Decadal Supbolar Gyre Variability Linked to MWA-LIA Climate Regime Shift in the North Atlantic

Abstract:

Recent paleoceanographic reconstructions of subpolar North Atlantic variability during the last millennium describe oceanic conditions associated with a stronger subpolar gyre (SPG) during the Medieval Climate Anomaly (950-1250 CE) followed by a weak phase during the Little Ice Age (1450-1850 CE). Yet the mechanism behind this relatively abrupt SPG shift remains unclear. Here, we investigate the SPG dynamics during the last millennium in an ensemble of three transient climate simulations performed with the Max Planck Institute-Earth system model. In particular, we focus on the dynamics underlying a decadal-scale SPG transition from an initial strong to a later weak state, which is found in one of the simulations and whose features are similar to the reconstructed event.

Our results indicate that the simulated shift is triggered by a rapid increase in the freshwater transport from the Arctic toward the subpolar North Atlantic, which causes a broad freshening of the upper Labrador Sea. As a result, upper ocean densities largely decrease in the area, leading to a shut-down of oceanic deep mixing and, eventually, to a substantial weakening of the SPG. This event triggers a series of long-lasting feedbacks relating anomalous oceanic and atmospheric circulations, sea-ice extent, deep water formation in the Labrador Sea, and upper-ocean east-west density gradient within the SPG, all contributing to maintain the North Atlantic in an anomalous state for at least 200 years. A reorganization of the North Atlantic/Arctic ocean-atmosphere coupled system sustained by internal feedbacks acting on multicentennial time scales can therefore be initiated by changes in the SPG. Such reorganization, by contrast, does not depend on a long-lasting weakening of the Atlantic Meridional overturning circulation or a shift in large-scale modes of atmospheric variability, such as the North Atlantic Oscillation. The simulated SPG shift occurs right after a cluster of relatively small volcanic eruptions, which induces prominent thickening of the Arctic ice cap previous to the SPG shift. However, sensitivity experiments in which the volcanic forcing is removed can also show a similar abrupt SPG shift, pointing to a central role of internal climate variability in its inception.