Observational constraints on the response function of Southern Ocean SST to SAM forcing

Abstract:

Recent coupled model studies of the polar Southern Ocean (SO) revealed an initial (fast) cooling, but longer-term (slow) and equilibrium warming, of sea surface temperature (SST) in response to stratospheric ozone depletion and the concurrent shift of the Southern Annular Mode (SAM) to its positive phase. Yet there is much spread across models in the amplitude and time scale of the equilibration, so that even the sign of the implied recent-decade SST response to ozone depletion is not robust.

Here we use the framework of a simple layered model (representing mixed layer, seasonal thermocline and upper permanent thermocline of the SO south of the polar front) combined with observations of the SO, to derive constraints on the equilibrium response of the real-world SO to annually-repeating seasonal SAM forcing. We obtain simple expressions for the equilibrium response in terms of the SAM-induced air-sea fluxes of heat and momentum and the SO horizontal and vertical temperature stratifications. These are then evaluated using satellite observations and atmospheric reanalysis data, as well as in-situ ocean climatologies.

Our estimates suggest that, for observed characteristics (mixed layer depths, stratification, phasing of the SAM-forcing in season and space), the well-documented surface-forced fast SO SST cooling is large in comparison to the dynamically-induced subsurface-forced warming, and thus also largely sets the sign and amplitude of the equilibrium response.
Exploration of the parameter space of coupled model versus observed ratios of horizontal to vertical stratifications provides a rationale for the discrepant equilibrium responses.