Hydrography, transport and mixing of the West Spitsbergen Current: the Svalbard Branch in summer 2015

Abstract

Measurements of ocean currents, stratification and microstructure were made in August 2015, northwest of Svalbard, downstream of the Atlantic inflow in Fram Strait in the Arctic Ocean. Observations in three sections are used to characterize the evolution of the West Spitsbergen Current (WSC) along a 170 km downstream distance. Two alternative calculations imply 1.5 to 2 Sv (1 Sv = 10 6 m 3 s −1 ) is routed to recirculation and Yermak branch in Fram Strait, whereas 0.6 to 1.3 Sv is carried by the Svalbard branch. The WSC cools at a rate of 0.20 • C per 100 km, with associated bulk heat loss per along-path meter of (1.1 − 1.4) × 10 7 W m −1 , corresponding to a surface heat loss of 380-550 W m −2 . The measured turbulent heat flux is too small to account for this cooling rate. Estimates using a plausible range of parameters suggest that the contribution of diffusion by eddies could be limited to one half of the observed heat loss. In addition to shear-driven mixing beneath the WSC core, we observe energetic convective mixing of an unstable bottom boundary layer on the slope, driven by Ekman advection of buoyant water across the slope. The estimated lateral buoyancy flux is O(10 −8 ) W kg −1 , sufficient to maintain a large fraction of the observed dissipation rates, and corresponds to a heat flux of approximately 40 W m −2 . We conclude that -at least in summer -convectively driven bottom mixing followed by the detachment of the mixed fluid and its transfer into the ocean interior can lead to substantial cooling and freshening of the WSC.

Eddies in the North Greenland Sea and Fram Strait From Satellite Altimetry, SAR and High‐Resolution Model Data

“…The general pattern shows an intensification of eddy number and dynamic properties along the main currents, consistent with previous studies (Trodahl & Isachsen, 2018). We also note the same intensification of EKE toward the Fram Strait, where high eddy azimuthal velocities were observed (Hattermann et al, 2016;Kolas & Fer, 2018;Nilsen et al, 2006;von Appen et al, 2015).…”

“…In the eastern part of the region, the Atlantic Water splits between two major pathways: the branches of the West Spitsbergen Current (WSC) in the Arctic Ocean and the southwestward recirculation patterns in the Fram Strait, which further merge with the East Greenland Current (EGC). This recirculating Atlantic Water strongly influences the interannual variations of deep convection in the Greenland Sea , as well as the heat flux to the Arctic (Hattermann et al, 2016;Kolas & Fer, 2018). Both processes have a major climatic significance, which highlights the importance of investigating this region.…”

“…Estimates of heat fluxes from in situ, satellite observations and high-resolution ocean models (Hattermann et al, 2016;von Appen et al, 2015von Appen et al, , 2016 showed that the observed cooling of the WSC, as well as diving of its westward recirculation branch below the sea surface, cannot be solely attributed to the heat loss to the atmosphere, ice melt, and diapycnal mixing with the Polar Water (Kolas & Fer, 2018). Analysis of the 16 years of in situ observations at the long-term mooring array in the Fram Strait at~78°50′N, as well as other data along the northwestern Spitsbergen, permitted to suggest a principal role of eddies in driving the recirculation of Atlantic Water in the Fram Strait (Boyd & D'Asaro, 1994;Hattermann et al, 2016;Kolas & Fer, 2018;Nilsen et al, 2006;von Appen et al, 2015von Appen et al, , 2016. In particular, the in situ observations show a high level of eddy kinetic energy (EKE) in the WSC, reaching 200 cm 2 s −2 in winter.…”

Variability of the thermohaline structure and transport of Atlantic water in the Arctic Ocean based on NABOS (Nansen and Amundsen Basins Observing System) hydrography data

“…This suggests stronger transformation and mixing of, primarily, the BSBW. The BSBW transformation can be due to various reasons, including mixing with the FSBW caused by thermohaline intrusive layering at absolutely stable stratification (Merryfield, 2002;Kuzmina et al, 2014;Kuzmina, 2016), the influence of the slope topography, the impact of local counterflows near the slope (see, for example, Pnyushkov et al, 2015), lateral convection (Ivanov and Shapiro, 2005;Ivanov and Golovin, 2007;Walsh et al, 2007), the impact of the Arctic Shelf Break Water (Aksenov et al, 2011;Ivanov and Aksenov, 2013), and mixing due to eddies (Schauer et al, 2002;Dmitrenko et al, 2008;Aagaard et al, 2008;Pnyushkov et al, 2018a). The understanding of the processes of transformation and mixing of the BSBW and FSBW is necessary to verify an important concept proposed by Rudels et al (2015) that the BSBW supplies the major part of the AW to the Amundsen, Makarov, and Canadian basins, while the FSBW remains almost fully in the Nansen Basin.…”

Time Dependent Flow of Atlantic Water on the Continental Slope of the Beaufort Sea Based on Moorings

“…of the strait: the Yermak Plateau branch (Meyer et al, 2017), the Yermak Pass Branch (Koenig et al, 2017), and the Svalbard slope branch (Cokelet et al, 2008;Kolas & Fer, 2018;Pérez-Hernández et al, 2017;Våge et al, 2016). These branches are believed to subsequently merge back into a single boundary along the northern continental slope of Svalbard.…”