Heat fluxes across 7°30′N and 4°30′S in the Atlantic Ocean

Abstract

Heat fluxes are estimated across transatlantic sections made at 4°30′S and 7°30′N in January–March 1993, following Hall and Bryden (1982. Deep-Sea Research 29, 339–359). Particular care is given to the computation of Ekman volume and heat fluxes, which are assessed both (a) from the windstress data for the period of the cruise and (b) from the comparison between geostrophic and Vessel Mounted Acoustic Doppler Current Profiler (VM-ADCP) velocities. In contrast with previous studies, the two estimates for Ekman fluxes do not converge for either section: (a) (11.5±0.5 Sv; 1.01±0.05 PW) across 7°30′N and (−9.3±1.2 Sv; −0.85±0.12 PW) across 4°30′S when windstress data at the date of the hydrographic stations are used; (b) (6.3±1.1 Sv; 0.56±0.09 PW) across 7°30′N and (−3.4±3.0 Sv; −0.35±0.24 PW) across 4°30′N when the ageostrophic transport above the thermocline is used. The divergence would have been even greater at 4°30′S if the strong ageostrophic signal beneath the thermocline, which brings a transport of (8.4 Sv; 0.82 PW), had been considered. The corresponding total meridional heat fluxes are: (a) 1.40±0.16 PW and (b) 0.95±0.20 PW across 7°30′N, (a) 1.05±0.12 PW and (b) 1.67±0.14 PW (2.39±0.14 PW when the subthermocline ageostrophic transport is taken into account) across 4°30′S. The estimates based on windstress data are compared with the results from an inverse model (Lux and Mercier, 1999) to show the importance of the heat flux due to the deviation of the local depth-averaged potential temperature from its average over the section, which is neglected in the Hall and Bryden (1982. Deep-Sea Research 29, 339–359) method but is not negligible in our computation in which we do not isolate the transport of the western boundary current east of the 200 m isobath; this corrective flux amounts here to −0.19 PW across 7°30′N and 0.33 PW across 4°30′S. The seasonal variability of the meridional heat flux across 7°30′N is studied through the hydrographic data collected during the ETAMBOT 1–2 cruises, which repeated the 7°30′N section west of 35°W in September 1995 and April 1996. When the section is completed east of 35°W with CITHER 1 data and when windstress data are used for the computation of the Ekman transport, the estimates for the meridional heat fluxes are 0.20±0.14 PW in September 1995 and 1.69±0.27 PW in April 1996. The estimates fit well with results from numerical models.