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

Abstract

<p>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.</p><p>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<sup>-2</sup>, 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.</p>

Adaptive Sampling of Surface Fronts in the Arctic Using an Autonomous Underwater Vehicle

“…The front is shown as an iso-surface with its outline marked for clarification. The front has an intrusion of warm water at 40 m that arises from the frontal dynamics; this is further discussed in detail in [7]. Temperature Fig.…”

“…Figure 9 presents cross-front sections of the different parameters, with the warm side located at distance y < 0 km and the cold side at y > 0 km, where y is the cross-front distance in km. Details about the construction of the sections can be found in [7]. The AUV mission was designed to cross the surface temperature front, but it also criss-crossed a deeper front.…”

“…Satellite imagery from polar orbiting satellites have been extensively used to observe ocean surface fronts, see e.g. [7]. As there is an inherit latency between capture and availability, surface coverage only, cloud dependence, combined with the tidal movements and ocean dynamics, the use of satellite-based remote sensing disallow direct use in resolving subsurface details and tracking the front.…”