Tropical teleconnections to Antarctic circulation during the millennial-scale Dansgaard-Oeschger climate cycles.
Abstract:The abrupt Dansgaard-Oeschger events define Northern Hemisphere (NH) climate variability during the last glacial period. They are out of phase with gradual temperature variations recorded in Antarctic ice cores. Ocean teleconnections are thought to link the temperatures of both hemispheres via the thermal bipolar seesaw. Additionally, atmospheric teleconnections link the NH to tropical circulation – in particular the position of the Inter-tropical Convergence Zone (ITCZ). Recent modeling has demonstrated the influence of the ITCZ on the Southern Hemisphere (SH) jets. In the context of millennial variability, this suggests that changes in high latitude SH atmospheric circulation could have been in-phase with the distant D-O signal, rather than local temperatures. To date, there has been little proxy evidence spanning the last glacial period to support this prediction.
We examine phasing between δ18O, methane concentration, and deuterium excess data form the new West Antarctic Ice Sheet (WAIS) Divide ice core. Deuterium excess is a second order isotope parameter, sensitive to ocean surface conditions and atmospheric circulation. We resolve a long-standing issue by redefining this parameter, revealing zonally coherent millennial-scale sea surface temperature changes in the Southern Ocean. Superimposed upon this variability, we find evidence for changes in SH atmospheric circulation in phase with the D-O events. This is found in the strong and in-phase coherence between deuterium excess and atmospheric methane. Methane records tropical hydrological changes and is identical to the abrupt D-O signal. WAIS deuterium excess tracks methane during D-O events, showing large variability significantly before local temperature change. We interpret Antarctic deuterium excess to be driven by two primary signals: 1) variability in Southern Ocean sea surface temperatures at fixed locations, and 2) variability in the spatial sampling of that surface water. The latter is driven by atmospheric circulation and teleconnetions to the tropics. This interpretation is supported by water tracing atmospheric model experiments. Our results highlight the role for atmospheric processes, and tropical teleconnections in particular, in addition to oceanic processes in the machinery of millennial climate change.