Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.
The Rockall Trough is one of the main conduits for warm Atlantic Water to the Nordic Seas. Ocean heat anomalies, originating from the eastern subpolar gyre, are known to influence Arctic sea ice extent, marine ecosystems, and continental climate. Knowledge of the transport through this basin has previously been limited to estimates from hydrographic sections which cannot characterize the intra-annual and multiannual variability. As part of the Overturning in the Subpolar North Atlantic Programme (OSNAP), a mooring array was deployed in the Rockall Trough in order to obtain the first continuous measurements of transport. Here, we define the methodology and the errors associated with estimating these transports. Results show a 4-year mean northward transport of 6.6 Sv (1 Sv = 10 6 m 3 /s) by the North Atlantic Current (NAC) in the east and interior of the Rockall Trough (2014-2018). A mean transport of −2.0 Sv (southward) is observed in the west of the basin, which could be part of a recirculation around the Rockall Plateau. The 90-day low-pass-filtered transport shows large subannual and interannual variability (−1.6 to 9.1 Sv), mostly resulting from changes in the midbasin geostrophic transport. Satellite altimetry reveals the periods of low and high transport are associated with significant changes in the Rockall Trough circulation. There is a detectable seasonal signal, with the greatest transport in spring and autumn. Plain Language Summary There is mounting evidence that the North Atlantic Current (eastward extension of the Gulf Stream) heavily influences the European and Arctic climate. To adequately measure this current and understand its dynamics, an array of underwater instruments was deployed in the Rockall Trough, a remote region of the eastern North Atlantic. Over a 4-year period, these instruments continuously collected measurements of temperature, salinity, pressure, and velocity data. Analysis of these data provides a new and more accurate description of the North Atlantic Current in this region. This study reveals a surprisingly large variability in the eastern North Atlantic circulation. The combined analysis of underwater measurements and satellite data indicates that this variability is due to changes of the North Atlantic Current system.
A high‐resolution numerical hydrodynamic model of Kangerdlugssuaq Fjord and the adjacent southeast Greenland shelf region was constructed in order to investigate the dynamics of fjord‐shelf exchange. Recent studies have suggested that rapid exchange flows, driven by along‐shelf barrier wind events, are the dominant agent of exchange between fjord and shelf. These events are prone to occur during the winter, when freshwater forcing is minimal and observations of the fjord interior are scarce. Subglacial freshwater discharge was held at zero, so that any buoyancy‐driven overturning circulation was driven by melting alone. The model described a geostrophically balanced background flow transporting water masses between the fjord mouth and the glacier terminus, indicating that rotational effects are of order‐one importance. Barrier wind events were found to trigger coastally trapped internal wave activity within fjord, temporarily enhancing exchange and vertical mixing, and causing warm water to oscillate in the along‐fjord direction. These internal waves were also found to enhance the background flow via Stokes' drift. Heat delivery through the fjord mouth was smaller than that recorded in summer observations, however the system is more effective at delivering this heat to the head of the fjord. There exists the potential for wintertime melting at the ice‐ocean interface to be significant to the same order as summertime melting.
A realistic numerical model was constructed to simulate the oceanic conditions and circulation in a large southeast Greenland fjord (Kangerdlugssuaq) and the adjacent shelf sea region during winter 2007–2008. The major outlet glaciers in this region recently destabilized, contributing to sea level rise and ocean freshening, with increased oceanic heating a probable trigger. It is not apparent a priori whether the fjord dynamics will be influenced by rotational effects, as the fjord width is comparable to the internal Rossby radius. The modeled currents, however, describe a highly three‐dimensional system, where rotational effects are of order‐one importance. Along‐shelf wind events drive a rapid baroclinic exchange, mediated by coastally trapped waves, which propagate from the shelf to the glacier terminus along the right‐hand boundary of the fjord. The terminus was regularly exposed to around 0.5 TW of heating over the winter season. Wave energy dissipation provoked vertical mixing, generating a buoyancy flux which strengthened overturning. The coastally trapped waves also acted to strengthen the cyclonic mean flow via Stokes' drift. Although the outgoing wave was less energetic and located at the opposite sidewall, the fjord did exhibit a resonant response, suggesting that fjords of this scale can also exhibit two‐dimensional dynamics. Long periods of moderate wind stress greatly enhanced the cross‐shelf delivery of heat toward the fjord, in comparison to stronger events over short intervals. This suggests that the timescale over which the shelf wind field varies is a key parameter in dictating wintertime heat delivery from the ocean to the ice sheet.
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