Integration of a Global Set of Millennial-Scale Climate Records: Insights From EOF Analysis

Paleoceanographic records suggest that large changes in North Atlantic climate were associated with transitions among one of three possible modes of North Atlantic deepwater (NADW) formation: a modern mode, a glacial mode, and a Heinrich mode. Transitions from modern to glacial modes of NADW are manifested as dramatic fluctuations of North Atlantic climate (Dansgaard-Oeschger (D/O) events). Atmospheric transmission of the North Atlantic D/O signal around the globe is suggested by high-resolution climate records that display the same structure of change that, within dating uncertainties, is synchronous with the North Atlantic signal. A large reduction in the THC during the Heinrich mode causes no additional cooling in the North Atlantic region, while contemporaneous warming observed in some regions of the southern hemisphere indicates that the large reduction in NADW formation decreased meridional heat transport from the South Atlantic. A similar "seesaw" effect may hold for the Younger Dryas event, but the absence of an antiphased southern signal associated with older D/O events suggests that any changes in oceanic heat transport accompanying the changes from modern to glacial circulation modes were either too small or too short to be clearly registered in existing climate records. We thus expect to see two modes of variability in climate related to changes in the THC, one associated with atmospheric transmission and one associated with an oceanic seesaw. We use empirical orthogonal function (EOF) analysis to reduce well-dated time series of climate change into spatially coherent, orthogonal eigenvectors. Each eigenvector represents an independent mode of variability within the data set, which can then be mapped geographically. Our EOF analysis indicates that the last deglaciation was dominated by two climate responses. The first EOF (68% of variance) captures the global warming from glacial to interglacial conditions that was interrupted in many places by a cooling centered at 12.5 kyr BP, or about the time of the Younger Dryas. In contrast, the second EOF (15% of variance) represents a millennial-scale response whereby the EOF scores are positive over Antarctica (except Taylor Dome) and in the South Atlantic, whereas they are negative at all other sites. A similar pattern is suggested for isotope stage 3. This spatial pattern is consistent with an atmospheric transmission of the North Atlantic signal except for those areas in the Southern Hemisphere where operation of the seesaw produced an antiphased response as predicted by a large change in the THC.