Meridional transport and heat flux variations in the subtropical North Atlantic

Abstract

Three sets of hydrographic data from October 1957, August 1981, and July - August 1992 are used to examine the variations in meridional circulation and ocean heat transport at 24.5°N in the subtropical North Atlantic. From these sections general warming has been found at intermediate levels between 800 and 2700 dbar in the middle of the North Atlantic subtropical gyre, over the entire 35-year period in a zonally uniform manner. On isopycnal surfaces, the waters have become warmer and saltier nearly everywhere above 2000 db. On level surfaces, maximum warming of 0.1°C per decade occurs at 1100 dbar; on isopycnal surfaces the salinity in the main thermocline is increasing at a rate of 0.018 per decade. By requiring that the ocean interior transport balances the Gulf Stream transport through the Straits of Florida and the Ekman transport over the Atlantic (Hall and Bryden, 1982), we have performed a heat flux calculation using similar methods on each dataset. Even though the temperature and salinity have changed, the three cruises exhibit similar features in the large-scale velocity fields and similar zonally averaged meridional transport. Ocean heat transport performed by similar calculations on the three cruises are indistinguishable, which is the same conclusion reached by Roemmich and Wunsch (1985) in their comparison between the 1957 and 1981 sections based on a number of inverse models.