A practical indicator for surface ocean heat and freshwater buoyancy fluxes and its application to the NCEP reanalysis data

Abstract

A B S T R A C TThe buoyancy flux at the air/sea interface plays a key role in water mass transformation and mixing as it modifies surface water density and in turn drives overturning and enhances stratification. It is the interplay of these two independent heat and freshwater buoyancy flux components that is of central importance when analysing mechanisms of the ocean/atmosphere interaction. Here, a diagnostic quantity ( B ) is presented that allows to capture the relative contribution of both components on the buoyancy flux in one single quantity. Using NCEP reanalysis of heat and freshwater fluxes demonstrates that B is a convenient tool to analyse both the temporal and spatial variability of their corresponding buoyancy fluxes. For the global ocean the areal extent of buoyancy gain and loss regions changed by 10%, with the largest extent of buoyancy gain during the 1970-1990 period. In the subpolar North Atlantic, and likewise in the South Pacific, decadal variability in freshwater flux is pronounced and, for the latter region, takes control over the total buoyancy flux since the 1980s. Some of the areal extent time series show a significant correlation with large-scale climate indices.

Upper Ocean Response to Rain Observed From a Vertical Profiler

“…The stability of the OSBL is determined by the density profile and changes therein due to the fluxes of heat and moisture through the air‐sea interface. This can be described by the surface buoyancy flux B 0 , defined as where g is the gravitational acceleration, ρ 0 a reference sea surface density, and Q ρ the density flux computed following Schmitt et al () and Karstensen and Lorbacher (): where Q net is the sum of the turbulent fluxes of sensible ( Q S ) and latent ( Q L ) heat, as well as the longwave and shortwave radiative forcing Q LW and Q SW , and is here defined positive into the ocean. The coefficients α and β represent thermal expansion and haline contraction, respectively, and c pw is the specific heat of seawater at constant pressure.…”