Observed Variability of the North Atlantic Current in the Rockall Trough From 4 Years of Mooring Measurements

Abstract

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.

The North Atlantic Glacial Eastern Boundary Current as a Key Driver for Ice‐Sheet—AMOC Interactions and Climate Instability

“…Recent observations from the Hebridean region reveal that decadal scale variability in the strength of the European Slope Current is, to first order, directly connected to that of the cyclonic subpolar gyre (SPG; and associated meridional density gradients) with a strong (weak) SPG associated with a strong (weak) slope current (Marsh et al., 2017). These observations support the idea that the European Slope Current is largely recruited from the eastern North Atlantic and is connected, north of ∼55°N (i.e., Rockall Trough), to the upper part of the NAC (the NAC being the eastern limb of the SPG) (Houpert et al., 2020; Lozier et al., 2019). Further south, in the central North Atlantic (∼46–53°N), the NAC flows eastward in three main branches with circulation pathways strongly constrained by the Mid‐Atlantic Ridge (MAR) (Daniault et al., 2016).…”

Circulation and overturning in the eastern North Atlantic subpolar gyre

“…While Petit et al's (2019) transport estimate is highly accurate, it is from a single synoptic study in a region of high temporal variability (Sarafanov et al, 2012, Koman et al, 2020. The estimates from Koman et al (2020) are a time-mean calculation but using altimetry as a reference velocity may not fully resolve finer mesoscale features near topography, potentially resulting in an underestimate of velocity (Chafik et al, 2014;Pujol et al, 2016;Houpert et al, 2020;Koman et al, 2020). Koman et al (2020) also analyzed three OSNAP cruise sections along the Reykjanes Ridge and found that those synoptic realizations of the flow over the ridge varied widely (their Fig.…”

“…1) propagates waters of subtropical origin into the gyre as part of the upper limb of the AMOC. These waters flow into the basin at two primary locations: a smaller buoyancy-driven current in the east confined to the flank of the continental shelf at depths < 1000 m, and a larger flow in the basin's interior (Houpert et al, 2020). Studies from the Extended Ellett Line program (Holliday et al, 2000;Holliday et al, 2015) found a net northward transport of 3-4 Sv of the upper AMOC through the Rockall Trough using a mid-depth level of no motion.…”

“…Studies from the Extended Ellett Line program (Holliday et al, 2000;Holliday et al, 2015) found a net northward transport of 3-4 Sv of the upper AMOC through the Rockall Trough using a mid-depth level of no motion. More recently, results from the first continuous observations in the Rockall Trough from OSNAP have found stronger net transports of 5.2 Sv (Lozier et al, 2019) from 21 months of data (2014-2016) and 4.5 ± 0.8 Sv (Houpert et al, 2020) from 4 years of data (2014)(2015)(2016)(2017)(2018). The latter study also found notable seasonality with an increased transport of 6.3 Sv in October followed by a rapid spin-down to 2.8 Sv in January associated with a diversion of the North Atlantic Current from the Rockall Trough entrance to the west of the Rockall Bank.…”

“…The latter study also found notable seasonality with an increased transport of 6.3 Sv in October followed by a rapid spin-down to 2.8 Sv in January associated with a diversion of the North Atlantic Current from the Rockall Trough entrance to the west of the Rockall Bank. This study will use the 4.5 ± 0.8 Sv value from Houpert et al (2020) for the best estimate of transport through the Rockall Trough because it is derived from the longest continuous time series in the basin.…”