Variability of the water masses in the North Atlantic subtropical gyre

Abstract

At 24.5ºN the global heat transport is maximum, and most of this transport is due to the Atlantic Ocean. Changes in the oceanic heat transport can be related to changes in the water masses properties. This study focuses in understanding long term-changes and interannual variability of water masses properties in this latitude embedded on the North Atlantic Subtropical Gyre (NASG). Firstly, three hydrographic cruises at the 24.5oN section carried out in 1981, 1998 and 2015 are selected to understand the full basin water masses changes of the NASG. Secondly, a deeper study focussing on the eastern boundary of the NASG is done. For this purpose, the eastern side of the 24.5ºN section is used together with four high-resolution hydrographic cruises carried out in 1997, 2007, 2014 and 2017 north of the Canary Islands (29ºN). The Bindoff & McDougall (1994) analysis is done to identify whether the observed changes are due to a vertical displacement of isoneutrals or due to changes along isoneutrals (differences in water properties). The results show a continuous warming and salinity increase of the NACW from 1957 to 2015 (0.1°C decade-1 and 0.01 decade-1 respectively) that comes with a temperature-diven deepening of the layer. At the Eastern boundary this trend higher but also consistent between 1997 and 2017. A 0.1°C decade-1 warming is also observed in IW from 1957 to 2004. Trends in the Canary area have been more difficult to infer due to high seasonality and the presence of mesoscale processes. Nevertheless, a decrease on the Mediterranean Water properties is observed for nearly two decades. For both A05 and Eastern Boundary sections, we find a weak cooling and freshening in the deep waters.