Acoustic Mapping of Thermohaline Staircases in the Arctic Ocean

Abstract

Although there is enough heat contained in inflowing warm Atlantic Ocean water to melt all Arctic sea ice within a few years, a cold halocline limits upward heat transport from the Atlantic water. The amount of heat that penetrates the halocline to reach the sea ice is not well known, but vertical heat transport through the halocline layer can significantly increase in the presence of double diffusive convection. Such convection can occur when salinity and temperature gradients share the same sign, often resulting in the formation of thermohaline staircases. Staircase structures in the Arctic Ocean have been previously identified and the associated double diffusive convection has been suggested to influence the Arctic Ocean in general and the fate of the Arctic sea ice cover in particular. A central challenge to understanding the role of double diffusive convection in vertical heat transport is one of observation. Here, we use broadband echo sounders to characterize Arctic thermohaline staircases at their full vertical and horizontal resolution over large spatial areas (100 s of kms). In doing so, we offer new insight into the mechanism of thermohaline staircase evolution and scale, and hence fluxes, with implications for understanding ocean mixing processes and ocean-sea ice interactions.

Acoustic mapping of mixed layer depth

“…This agreement among reflection coefficients is consistent with observations of deeper thermohaline staircase 5 stratification from the central Arctic Ocean presented in Stranne et al (2017). In the SWERUS-C3 data, biological scatterers are generally identified at CTD stations closer to the coast.…”

“…The deep depth limit for detecting ocean stratification with this particular EK80 setup appears to be around 300 m 35 (Stranne et al, 2017) while the shallow depth limit depends on the draft of the hull-mounted transducer and the pulse length. On the Oden, the EK80 transducer is mounted at a draft of 7 m and, depending on pulse length, we generally observe useful data starting at 7.5-12 m depth from the surface (0.5-5 m from the transducer, Fig.…”

“…Custom-built echosounders utilizing wideband frequency-modulated pulses have been deployed since the 1970s (e.g. Holliday, 1972), but have received renewed attention as they have become commercially available (Duda et al, 2016;Lavery et al, 2010;Stranne et al, 2017).…”

“…20 Stranne et al (2017) were able to acoustically image individual thermohaline steps resulting from the intrusion of warm and salty Atlantic waters into the colder and less saline Arctic waters. The range resolution provided by the wideband sonar enabled the detection of individual density layers separated by less than 0.5 m to depths of about 300 m. These thermohaline layers represent change in temperature of typically 0.05 °C and change in salinity of 0.015, with corresponding acoustic reflection coefficients at the layer interface as low as 2·10 -5 .…”

“…The increased vertical resolution (from ~10 m with multi-channel seismic data to <0.5 m with wideband 10 acoustics (Stranne et al, 2017)) facilitates the detection of much finer thermohaline structures in the water column, including the MLD, and can potentially vastly improve these methods.…”