Tidally Forced Lee Waves Drive Turbulent Mixing Along the Arctic Ocean Margins

Abstract

In the Arctic Ocean, limited measurements indicate that the strongest mixing below the atmospherically forced surface mixed layer occurs where tidal currents are strong. However, mechanisms of energy conversion from tides to turbulence and the overall contribution of tidally driven mixing to Arctic Ocean state are poorly understood. We present measurements from the shelf north of Svalbard that show abrupt isopycnal vertical displacements of 10–50 m and intense dissipation associated with cross‐isobath diurnal tidal currents of ∼0.15 m s−1. Energy from the barotropic tide accumulated in a trapped baroclinic lee wave during maximum downslope flow and was released around slack water. During a 6‐hr turbulent event, high‐frequency internal waves were present, the full 300‐m depth water column became turbulent, dissipation rates increased by a factor of 100, and turbulent heat flux averaged 15 W m−2 compared with the background rate of 1 W m−2.

Turbulent Mixing and the Formation of an Intermediate Nepheloid Layer Above the Siberian Continental Shelf Break

“…Based on an extensive observational data set including a 24 h time series of temperature, salinity, dissipation rate and current velocity profiles, Fer et al. (2020) report enhanced mid‐water turbulence over a period of 6 h, following the downslope flow phase of a diurnal tidal current above Yermak Plateau. The topographic setting (slope and water depth), and the magnitude of the downslope velocity discussed here are comparable to the situation described in Fer et al.…”

Changes in Atlantic Water Circulation Patterns and Volume Transports North of Svalbard Over the Last 12 Years (2008–2020)

“…grid size of 4 km, and Rossby deformation radius of ~10 km; Crews et al, 2017) and does not simulate tides which are important above and on the slopes of the Yermak Plateau (e.g. Padman et al, 1992;Koenig et al, 2017b;Fer et al, 2020). Further comparisons with a 32 month-long (2017-2020) time series of temperature and salinity at 350 m in the Yermak Pass (Labaste et al, 2020) and with a CNES/CLS multi-mission altimetry product prototype for the Arctic Ocean (2016-2018) confirmed the good performance of PSY4 in recent years.…”

“…Along its paths, the AW is cooled and freshened through ice‐melt, heat loss to the atmosphere and mixing with shelf waters (Boyd & D'Asaro, 1994; Fer et al., 2020; Onarheim et al., 2014; Renner et al., 2018; Rudels et al., 2015). In the Western Nansen Basin (WNB) in particular, deep winter mixed layers reaching 500 m (V. Ivanov et al., 2018; Pérez‐Hernández et al., 2019), exchanges with fjords west of Svalbard (Koenig et al., 2018) and troughs outflows from the Barents Sea (Athanase et al., 2020; Schauer et al., 1997) contribute to alter AW properties.…”

On the Along‐Slope Heat Loss of the Boundary Current in the Eastern Arctic Ocean

“…Fer et al. (2020) hypothesize that the contribution of spatially confined tidally driven slope mixing to the heat loss from the AW layer is comparable to the Arctic‐wide heat loss by double diffusion. North of Svalbard, where the AW still resides close to the ocean surface, reported values of the mean heat flux over the AW thermocline are 17 W m −2 (Meyer et al., 2017, N‐ICE2015 campaign January to June 2015), with much higher values of more than 100 W m −2 during storm events.…”

“…Renner et al (2018) suggest that tidal mixing on the upper slope is an important factor for the cooling of the AW Boundary current north of Svalbard. A mechanism for the conversion of tidal energy to turbulent mixing on the Arctic continental slope is the generation of trapped lee waves by the displacement of isopycnals during cross-slope tidal flows, and the subsequent energy release as described in Fer et al (2020). The isopycnal displacement associated with this process generates a surface signal that can be identified in satellite images, showing the frequent occurrence along the Arctic shelves.…”