Southern Ocean mean state and internal variability responses to sea ice albedo perturbations

Liping Zhang, NOAA/GFDL, UCAR, Princeton, NJ, United States, Thomas L Delworth, NOAA/GFDL, Princeton, NJ, United States and William Cooke, Geophysical Fluid Dynamics Laboratory, Princeton, NJ, United States
While Arctic sea ice is rapidly decreasing in association with increasing surface air temperature, the Southern Ocean (SO) clearly exhibits cold SST and sea ice increasing trends in 1979-2015. This study explores how the Southern Ocean (SO) mean state and internal variability respond to surface sea ice albedo change. The mean state response is found to comprise both fast and slow processes. The fast response comprises an increased upward shortwave radiation in response to a positive sea ice albedo anomaly, which induces cold SST anomalies and sea ice formations in the surface. The persistent cold SST and sea ice increase in the SO gradually increase surface density, weaken ocean stratification and in turn enhance the SO deep convection. The deep convection brings subsurface warm water to the surface and causes a warm SST anomaly. This slow time scale warming from below eventually dominates over the cooling from initial shortwave anomalies. The weak ocean stratification mean state further affects the SO internal variability associated with the deep convection fluctuations. It’s found that the weak ocean stratification corresponds to internal variations with small amplitude and short period. The weak ocean stratification background erodes the subsurface heat reservoir through enhanced gravitational instability and enhanced vertical mixing, thereby prohibiting the amplitude increase of internal variability. The subsurface heat and surface freshwater take a shorter time to destabilize the ocean stratification and are more prone to cause the occurrence of deep convection under a weaker stratification mean state, which corresponds to a short period of internal variability. The SO SST and the associated climate impacts have less predictability because of changes in the internal variability. The opposite is also established for an opposite perturbation in sea ice albedo.