Observations of Turbulence at a Near‐Surface Temperature Front in the Arctic Ocean

Abstract

High-resolution ocean temperature, salinity, current, and turbulence data were collected at an Arctic thermohaline front in the Nansen Basin. The front was close to the sea ice edge and separated the cold and fresh surface melt water from the warm and saline mixed layer. Measurements were made on 18 September 2018, in the upper 100 m, from a research vessel and an autonomous underwater vehicle. Destabilizing surface buoyancy fluxes from a combination of heat loss to the atmosphere and cross-front Ekman transport by down-front winds reduced the potential vorticity in the upper ocean. Turbulence structure in the mixed layer was generally consistent with turbulence production through convection by heat loss to atmosphere and mechanical forcing by moderate winds. Conditions at the front were favorable for forced symmetric instability, a mechanism drawing energy from the frontal geostrophic current. A clear signature of increased dissipation from symmetric instability could not be identified; however, this instability could potentially account for the increased dissipation rates at the front location down to 40 m depth that could not be explained by the atmospheric forcing. This turbulence was associated with turbulent heat fluxes of up to 10 W m −2 , eroding the warm and cold intrusions observed between 30 and 60 m depth. A Seaglider sampled across a similar frontal structure 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. Plain Language SummaryOn 18 September 2018, high-resolution temperature, salinity, current, and turbulence data were collected near a surface temperature and salinity front in the Nansen Basin north of Svalbard, within the framework of the Nansen Legacy Project. Measurements were performed from the research icebreaker Kronprins Haakon and using an autonomous underwater vehicle. The front separates warm and salty Atlantic-origin waters from cold and fresh Arctic-origin waters. Energetic turbulence in the upper 30 m was consistent with convection by heat loss to the atmosphere and mechanical forcing by moderate winds at the surface of the ocean. At the front, the conditions were favorable for forced symmetric instability, a mechanism drawing energy from the large-scale current. The contribution to turbulence from this instability could not be clearly identified but can be potentially important. Such observations linking 1-3 km scales to turbulence in the Arctic Ocean are rare. Turbulence at a front in the Arctic has consequences on the heat and nutrient transport from the slope to the deep Arctic Basin. A Seaglider sampled across a similar frontal structure in the same region end of September 2018. Observed frontal structure and mixing processes can be common in the Atlantic sector of the Arctic Ocean.

Stirring of Sea‐Ice Meltwater Enhances Submesoscale Fronts in the Southern Ocean

“…Observations of wind-front interactions in ice-free MIZs yield contrasting results between the studies. Under winter conditions near the Arctic MIZ, Koenig et al (2020) provide evidence of sustained turbulence by forced symmetric instabilities associated with downfront winds and heat loss from the surface layer, which extract potential vorticity from the water column at submesoscale fronts. Conversely, Brenner et al (2020) found that upfront winds lead to frontogenesis on time scales shorter than Ekman dynamics.…”

“…However, it has been demonstrated by theory and models that submesoscale flows in MIZs interact with horizontal gradients in density to modify ML structure, ESSOAr | https://doi.org/10.1002/essoar.10504395.1 | Non-exclusive | First posted online: Thu, 24 Sep 2020 09:41:41 | This content has not been peer reviewed. manuscript submitted to JGR: Oceans et al, 2016;Lu et al, 2015;Manucharyan & Thompson, 2017), and by observations in the Arctic (Timmermans & Winsor, 2013;Koenig et al, 2020;Brenner et al, 2020) and more recently in the Antarctic (e.g. (Swart et al, 2020;.…”

“…One-dimensional mixing processes associated with local sea-ice melt and vertical mixing have been shown to dominate upper ocean physics in MIZs (Dewey et al, 2017;Smith et al, 2018). However, it has been demonstrated by theory and models that submesoscale flows in MIZs interact with horizontal gradients in density to modify ML structure (Horvat et al, 2016;Lu et al, 2015;Manucharyan & Thompson, 2017), and by observations in the Arctic (Brenner et al, 2020;Koenig et al, 2020;Timmermans & Winsor, 2013) and more recently in the Antarctic (e.g., Swart et al, 2020).…”

Stepwise Subduction Observed at a Front in the Marginal Ice Zone in Fram Strait

“…Furthermore, symmetric instability can be sustained by down-front winds or destabilizing convection, which would favor a forward energy cascade. Increased turbulence, which may have been caused by forced symmetric instability, was observed along a front, which we would categorize as between an AW mixed layer and a stratified SW layer by Koenig et al (2020) in the Nansen Basin of the Arctic Ocean.…”

“…Most of these studies additionally utilized a variety of instruments, such as a vessel‐mounted Acoustic Doppler Current Profiler (ADCP), a thermosalinograph, a conductivity‐temperature‐depth (CTD) rosette, water sampling, atmospheric observations, and reanalysis or model data. The survey durations ranged from a few hours (Archer et al., 2020; Tippenhauer et al., 2021; Wulff et al., 2016) to 1–1.5 days (von Appen et al., 2018; Brenner et al., 2020; Koenig et al., 2020; von Appen et al., 2020), with more comprehensive studies such as Hosegood et al. (2017), Mahadevan et al.…”