Late Quaternary dynamics of the Southern Westerlies over southern South America and link to paleoceanographic changes in the South Pacific

Atmosphere-ocean interactions play an important role for understanding processes and feedbacks in the Southern Ocean (SO) and are relevant for changes in Antarctic ice-sheets and atmospheric CO₂ concentrations. The most important atmospheric forcing at high and mid-latitudes of the Southern Hemisphere is the westerly wind belt (SWW), which strongly affects the strength and extension of the Antarctic Circumpolar Current (ACC), upwelling of deep-water masses, and controls the back-flow of intermediate waters to the tropics. In order to address orbital and millennial-scale changes of the SWW and the ACC, we present sediment proxy records from the Pacific SO including the Chilean Margin and the Drake Passage.

The Drake Passage (DP) represents the most important oceanic gateway along the ACC. Based on grain-size and geochemical properties of sediment records from the southernmost continental margin of South America, we reconstruct changes in DP throughflow dynamics over the past 65,000 years. We argue for a considerable total reduction of DP transport and reveal an up to 40% decrease in flow speed along the northernmost ACC pathway entering the DP during glacial times. Superimposed on this long-term decrease are high-amplitude millennial-scale variations, which parallel Southern Ocean and Antarctic temperature patterns. The glacial intervals of strong weakening of the ACC entering the DP imply a reduced Pacific-Atlantic exchange via the DP ("cold-water route").

The reduced Drake Passage glacial throughflow was accompanied by a pronounced northward extension of the Antarctic cold-water sphere in the Southeast Pacific sector and stronger export of northern ACC water into the South Pacific gyre. These oceanographic changes are consistent with reduced SWW within the modern maximum wind strength zone over the subantarctic ACC and reduced wind forcing due to extended sea-ice further south. Further north, at the southern boundary of the Atacama Desert, rainfall was enhanced during the last glacial maximum, related to a stronger subtropical jet over the mid-latitude South Pacific. Taken together our data indicate antiphased changes of the strength of the mid-latitude and subpolar core westerlies at orbital time-scales.