Heat Flux Estimates for the Western North Atlantic. Part II: The Upper-Ocean Heat Balance

Abstract

The assimilation of temperature and altimetric velocity into a numerical model of the upper-ocean mixed layer in Part I allowed an analysis of the upper ocean heat budget for the western North Atlantic Ocean over the 2.5-year period of the Geosat Exact Repeat Mission (November 1986?April 1989). The balance of terms varied regionally: south of the Gulf Stream advection was relatively unimportant in the heat budget, and the ocean responded passively to changes in surface flux. Within the Gulf Stream and to the north of it, cooling of the upper ocean by advection was as large as 0.15°C/day for periods of several weeks. An analysis of the advection term showed that cooling by Ekman transport was opposed by warming from the geostrophic currents of the Gulf Stream, with cooling typically stronger by a factor of 2 because nonuniform Ekman transport disrupted the normal alignment between isotherms and sea surface height contours. There is a complex ocean-atmosphere coupling in this region: in addition to its increase during strong wind events, warming by geostrophic currents is a function of the strength of the Gulf Stream and its recirculation gyres. Over the 2.5-year period, the winds became progressively stronger, causing an increase in cooling by Ekman transport. Advective cooling was balanced by an increasingly positive surface flux (warming of the ocean by the atmosphere) at the rate of about 20% of the annually averaged surface flux per year. This positive trend in the surface flux was also observed in the estimates from the atmospheric general circulation model of the ECMWF.