Time-dependent response of the overturning circulation and pycnocline depth to Southern Ocean surface wind stress changes

Abstract

Abstract Changes in the Southern Ocean (SO) surface wind stress influence both the meridional overturning circulation (MOC) and stratification not only in the SO but in the global oceans, which can take multiple millennia to fully equilibrate. We use a hierarchy of models to investigate the time-dependent response of the MOC and low-latitude pycnocline depth (which quantifies the stratification) to SO wind stress changes: a two-layer analytical theory, a multi-column model (PyMOC), and an idealized general circulation model (GCM). We find that in both the GCM and PyMOC, the MOC has a multi-decadal adjustment timescale while the pycnocline depth has a multi-centennial timescale. The two-layer theory instead predicts the MOC and pycnocline depth to adjust on the same, multi-decadal timescale. We argue that this discrepancy arises because the pycnocline depth depends on the bulk stratification, while the MOC amplitude is sensitive mostly to isopycnals within the overturning cell. We can reconcile the discrepancy by interpreting the “pycnocline depth” in the theory as the depth of a specific isopycnal near the maximum of the MOC. We also find that SO stationary eddies respond very quickly to a sudden wind stress change, compensating for most of the change in the Ekman-driven MOC. This effect is missing in the theory, where the eddy-induced MOC only follows the adjustment of the pycnocline depth. Our results emphasize the importance of depth-dependence in the oceans’ transient response to changes in surface boundary conditions, and the distinct role played by stationary eddies in the SO.