Heat Budget in the Gulf Stream Region: The Importance of Heat Storage and Advection

Abstract

A simple three-dimensional thermodynamic model is used to study the heat balance in the Gulf Stream region (30°?45°N, 40°?75°W) during the period from November 1992 to December 1999. The model is forced by surface heat fluxes derived from NCEP variables, with geostrophic surface velocity specified from sea surface height measurements from the TOPEX/Poseidon altimeter and Ekman transport specified from NCEP wind stress. The mixed layer temperature and mixed layer depth from the model show good agreement with the observations on seasonal and interannual time scales. Although the annual cycle of the upper-ocean heat content is underestimated, the agreement of the interannual variations in the heat content and the sea surface height are good; both are dominated by the large decrease from 1994 to 1997 and the increase afterward. As expected from previous studies, the surface heat flux dominates the seasonal and interannual variations in the mixed layer temperature. However, interannual variations in the upper-ocean heat content are dominated by the advection? diffusion term. Within the advection term itself, the largest variations are from the geostrophic advection anomaly. In the western Gulf Stream region the largest component of anomalous advection is the advection of the anomalous temperature by the mean current; elsewhere, the advection of the mean temperature by the anomalous current is also important. Other studies have shown that upper-ocean heat content is a more robust indicator of the potential contribution of the ocean to interannual heat flux anomalies than is sea surface temperature. The analysis here shows that the dominant term in interannual variations in heat content in the Gulf Stream region is anomalous advection by geostrophic currents. In fact, these ocean-forced variations in heat content appear to force air?sea fluxes: the surface heat flux anomalies in the western Gulf Stream region are negatively correlated with the anomalous upper-ocean heat content, that is, a large heat loss to the atmosphere corresponding to a positive heat content anomaly.