Dynamics of the Three-Dimensional Ocean Overturning: Glacial Transitions and Inter-Hemispheric Coupling

Abrupt climate transitions during the last glacial period are apparent in Greenland ice cores. These Dansgaard-Oeschger (DO) events are the manifestation of a climate system with multiple steady states and the ability to transition rapidly between them. High-resolution Southern Hemisphere ice cores have complementary oscillations known as Antarctic Isotope Maximum (AIM) events, and the anti-phase correspondence of these Northern and Southern Hemisphere oscillations is known as the bipolar seesaw. DO events have been linked to freshwater perturbations in the North Atlantic, which lead to changes in North Atlantic Deep Water formation strength. There is currently no accepted physical mechanism for the bipolar seesaw and no explanation for the inter-hemispheric timing between DO and AIM events, despite the fact that they are thought to be dynamically linked.

This study builds upon a recent steady-state multi-basin model model of the global overturning circulation by extending the governing equations to include transient dynamics. This isopycnal box model determines the stratification and diabatic water mass transformations in each basin. The idealization to multiple, two-dimensional basins permits zonal mass transport along isopycnals in a Southern Ocean–like channel, while retaining the dynamical framework of residual-mean theory. Our model exhibits abrupt transitions in the overturning structure related to transitions in the Southern Ocean surface buoyancy forcing; hysteresis also occurs. The dominant mechanism for these transitions is a shift in the location of outcropping Lower Circumpolar Deep Water density classes between regions of negative buoyancy forcing in the south (region of sea ice formation) and a region of positive buoyancy forcing located further north in the Antarctic Circumpolar Current (region of sea ice melt and net precipitation). These outcropping transitions respond to transient changes in the vertical stratification of the northern basins following a perturbation in North Atlantic Deep Water strength. This stratification adjustment occurs over a timescale which closely matches the observed 200-year time lag between DO events and AIM events.