Submesoscale Fronts in the Antarctic Marginal Ice Zone and Their Response to Wind Forcing

Abstract

Submesoscale flows in the ocean are energetic motions, O(1–10 km), that influence stratification and the distributions of properties, such as heat and carbon. They are believed to play an important role in sea‐ice‐impacted oceans by modulating air‐sea‐ice fluxes and sea‐ice extent. The intensity of these flows and their response to wind forcing are unobserved in the sea‐ice regions of the Southern Ocean. We present the first submesoscale‐resolving observations in the Antarctic marginal ice zone (MIZ) collected by surface and underwater autonomous vehicles, for >3 months in austral summer. We observe salinity‐dominated lateral density fronts occurring at sub‐kilometer scales. Surface winds are shown to modify the magnitude of the mixed‐layer density fronts, revealing strongly coupled atmosphere‐ocean processes. We posture that these wind‐front interactions occur as a continuous interplay between front slumping and vertical mixing, which leads to the dispersion of submesoscale fronts. Such processes are expected to be ubiquitous in the Southern Ocean MIZ.

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

“…We observed salinity-driven lateral fronts at submesoscales during summer in the ice free Antarctic MIZ, with increased prevalence occuring with close proximity to the sea ice melt out (occurring at this study site on the 10 th December 2018). Concurring with surface observations by Swart et al (2020), evidence for an active submesoscale regime during early summer is reflected in the ML density variability, representative of potential energy in the ML (Figure 8). As the summer season progresses, the magnitude of density variability decreases and the reduction in variance is greater at smaller scales.…”

“…The time series we present here extends beyond that of Swart et al (2020), in which they show that submesoscale fronts persist into summer in an interplay between thermohaline slumping by winds and frontogenesis. Corresponding with their observations, lateral buoyancy gradients observed here persist from December to February, but eventually reduce in magnitude from mid-February until the end of March when the surface heat flux turns negative ( Figure 2) and the supply of freshwater from the south decreases ( Figure 6).…”

“…Acknowledging this is not the case, a statistical analysis shows that the lateral gradients are underestimated by a factor of 0.64. Compared to the lateral gradients captured by a Sailbuoy (measuring values of b x up to 0.06 kg m −3 km −1 , Swart et al, 2020), which was sampling the study site concurrently, the lateral gradients observed by the Seaglider are small (b x up to 0.015 kg m −3 km −1 ), but nevertheless comparable to those observed in the Arctic MIZ by Seagliders (∼ 0.03 kg m −3 km −1 , Timmermans and Windsor, 2013). Furthermore, using averaged ML lateral gradients also reduces the magnitude of lateral gradients, compared to taking the lateral gradients in the middle of the ML (Thompson et al, 2016).…”

“…This results in ML (z ∼ 10 -40 m) profiles which are captured at high resolution (x ∼ 300 m) between a climb profile and a dive profile, and profiles which are further apart (x ∼ 4 km) between a dive profile and a climb profile. An indepth analysis of the implications of this sampling pattern in capturing lateral gradients in the ML is provided by (Swart et al, 2020) (see their Supplementary Figure 2).…”

“…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;.…”