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Southern Ocean Seasonal Restratification Delayed by Submesoscale Wind–Front Interactions

Journal article
Authors Marcel Du Plessis
Sebastiaan Swart
Isabelle Ansorge
Amala Mahadevan
Andrew Thompson
Published in Journal of Physical Oceanography
Volume 49
Issue 4
Pages 1035-53
ISSN 0022-3670
Publication year 2019
Published at Department of marine sciences
Pages 1035-53
Language en
Links doi.org/10.1175/JPO-D-18-0136.1
Keywords mixed-layer depth, potential vorticity, mesoscale, transport, instability, turbulence, impacts, spectra, models
Subject categories Oceanography

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.

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