Physics and dynamics of density compensated temperature and salinity anomalies

Subducted density compensated temperature anomalies have been invoked as a possible way for mid-latitudes to alter the climate variability of the equatorial regions through the so-called `thermocline bridge', both in the Pacific and Atlantic oceans. The validity of this mechanism, however, depends crucially on whether such temperature anomalies can reach the equatorial regions sufficiently undamped to alter significantly the local heat balance.

The purpose of this talk is to provide theoretical results describing some basic theoretical properties of density compensated temperature and salinity anomalies for a realistic non-Boussinesq stratified ocean with a nonlinear equation of state. The main theoretical predictions are: 1) density compensated temperature and salinity anomalies propagate along mean trajectories at leading order; 2) the temperature (salinity) signatures of density compensated anomalies are systematically and significantly attenuated (conserved or amplified) when propagating equatorward from mid-and high-latitudes. The underlying physical mechanism for the amplitude variations of the temperature and salinity signatures of the density compensated anomalies are the spatial variations of the thermodynamic coefficients of sea-water, as well as the coupling of density and salinity anomalies at the gyre scale. The potentially important result of this study is to raise the intriguing possibility that subducted salinity anomalies, rather than their temperature counterparts, are the ones more likely to affect decadal tropical variability through their slow modulation of the equatorial mixed layer, by affecting for instance barrier layers by which salinity is known to strongly affect local heat fluxes and heat content.