Abstract
Abstract. Observations from moored instruments are analyzed to describe the Norwegian Atlantic Slope Current at the Lofoten Escarpment (13∘ E, 69∘ N). The data set covers a 14-month period from June 2016 to September 2017 and resolves the core of the current from 200 to 650 m depth between the 650 and 1500 m isobaths. The along-isobath current, vertically averaged between 200 and 600 m depth, has an annual cycle amplitude of 0.1 m s−1, with the strongest currents in winter, and a temporal average of 0.15 m s−1. Higher-frequency variability is characterized by fluctuations that reach 0.8 m s−1, lasting for 1 to 2 weeks, and extend as deep as 600 m. In contrast to observations in Svinøy (2∘ E, 63∘ N), the slope current is not barotropic and varies strongly with depth (a shear of 0.05 to 0.1 m s−1 per 100 m in all seasons). Within the limitations of the data, the average volume transport of Atlantic Water is estimated at 2.0±0.8 Sv (1 Sv =106 m3 s−1), with summer and winter averages of 1.6 and 2.9 Sv, respectively. The largest transport is associated with the high temperature classes (>7 ∘C) in all seasons, with the largest values of both transport and temperature in winter. Calculations of the barotropic and baroclinic conversion rates using the moorings are supplemented by results from a high-resolution numerical model. While the conversion from mean to eddy kinetic energy (e.g., barotropic instability) is likely negligible over the Lofoten Escarpment, the baroclinic conversion from mean potential energy into eddy kinetic energy (e.g., baroclinic instability) can be substantial, with volume-averaged values of (1–2)×10-4 W m−3.
Highlights
The relatively mild climate of Norway is largely attributed to the northern extension of the North Atlantic Drift, the Norwegian Atlantic Current that transports warm and saline water masses toward the Arctic Ocean (Seager et al, 2002; Rhines et al, 2008)
The circulation pattern is organized in two main branches originating from the Iceland–Faroe and Faroe–Shetland gaps (Poulain et al, 1996; Orvik and Niiler, 2002) (Fig. 1a): the Norwegian Atlantic Slope Current and the Norwegian Atlantic Front Current
Atmospheric forcing was obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERAInterim (Dee et al, 2011) reanalysis over the historical time period from 1979 to 2018 and from the higher-resolution ERA-5 reanalysis (Copernicus Climate Change Service, 2017) over the mooring observation period
Summary
The relatively mild climate of Norway is largely attributed to the northern extension of the North Atlantic Drift, the Norwegian Atlantic Current that transports warm and saline water masses toward the Arctic Ocean (Seager et al, 2002; Rhines et al, 2008). Using time-averaged fields of an eddy-resolving numerical ocean simulation, Isachsen (2015) showed that the steep Lofoten Escarpment exhibits enhanced unstable baroclinic growth rates and large velocity variability, suggesting high lateral diffusion rates. Observations from multiple cruises, gliders and subsurface floats were analyzed and reported elsewhere with focus on AW transformation (Bosse et al, 2018), the permanent Lofoten Basin Eddy (Fer et al, 2018; Bosse et al, 2019) and the frontal structure across the Mohn Ridge (Bosse and Fer, 2019). We report the first observations of the volume transport rates, energetics and their variability from weekly to seasonal timescales based on the mooring records
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