Contribution of Horizontal Advection to the Interannual Variability of Sea Surface Temperature in the North Atlantic

Abstract

The interannual variability of sea surface temperature (SST) in the North Atlantic is investigated from October 1992 to October 1999 with special emphasis on analyzing the contribution of horizontal advection to this variability. Horizontal advection is estimated using anomalous geostrophic currents derived from the TOPEX/Poseidon sea level data, average currents estimated from drifter data, scatterometer-derived Ekman drifts, and monthly SST data. These estimates have large uncertainties, in particular related to the sea level product, the average currents, and the mixed-layer depth, that contribute significantly to the nonclosure of the surface temperature budget. The large scales in winter temperature change over a year present similarities with the heat fluxes integrated over the same periods. However, the amplitudes do not match well. Furthermore, in the western subtropical gyre (south of the Gulf Stream) and in the subpolar regions, the time evolutions of both fields are different. In both regions, advection is found to contribute significantly to the interannual winter temperature variability. In the subpolar gyre, advection often contributes more to the SST variability than the heat fluxes. It seems in particular responsible for a low-frequency trend from 1994 to 1998 (increase in the subpolar gyre and decrease in the western subtropical gyre), which is not found in the heat fluxes and in the North Atlantic Oscillation index after 1996.