Influence of interhemispheric thermal gradient on the tropical climate state and Southern Hemisphere Westerlies

A recent paleoclimate proxy study (Anderson et al., 2009) indicated enhanced upwelling rates in the Southern Ocean during the late Pleistocene glacial termination that coincided with the deglacial warming in Antarctica and the rise in atmospheric CO2. Anderson et al. (2009) suggested the possibility of robust atmospheric teleconnections from the Northern Hemisphere to the Southern Hemisphere, specifically affecting the midlatitude westerlies. Our previous modeling study (Lee et al. 2011) showed the viability of such a teleconneciton, albeit in a relatively simplified modeling framework (an atmospheric general circulation model coupled to a simplified ocean) and using idealized experiments. In the study, we explore the climate connection between interhemispheric thermal gradient, tropical climate state, Southern Ocean wind response, and ocean overturning circulation in a realistic 21,000 year transient coupled ocean-atmosphere climate model simulation encompassing the most recent deglaciation and the Holocene (the TraCE-21k simulation). We find a significant connection between the interhemispheric surface temperature gradient and the upper level westerlies over the Southern Ocean, in particular in the South Pacific during austral winter. In tropics, we observe a marked southward shift of the tropical precipitation, indicating an alteration of the Hadley circulation. The climate response resembles our previous idealized study (Lee et al., 2011), albeit with smaller magnitude. Our results provide further evidence for the hypothesis that northern hemisphere climate changes can influence the southern hemisphere westerlies via the tropics through an atmospheric teleconnection.