Seasonal and low-frequency variability of the meridional heat flux at 36 degrees N in the North Atlantic

Historical hydrographic sections are used to investigate the seasonal and i nterannual variability in the meridional heal flux at 36 degrees N in the N orth Atlantic. The data consist of ten transatlantic sections and sections from four sectors, which combined, cross the entire basin. These sectors ar e 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 al l three regions, but their properties are examined individually. To improve estimates of the Gulf Stream contribution to the total heat flux, a tangen t hyperbolic model of the current's temperature field is used to retain its structure in the temperature flux integrations even when only a few statio ns are available. The technique removes biases due to undersampling that av erages about 0.3 PW. The temperature flux of the upper layer is estimated for the four sectors p lus 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 trans ect can be accomplished by assuming that in the deep layer an equivalent am ount 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 degrees 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 tempe rature 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 sta tement, 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 pr esence 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 summe r 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 se ctions preferably with a uniform distribution throughout the year.