Atmospheric teleconnection of North Atlantic cooling to the Southern Hemisphere westerlies

The realization from paleoclimate studies that North Atlantic cooling can significantly alter the global climate has motivated a number of modeling and theoretical studies over the last few years to understand how and why these teleconnections come about. In particular, recent proxy studies suggest an intensification of wind-driven upwelling in the Southern Ocean during intense North Atlantic cooling events, suggesting the possibility of robust atmospheric teleconnections from the Northern Hemisphere affecting the Southern Hemisphere westerlies. We explore the latter hypothesis using simulations of an AGCM coupled to a reduced-gravity ocean, and with reference to current thinking regarding extratropical-tropical atmospheric dynamical linkages.

When we impose a North Atlantic cooling event in our model, we find a significant strengthening of the southern midlatitude westerlies, in particular during the austral winter, and in the South Pacific. The other pronounced climate change is a marked southward shift of the tropical rainbelt, indicating alteration of the Hadley circulation. Our analysis indicates that the teleconnection can be broken into two parts: first, the northern hemisphere cooling shifting the ITCZ southwards with a pronounced effect on the Hadley circulation, and then the altered Hadley circulation in turn affecting the southern midlatitude westerlies through the former's control of the southern subtropical westerlies and subsequent effect on the eddy-driven midlatitude westerlies, in particular on the 'split-jet' structure of the South Pacific westerlies.

We will also briefly touch upon the possible implications of this teleconnection on understanding atmospheric CO2 variations in glacial climates.