Meridional overturning transports at 7.5N and 24.5N in the Atlantic Ocean in 1992-93 and 2010-11

Abstract

Transatlantic hydrographic sections along latitudes 7.5N and 24.5N have been repeated with an about 20 years difference, at the beginning of the 90s and 2010s. A same methodology, a box inverse model, has been applied to the box bounded for the stations carried out in each latitude and year. The inverse model consists of mass conservation in the whole water column, in each layer defined by neutral density and several constraints in specific range of longitudes and depths. These last come from previous independent studies or from our LADCP data. As result, velocities at the reference layer for each station pair and dianeutral velocities between layers are estimated as well as the adjustment of the Ekman transport. Dianeutral velocities show a downwelling transport from Upper North Atlantic Deep Water (UNADW) to Lower North Atlantic Deep Water (LNADW) resulting that the two lobes at 24.5N merge at 7.5N with a larger vertical extension. At 24.5N, the increase of southward transport of UNADW from 1992 to 2011 is compensated with a decrease of southward transport of LNADW. The Atlantic Meridional Overturning Circulation (AMOC) at 24.5N is larger in 1992 (24.7±1.7 Sv) than in 2011 (20.1±1.4 Sv). This difference comes from the transport at intermediate layers consisting of Antarctic Intermediate Waters (AAIW) and Mediterranean Water (MW). In contrast, heat transport at 24.5N is not significantly different in 1992 (1.4±0.1 PW) and 2011 (1.2±0.1 PW)