Observations of turbulence at a near-surface temperature front in the Arctic Ocean

Zoé Koenig,Ilker Fer,Trygve Fossum,Martin Ludvigsen,Eivind Kolås,Petter Norgren

doi:10.5194/egusphere-egu2020-7026

Abstract

&lt;p&gt;Measurements made at an Arctic thermohaline front show turbulence production through convection and forced symmetric instability, a mechanism drawing energy from the frontal geostrophic current. Destabilizing surface buoyancy fluxes from a combination of heat loss to the atmosphere and cross-front Ekman transport by down-front winds reduce the potential vorticity in the upper ocean. The front, located in the Nansen Basin close to the sea ice edge, separates the cold and fresh surface melt water from the warm and saline mixed layer. High resolution temperature, salinity, current and turbulence data were collected in the upper 100 m, on 18 September 2018 across the front from a research vessel and an autonomous underwater vehicle. The AUV was deployed to autonomously collect high resolution data across the front using adaptive sampling. Both front detection and sampling location were decided by a state-based autonomous agent running onboard the AUV, optimizing data collection across and along the front.&lt;/p&gt;&lt;p&gt;In addition to convection by heat loss to atmosphere and mechanical forcing by moderate wind in the mixed layer, forced symmetric instability contributed with comparable magnitude in generation of turbulence at the front location down to 40 m depth. This turbulence was associated with turbulent heat fluxes of up to 10 W.m&lt;sup&gt;-2&lt;/sup&gt;, eroding the warm and cold intrusions observed at respectively 35 and 55 m depth. A similar frontal structure has been crossed by a Seaglider in the same region 10 days after our survey. The submesoscale-to-turbulence scale transitions and resulting mixing can be widespread and important in the Atlantic sector of the Arctic Ocean.&lt;/p&gt;

Highlights

Warm Atlantic Water is the main source of oceanic heat to the Arctic Ocean (Carmack et al, 2015; Rudels et al, 2015)
High-resolution ocean temperature, salinity, current, and turbulence data were collected at an Arctic thermohaline front in the Nansen Basin
In this study we present detailed observations characterizing the structure of hydrography, currents, and turbulence at a near-surface front in the Arctic Ocean in the Nansen Basin away from topography, 4–5 km away from the sea ice edge (Figure 1a)

Summary

Introduction

Warm Atlantic Water is the main source of oceanic heat to the Arctic Ocean (Carmack et al, 2015; Rudels et al, 2015). As the Atlantic Water boundary current flows cyclonically along the upper continental slope, it mixes vertically and laterally, spreading its heat to the interior Arctic basins; the processes of vertical and lateral mixing, are not well understood (Carmack et al, 2015). In the central Arctic, sea ice limits the energy input from wind, and the vertical mixing is weak (Fer, 2009; Rainville & Winsor, 2008). Double diffusive heat fluxes are, weak about 0.1 to 1 W m−2 in the central Arctic (Guthrie et al, 2013; Sirevaag & Fer, 2012) and 1 to 10 W m−2 in the eastern Arctic Ocean (Polyakov et al, 2019). Episodic energetic vertical mixing in the interior ocean has been observed in response to strong winds and ice divergence. North of Svalbard, winter storms increased the pycnocline fluxes by a factor of 2 (Meyer, Fer, et al, 2017) and the ocean-to-ice fluxes by a factor of 3 (Graham et al, 2019)

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Conclusion