Meridional Overturning Transports at 7.5N and 24.5N in the Atlantic Ocean during 1992-93 and 2010-11

Abstract

Transatlantic hydrographic sections along latitudes 7.5ºN and 24.5ºN have been repeated with about 20 years difference, at the beginning of the 1990s and 2010s. For each period, an inverse model is applied to the box bound by both sections. The model imposes mass conservation for individual layers, defined by isoneutral surfaces, and the whole water column, using surface Ekman transport and several transport constraints for specific ranges of longitudes and depths. As a result, the velocities at the reference layer for each station pair and the dianeutral velocities between layers are estimated, and the horizontal velocity fields and the water, heat and freshwater transports are calculated. The dianeutral velocities show downwelling from the Upper North Atlantic Deep Water (UNADW) stratum to the Lower North Atlantic Deep Water (LNADW) stratum, resulting that the two lobes at 24.5ºN merge at 7.5ºN with a larger vertical extension. At 24.5ºN, the 1992 to 2011 increase in southward UNADW transport is compensated with a decrease of southward LNADW transport. The Atlantic Meridional Overturning Circulation (AMOC) at 24.5ºN is larger in 1992 (24.7±1.7 Sv) than in 2011 (20.1±1.4 Sv). Much of the observed AMOC variability arises because of a changing northward flow of Antarctic Intermediate Water (AAIW), which was much more intense in 1992/93 than in 2010/11. In contrast, heat transport at 24.5ºN is not significantly different in 1992 (1.4±0.1 PW) and 2011 (1.2±0.1 PW).