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 7.5N and 24.5N 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 closed 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; in particular, we find that mass transport per stratum at 24.5N in 2011 is in good agreement with the transport estimates from the RAPID-Watch array. During both realizations the dianeutral velocities downwell from the Upper North Atlantic Deep Water (UNADW) to the Lower North Atlantic Deep Water (LNADW) strata, resulting in the merging of the two southward flowing strata at 24.5N into one deep southward-moving stratum at 7.5N. At 24.5N, there is an increase in southward UNADW transport between 1992 and 2011, compensated by a decrease of southward LNADW transport; a descent in the upper limit of the Antarctic Bottom Water (AABW) from 1992 to 2011 is also inferred. The Atlantic Meridional Overturning Circulation (AMOC) is larger in 1992–93 than in 2010–11, decreasing from 24.7 ± 1.7 to 20.1 ± 1.4 Sv at 24.5N and from 29.2 ± 1.7 to 16.9 ± 1.5 Sv at 7.5N. Much of this decrease arises because of the northward flow of Antarctic Intermediate Water (AAIW), which was much more intense in 1992–93 than in 2010–11. As a consequence, heat transport at 24.5N is not significantly different in 1992 (1.4 ± 0.1 PW) and 2011 (1.2 ± 0.1 PW). The estimation of heat transport at 7.5N strongly depends on the magnitude of the North Brazil Current over the American continental platform. The freshwater flux into the box bounded by the two transoceanic sections decreases significantly from 1992–93 (−0.45 ± 0.05 Sv) to 2010–11 (−0.36 ± 0.3 Sv), meaning a decrease in net evaporation over this same area.