Abstract
<p class="MsoNormal" style="margin: 0in; font-size: 12pt; font-family: "Times New Roman", serif; text-indent: 0.25in; line-height: 32px;">In this dissertation, new observations obtained as part of the Overturning in the Subpolar North Atlantic Program (OSNAP) are used to describe and quantify the circulation of the eastern subpolar North Atlantic and its role in large-scale North Atlantic overturning. The first part of this research is focused on the East Reykjanes Ridge Current (ERRC), which effectively acts as a western boundary current in the Iceland Basin on the eastern flank of the Reykjanes Ridge. The observations obtained by the OSNAP program are the first continuous multi-year observations of the ERRC and reveal it to be a highly variable, mostly barotropic southwestward flow with a mean transport of 10-13 Sv. The OSNAP measurements cover the period from July 2014 to August 2018 and include time series data from ADCPs, current meters and dynamic height moorings at six mooring sites near 58°N. Together with satellite altimetry and Argo profile and drift data, the mean transport, synoptic variability, water mass properties and upstream and downstream pathways of the ERRC are examined. Results show that the ERRC forms in the northeastern Iceland Basin at the convergence of surface waters from the North Atlantic Current and deeper Icelandic Slope Water formed along the Iceland-Faroe Ridge. The ERRC becomes denser as it cools and freshens along the northern and western topography of the Basin before retroflecting over the Reykjanes Ridge near 59°N into the Irminger Current. Analysis of the flow-weighted density changes along the ERRC’s path reveals that it is responsible for about one-third of the net potential density change of waters circulating around the rim of the subpolar gyre.<o:p></o:p></p><p class="MsoNormal" style="margin: 0in; font-size: 12pt; font-family: "Times New Roman", serif; text-indent: 0.5in; line-height: 32px;">The second part of the dissertation describes new transport estimates of the North Atlantic Current in the Iceland Basin, and uses these results along with other contemporaneous measurements to determine mass and overturning budgets for the eastern North Atlantic subpolar gyre. Estimates of the North Atlantic Current are determined using three full-depth dynamic height moorings spanning the Iceland Basin, and are supplemented by Argo and satellite altimetry data. Along with historical estimates of the exchanges over the Iceland-Scotland Ridge, additional OSNAP results from the Rockall Trough and Rockall-Hatton Bank regions are used to calculate transport budgets in different density layers over a broad portion of the eastern subpolar gyre. Results show that about 13-14 Sv of the North Atlantic Current (σ<sub>θ</sub> < 27.8 kg m<sup>-3</sup>) flow northward into the middle of the Iceland Basin through a primary baroclinic flow near 23.5°W and a secondary quasi-barotropic flow near 26°W. Together with the observed northward flow in the Rockall-Hatton area, we conclude that approximately 20 Sv of the upper limb of the Atlantic Meridional Overturning Circulation (σ<sub>θ</sub> < 27.56 kg m<sup>-3</sup>) flows into the region where nearly 40% of it (7.7 Sv) is converted into the lower limb through progressive water mass modification from atmospheric cooling. This transformation process accounts for nearly half (46%) of the strength of Atlantic Meridional Overturning Circulation defined by the full OSNAP array extending across the basin from Greenland to Scotland.<o:p></o:p></p>