Seasonal and Low-Frequency Variability of the Meridional Heat Flux at 36°N in the North Atlantic

Abstract

Historical hydrographic sections are used to investigate the seasonal and interannual variability in the meridional heat flux at 36°N in the North Atlantic. The data consist of ten transatlantic sections and sections from four sectors, which combined, cross the entire basin. These sectors are the slope water, the Gulf Stream, the Sargasso Sea, and the midocean. The data from the first three sectors actually come from sections that span all three regions, but their properties are examined individually. To improve estimates of the Gulf Stream contribution to the total heat flux, a tangent hyperbolic model of the current?s temperature field is used to retain its structure in the temperature flux integrations even when only a few stations are available. The technique removes biases due to undersampling that averages about 0.3 PW. The temperature flux of the upper layer is estimated for the four sectors plus the climatologically forced Ekman layer. The annual mean is 1.4 ± 0.3 PW with a range of 0.6 ± 0.1 PW. The zero net mass flux across the transect can be accomplished by assuming that in the deep layer an equivalent amount of water to that estimated for the upper layer flows in the southward direction presumably via the deep western boundary current. The temperature flux of the deep layer, with its mean temperature of 2.3°C, has an annual mean of ?0.20 ± 0.06 PW and a range of 0.05 ± 0.02 PW. The net annual mean of the meridional heat flux is 1.2 ± 0.3 PW and a range of 0.6 ± 0.1 PW. Its phase is dominated by the annual cycle of the Ekman temperature flux. The heat flux residual is examined for evidence of long-term change in the poleward heat flux. While the database is very limited for a conclusive statement, it appears that the residual for the pentads 1935?39, 1970?74, and 1975?79 agreed to within 0.1 PW. The tightness of these estimates in the presence of a 0.6 PW annual range makes it clear how important it is to know the latter accurately before statements about long-term change can be made. To date, most individual transoceanic sections were taken during the summer and spring. The standard deviation of the heat flux estimates is 0.3 PW, much of it due to eddy variability, making it essential to obtain repeat sections preferably with a uniform distribution throughout the year.