The dependence of internal multidecadal variability in the Southern Ocean on the ocean background mean state

Abstract:

Previous studies have shown the existence of internal multidecadal variability in the Southern Ocean using multiple climate models. This variability, associated with deep ocean convection, can have significant climate impacts. In this work, we use sensitivity studies to investigate the linkage of this internal variability with the background ocean mean state. We find that mean ocean stratification that is dominated by salinity change in the subpolar region influences whether this variability occurs, as well as its time scale. For background stratification states in which the variability occurs, weaker ocean stratification corresponds to shorter periods of variability and vice versa. The amplitude of convection variability is largely determined by the amount of heat that can accumulate in the subsurface ocean during periods of the oscillation without deep convection. A larger accumulation of heat in the subsurface reservoir corresponds to a larger amplitude of variability. The subsurface heat buildup is a balance between advection that supplies heat to the reservoir and vertical mixing/convection that depletes it. Subsurface heat accumulation can be intensified by both an enhanced horizontal temperature advection by the Weddell Gyre, and an enhanced ocean stratification leading to reduced vertical mixing and surface heat loss. The realism of this low frequency variability is discussed based on paleoclimate records over the Antarctica.