Structure and Forcing of Observed Exchanges across the Greenland–Scotland Ridge

Abstract

The Atlantic meridional overturning circulation and associated poleward heat transport are balanced by northern heat loss to the atmosphere and corresponding water-mass transformation. The circulation of northward-flowing Atlantic Water at the surface and returning overflow water at depth is particularly manifested—and observed—at the Greenland–Scotland Ridge where the water masses are guided through narrow straits. There is, however, a rich variability in the exchange of water masses across the ridge on all time scales. Focusing on seasonal and interannual time scales, and particularly the gateways of the Denmark Strait and between the Faroe Islands and Shetland, we specifically assess to what extent the exchanges of water masses across the Greenland–Scotland Ridge relate to wind forcing. On seasonal time scales, the variance explained of the observed exchanges can largely be related to large-scale wind patterns, and a conceptual model shows how this wind forcing can manifest via a barotropic, cyclonic circulation. On interannual time scales, the wind stress impact is less direct as baroclinic mechanisms gain importance and observations indicate a shift in the overflows from being more barotropically to more baroclinically forced during the observation period. Overall, the observed Greenland–Scotland Ridge exchanges reflect a horizontal (cyclonic) circulation on seasonal time scales, while the interannual variability more represents an overturning circulation.

Significance of Climate Indices to Benthic Conditions Across the Northern North Atlantic and Adjacent Shelf Seas

“…Less vigorous flow in Iceland-Scotland Overflow Water during a high NAO is consistent with results from a sediment core on the eastern flank of the Reykjanes Ridge (Boessenkool et al, 2007). Additionally, the anti-correlation between the eastern and western overflow branches is consistent with previous work that shows overflow volume transports are correlated with NAOtype changes in sea level pressure and wind stress curl, and that transports between the eastern and western routes can be out of phase (Biastoch et al, 2003;Bringedal et al, 2018).…”

Mechanisms of Ocean Heat Anomalies in the Norwegian Sea

“…Closer inspection of the region shows that the discontinuity in ν values across the ridge is much reduced if we consider unfiltered monthly time series. While exchanges between the Norwegian Sea and the North Atlantic south of the ridge are observed to occur on a broad range of time scales (Bringedal et al, ; Hansen & Østerhus, ), the processes driving local heat content change at the ridge is thus found to act predominantly on subannual time scales.…”

“…The origin of ocean circulation variability in the subpolar North Atlantic has been much studied (e.g., Häkkinen et al, ; Lohmann et al, ; Piecuch et al, ; Robson et al, ). Several studies have, for instance, previously related inflow changes to large‐scale wind forcing associated with the NAO (e.g., Bringedal et al, ; Hansen & Østerhus, ; Sandø et al, ). In ECCOv4, the correlation between and the NAO index is significant for the full time period (Figure ; r = 0.45).…”

Volume, Heat, and Freshwater Divergences in the Subpolar North Atlantic Suggest the Nordic Seas as Key to the State of the Meridional Overturning Circulation

“…Numerous studies of volume fluxes and estimates of the MOC based on hydrographic sections across the subpolar North Atlantic have been reported (McCartney & Talley, 1984;Bacon, 1997;Sarafanov et al, 2012;Talley, 2013;Mercier et al, 2015;Daniault et al, 2016;Bringedal et al, 2018;Østerhus et al, 2018;Lozier et al, 2019). A few studies also address heat and fresh water fluxes (Bacon, 1997;Wijffels, 2001;Trenberth & Fasullo, 2008;Lozier et al, 2019).…”