Mean transport and variability in the subpolar North Atlantic

The Atlantic Meridional Overturning Circulation (AMOC) plays an important role in the Earth’s heat and carbon budgets and sets the properties of the global water masses. Deep water formed in high latitudes, travel southwards as part of the AMOC’s lower limb and have properties that can be traced globally. As the AMOC’s upper limb transports warm water northwards it releases heat to the atmosphere, contributing to the moderate climate of Western Europe and impacts all aspects of high latitudinal climate. In the northern North Atlantic there is a lack of long-term transoceanic measurements, enabling estimates of the AMOC’s transport variability. For this reason, it is essential that quantification of the AMOC’s mean transports in the subpolar region are as accurate as possible. Furthermore, the possibility of quantifying the AMOC’s variability in this region could improve our understanding of the AMOC’s driving mechanisms. The Extended Ellett Line (EEL) is a repeat hydrographic section located between Iceland and Scotland and is the focus area for this study. This section captures 90% of the water flowing northwards into the Nordic Seas and half the returning dense water; two key parts of the AMOC. Data has been collected near-annually at the EEL since 1996, providing two decades of hydrographic and direct velocity measurements. The first objective of this study is to quantify the long-term averages of the currents that shape the AMOC across the EEL, using direct velocity measurements. Improved estimates of the long-term (1997-2015) mean absolute velocity field and volume, temperature and freshwater transports are presented. Analysis of the time-mean velocity field contributes to knowledge of the location and variability of the currents transporting water northwards. There is evidence that a branch of the North Atlantic Current is located at the Rockall-Hatton Plateau (RHP). The net transport at the EEL is 3.5 ± 0.9 Sv, of which there is 5.9 ± 1.4 Sv in the upper ocean and -3.3 ± 0.7 Sv in the overflow water. There is a temperature transport of 0.22 ± 0.04 PW, dominated by the transport strength, and a freshwater transport of 44.1 ± 8.8 mSv southwards. The slope current and RHP are identified as key routes for the transport of heat northwards. The second objective of this study is to assess the spatial resolution of the EEL over the slope current North of Scotland. Comparisons of the EEL spatial resolution with altimetry and the OSNAP array, shows that the slope current is better resolved in the former. The altimeters do represent the large-scale circulation pattern at the EEL, but they do not capture the narrow slope current. The OSNAP array does not have a high enough spatial resolution to resolve the key heat pathway of the slope current. This study shows the necessity of a higher sampling spatial resolution in this location.