The wind driven property variability of the Denmark Strait Overflow

• Main Author: Opher, Jake
• Format: Thesis
• Language: English
• Published: 2021
• Subjects: Greenland; Lofoten; Denmark Strait; East Greenland; east greenland current; Iceland; North Atlantic; North Atlantic oscillation; Sea ice
• Online Access: https://ueaeprints.uea.ac.uk/id/eprint/82740/; https://ueaeprints.uea.ac.uk/id/eprint/82740/1/2021OpherJPhD.pdf

The Denmark Strait Overflow (DSO) is the densest component of the Atlantic Meridional Overturning Circulation (AMOC) in the subpolar North Atlantic. This thesis demonstrates, for the first time, that the DSO exhibits a statistically significant seasonal salinity cycle. Sustained freshening of the DSO, which occurs in late winter and spring, is intensified upslope, in lighter classes of DSO (27.88 kg m−3 < σθ < 27.91 kg m−3) and is weaker downslope, in denser classes (σθ ≈ 27.94 kg m−3). The downstream evolution of fresh signals could shed light on the mixing between different water mass components of the AMOC. The freshening originates from advection of fresh water lenses, termed lids, present in the lighter classes of the DSO above the deep trough at Denmark Strait in spring, and hitherto not detected by moored observations. The freshening is linked to wintertime freshening and enhancement of the Shelfbreak East Greenland Current (EGC) 200 km to the north of Denmark Strait, driven in part by barrier winds, increasing the volume transport of fresh pycnocline water within the Shelfbreak EGC towards Denmark Strait, which forms the fresh lid. It is also shown that seasonality of Shelfbreak EGC volume transports in 2011-12 may explain around 50% of the DSO salinity seasonality. The DSO salinity is strongly controlled by northerly/northeasterly winds over the east Greenland shelfbreak around 70◦N, with an advective timescale of 2-3 months. Both the North Atlantic Oscillation and the Iceland Lofoten Difference atmospheric patterns influence DSO salinity, with the former exerting a dominant influence between 2010 and 2014, and the latter more important between 2005 and 2009. Finally, it is argued that the reduction in sea ice concentration between 1998 and 2015 in the wind forcing region has reduced the effective wind stress there, thus weakening the DSO freshening events.