Frontal movements and property fluxes: Contributions to heat and freshwater trends in the Southern Ocean

Abstract

We examine the synoptic variability of temperature and salinity in the Southern Ocean using a static gravest empirical mode (GEM) mapping based on historical hydrography and a time-evolving projection of the GEM created using satellite altimetry (satGEM). The GEM and satGEM projections allow the separation of observed trends into a component due to a shift of the circumpolar fronts (adiabatic) using the satGEM and a component due to changes in water masses (diabatic) expressed as a temporal trend in residual between historical hydrography and the static GEM. The mean southward movement of the Antarctic Circumpolar Current (ACC) fronts drives an adiabatic warming of 1.196 ± 0.090 W m-2 distributed over most of the ACC and at all depths. This is strongest where the meridional temperature gradient is largest, such as in the upper 1000 dbar of the Subantarctic Front (SAF). There is a weak adiabatic freshening of 6.57 ± 0.18 mm yr-1 m-2, concentrated mainly below the Antarctic Intermediate Water and south of the SAF. Residuals between historical hydrography and the static GEM field, driven by diabatic changes in the water mass structure, have temporal trends in temperature and salinity. When these trends are integrated over the whole ACC there is a net cooling of -0.628 ± 0.129 W m-2 and strong freshening of 30.27 ± 0.70 mm yr-1 m-2. By combining the diabatic and adiabatic components and integrating over the ACC the net increase in heat and freshwater in the region is 0.570 ± 0.099 W m-2 and 36.91 ± 0.72 mm yr-1 m-2, respectively. The sum of the adiabatic and diabatic changes are consistent with previous trends inferred from observations. Copyright 2011 by the American Geophysical Union.