Structure and drivers of ocean mixing north of Svalbard in summer and fall 2018

Abstract

Abstract. The Arctic Ocean is a major sink for heat and salt for the global ocean. Ocean mixing contributes to this sink by mixing the Atlantic- and Pacific-origin waters with surrounding waters. We investigate the drivers of ocean mixing north of Svalbard, in the Atlantic sector of the Arctic, based on observations collected during two research cruises in summer and fall 2018. Estimates of vertical turbulent heat flux from the Atlantic Water layer up to the mixed layer reach 30 W m−2 in the core of the boundary current, and average to 8 W m−2, accounting for ∼1 % of the total heat loss of the Atlantic layer in the region. In the mixed layer, there is a nonlinear relation between the layer-integrated dissipation and wind energy input; convection was active at a few stations and was responsible for enhanced turbulence compared to what was expected from the wind stress alone. Summer melting of sea ice reduces the temperature, salinity and depth of the mixed layer and increases salt and buoyancy fluxes at the base of the mixed layer. Deeper in the water column and near the seabed, tidal forcing is a major source of turbulence: diapycnal diffusivity in the bottom 250 m of the water column is enhanced during strong tidal currents, reaching on average 10−3 m2 s−1. The average profile of diffusivity decays with distance from the seabed with an e-folding scale of 22 m compared to 18 m in conditions with weaker tidal currents. A nonlinear relation is inferred between the depth-integrated dissipation in the bottom 250 m of the water column and the tidally driven bottom drag and is used to estimate the bottom dissipation along the continental slope of the Eurasian Basin. Computation of an inverse Froude number suggests that nonlinear internal waves forced by the diurnal tidal currents (K1 constituent) can develop north of Svalbard and in the Laptev and Kara seas, with the potential to mix the entire water column vertically. Understanding the drivers of turbulence and the nonlinear pathways for the energy to turbulence in the Arctic Ocean will help improve the description and representation of the rapidly changing Arctic climate system.

Diurnal and semidiurnal internal waves on the southern slope of the Yermak Plateau

“…A large proportion of energy is dissipated locally, especially for that in run M2A, which could explain the observed intense turbulent mixing on the YP 20 , 21 , 28 . Following previous studies 67 , we further examine the local dissipation coefficient q which is defined as the ratio of energy dissipation to conversion.…”

“…The following work 20 indicates that the internal tidal waves in this region are mainly in the diurnal and semidiurnal bands. Also, the measured dissipation rates at the YP are much higher than those in deep basin of AO 27 , 28 , 34 . The simulated barotropic-to-baroclinic energy conversion in this region reaches 1 GW 21 , which roughly accounts for 20% of the total conversion in the AO (~ 5 GW 7 , 10 ).…”

“…Because most of the AO is poleward of the critical latitudes (where the local inertial frequency is equal to the tidal frequency) of both semidiurnal and diurnal tides, internal tidal waves cannot freely propagate far away from their generation sites. Instead, they are trapped near their generation sites under the constraint of rotation and dissipated locally 23 – 26 , resulting in enhanced mixing in some slope regions in the AO 20 , 21 , 26 – 28 . The enhanced mixing further affects the heat budget in the AO and influences the global climate 15 , 29 , 30 .…”

“…The Yermak Plateau (YP) is the main passage of the Atlantic Water to the AO (Fig. 1 a; 28 , 31 ). The Atlantic Water carried by the West Spitsbergen Current could be an important source of heat and salt for the AO 32 .…”

“…The Atlantic Water carried by the West Spitsbergen Current could be an important source of heat and salt for the AO 32 . As the Atlantic Water flows over the YP, it gets cooled and freshened because of strong air-sea fluxes and turbulent mixing 15 , 21 , 28 . Given the presence of strong barotropic tidal flow and steep slopes around the YP 33 , breaking of tide-induced internal waves is considered to play a dominant role in driving the mixing near the YP 21 .…”