viability of the proposed atmospheric teleconnection hypothesis. The teleconnection appears to involves two distinct steps: first, the North Atlantic cooling shifts the Intertropical Convergence Zone southward, weakening the southern branch of the Hadley circulation, and second, how the altered Hadley circulation in turn modifies the structure of midlatitude westerlies in the South Pacific, via the former's influence on the Southern Hemisphere subtropical jet. This study underscores the control of the Northern Hemisphere has on southern midlatitude westerlies, mediating by tropical circulation, in contrast to past paleoclimate hypotheses that the magnitude and position of the southern midlatitude westerlies was controlled by global mean temperature. Our results do not preclude other potential mechanisms for affecting Southern Ocean ventilation, in particular through oceanic pathways.