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Giuliana A. Viglione, Andrew F. Thompson, M. Mar Flexas, Janet Sprintall, and Sebastiaan Swart

Abstract

Enhanced vertical velocities associated with submesoscale motions may rapidly modify mixed layer depths and increase exchange between the mixed layer and the ocean interior. These dynamics are of particular importance in the Southern Ocean, where the ventilation of many density classes occurs. Here we present results from an observational field program in southern Drake Passage, a region preconditioned for submesoscale instability owing to its strong mesoscale eddy field, persistent fronts, strong down-front winds, and weak vertical stratification. Two gliders sampled from December 2014 through March 2015 upstream and downstream of the Shackleton Fracture Zone (SFZ). The acquired time series of mixed layer depths and buoyancy gradients enabled calculations of potential vorticity and classifications of submesoscale instabilities. The regions flanking the SFZ displayed remarkably different characteristics despite similar surface forcing. Mixed layer depths were nearly twice as deep, and horizontal buoyancy gradients were larger downstream of the SFZ. Upstream of the SFZ, submesoscale variability was confined to the edges of topographically steered fronts, whereas downstream these motions were more broadly distributed. Comparisons to a one-dimensional (1D) mixing model demonstrate the role of submesoscale instabilities in generating mixed layer variance. Numerical output from a submesoscale-resolving simulation indicates that submesoscale instabilities are crucial for correctly reproducing upper-ocean stratification. These results show that bathymetry can play a key role in generating dynamically distinct submesoscale characteristics over short spatial scales and that submesoscale motions can be locally active during summer months.

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Yanzhou Wei, Sarah T. Gille, Matthew R. Mazloff, Veronica Tamsitt, Sebastiaan Swart, Dake Chen, and Louise Newman

Abstract

Proposals from multiple nations to deploy air–sea flux moorings in the Southern Ocean have raised the question of how to optimize the placement of these moorings in order to maximize their utility, both as contributors to the network of observations assimilated in numerical weather prediction and also as a means to study a broad range of processes driving air–sea fluxes. This study, developed as a contribution to the Southern Ocean Observing System (SOOS), proposes criteria that can be used to determine mooring siting to obtain best estimates of net air–sea heat flux (Q net). Flux moorings are envisioned as one component of a multiplatform observing system, providing valuable in situ point time series measurements to be used alongside satellite data and observations from autonomous platforms and ships. Assimilating models (e.g., numerical weather prediction and reanalysis products) then offer the ability to synthesize the observing system and map properties between observations. This paper develops a framework for designing mooring array configurations to maximize the independence and utility of observations. As a test case, within the meridional band from 35° to 65°S we select eight mooring sites optimized to explain the largest fraction of the total variance (and thus to ensure the least variance of residual components) in the area south of 20°S. Results yield different optimal mooring sites for low-frequency interannual heat fluxes compared with higher-frequency subseasonal fluxes. With eight moorings, we could explain a maximum of 24.6% of high-frequency Q net variability or 44.7% of low-frequency Q net variability.

Open access
Kevin M. Schmidt, Sebastiaan Swart, Chris Reason, and Sarah-Anne Nicholson

Abstract

Surface ocean wind datasets are required to be of high spatial and temporal resolution and high precision to accurately force or be assimilated into coupled atmosphere–ocean numerical models and to understand ocean–atmospheric processes. In situ observed sea surface winds from the Southern Ocean are scarce and, consequently, the validity of simulation models is often questionable. Multiple wind data products were compared to the first known high-resolution in situ measurements of wind speed from Wave Glider (WG) deployments in the Southern Ocean with the intent to determine which blended satellite or reanalysis product best represents the magnitude and variability of the observed wind field. Results show that the ECMWF reanalysis product is the most accurate in representing the temporal variability of winds, exhibiting consistently higher correlation coefficients with in situ data across all wind speed categories. However, the NCEP–DOE AMIP-II Reanalysis product matches in situ trends of deviation from the mean and performs best in depicting the mean wind state, especially during high wind states. The ECMWF product also leads to smaller differences in wind speeds from the in situ data, while CFSv2 showed slightly higher biases and a greater RMSE. The SeaWinds (SW) product consistently performed poorly at representing the mean or wind stress variability compared to those observed by the WG. Overall, the study shows autonomous surface vehicles provide valuable observations by which to validate, understand, and potentially assist in correcting satellite/reanalysis products, particularly in remote regions, where few in situ estimates exist.

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Marcel du Plessis, Sebastiaan Swart, Isabelle J. Ansorge, Amala Mahadevan, and Andrew F. Thompson

Abstract

Ocean stratification and the vertical extent of the mixed layer influence the rate at which the ocean and atmosphere exchange properties. This process has direct impacts for anthropogenic heat and carbon uptake in the Southern Ocean. Submesoscale instabilities that evolve over space (1–10 km) and time (from hours to days) scales directly influence mixed layer variability and are ubiquitous in the Southern Ocean. Mixed layer eddies contribute to mixed layer restratification, while down-front winds, enhanced by strong synoptic storms, can erode stratification by a cross-frontal Ekman buoyancy flux. This study investigates the role of these submesoscale processes on the subseasonal and interannual variability of the mixed layer stratification using four years of high-resolution glider data in the Southern Ocean. An increase of stratification from winter to summer occurs due to a seasonal warming of the mixed layer. However, we observe transient decreases in stratification lasting from days to weeks, which can arrest the seasonal restratification by up to two months after surface heat flux becomes positive. This leads to interannual differences in the timing of seasonal restratification by up to 36 days. Parameterizing the Ekman buoyancy flux in a one-dimensional mixed layer model reduces the magnitude of stratification compared to when the model is run using heat and freshwater fluxes alone. Importantly, the reduced stratification occurs during the spring restratification period, thereby holding important implications for mixed layer dynamics in climate models as well as physical–biological coupling in the Southern Ocean.

Open access