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

Eivind Kolås,Ilker Fer

doi:10.5194/os-14-1603-2018

Abstract

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 = 106 m3 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)×107 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.

Highlights

The Arctic Ocean contributes to the global ocean thermohaline circulation through exchanges in Fram Strait, which is the main connection to the Atlantic Ocean (Aagaard et al, 1985)
Observations made from drifting pack ice north of Svalbard in spring 2015 showed warmer and shallower Atlantic Water (AW) compared to the climatology (Meyer et al, 2016)
Observations from a cruise conducted in August 2015 provide a snapshot of the West Spitsbergen Current (WSC) hydrography, transport and mixing in ice-free regions west and north of Svalbard in a year with relatively warmer and saltier Atlantic Water (AW) compared to climatology

Summary

Introduction

The Arctic Ocean contributes to the global ocean thermohaline circulation through exchanges in Fram Strait, which is the main connection to the Atlantic Ocean (Aagaard et al, 1985). The total volume transport through Fram Strait is 9 ± 2 Sv (1 Sv = 106 m3 s−1) northward and 12 ± 1 Sv southward (Fahrbach et al, 2001; Schauer et al, 2004). A large fraction of the northward flow is the West Spitsbergen Current (WSC), located on the eastern side of Fram Strait, which is a northward-flowing extension of the Norwegian Atlantic Current. Between Fram Strait and the Lomonosov Ridge, the boundary current slows down from about 0.25 to 0.06 m s−1 and changes structure from a mainly barotropic flow to a baroclinic flow (Pnyushkov et al, 2015). AW transported by the WSC is the major heat and salinity source for the Arctic Ocean (Boyd and D’Asaro, 1994; Aagaard et al, 1985; Rudels et al, 2015), and Arctic conditions are highly influenced by changes in the AW inflow properties (Polyakov et al, 2017)

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