Residual-mean analysis of the air-sea fluxes and associated oceanic meridional overturning

Abstract

The dynamic response of the oceanic mixed-layer to the thermodynamic forcing at the sea surface is analyzed in order to describe the pattern of the oceanic meridional overturning. The technique proposed in this study is based on residual-mean theory, which takes into account the transport of buoyancy and tracers by transient eddies. From the observed air-sea density flux and mixed-layer density distributions, we estimate the two components of the Meridional Overturning Circulation (MOC) corresponding to the adiabatic (along-isopycnal) advection and the diabatic (cross-isopycnal) flux. Calculations are performed for the global ocean and, additionally, for each oceanic basin. The proposed method extends the Walin (1982) mass transformation theory, and permits, for the first time, assessment of the strength of the MOC adiabatic component from the sea surface data. This study offers a statistical description of the atmospheric and oceanic databases and gives some suggestions for the choice of specific datasets. In particular, the two most reliable atmospheric climatology databases (ECMWF and NCEP/NCAR re-analyses) are compared, and the impact of their inaccuracies on the MOC calculations is evaluated.