On the control of subantarctic stratification by the ocean circulation
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On the control of subantarctic stratification by the ocean circulation R. Justin Small1 · Alice K. DuVivier1 · Daniel B. Whitt1 · Matthew C. Long1 · Ian Grooms2 · William G. Large1 Received: 5 April 2019 / Accepted: 2 October 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract A shallow mixed layer depth bias in Austral winter in the Subantarctic Zone is a common feature of Coupled Model Intercomparison Project (CMIP5) models, including the Community Earth System Model (CESM). The bias is related to other deficiencies in the model solution, including too-weak Subantarctic Mode water formation and excessive leakage of Agulhas waters into the Atlantic instead of into the Indian Ocean and Subantarctic Frontal Zone. This work investigates the hypothesis that the shallow bias is due to errors in the simulated ocean circulation. Results from a model with low resolution ocean component (1° grid) are compared against: (i) an experiment with an eddy resolving (0.1°) grid and (ii) experiments using the 1° grid and employing an adiabatic nudging of the ocean pressure field to observations that is referred to as a semiprognostic method. For both the higher resolution and semi-prognostic experiments, improved horizontal advection of warm and salty water near the surface leads to a more realistic sea surface temperature (SST) and salinity front associated with the Agulhas Return Current and Antarctic Circumpolar Current. The warmer surface waters in the high-resolution model and semi-prognostic model lead to stronger air–sea heat loss, with a response of about 40 Wm−2 °C−1 in winter in the area of deep mixed layers. Budgets of the temperature and salinity stratification show that the deeper mixed layers in the highresolution experiment are primarily a result of the increased surface heat loss, whereas in the semi-prognostic case salinity advection is the main factor leading to destabilization of the water column. Both results indicate that ocean circulation is a key factor in wintertime deep Southern Ocean mixing, associated with advection of water masses and air–sea feedbacks.
1 Introduction 1.1 Background The Southern Ocean is an important region for air–sea exchange of heat and anthropogenic greenhouse gases (Sabine et al. 2004; Rodgers et al. 2014; Frölicher et al. 2015) due to a combination of strong winds and temperature gradients in the ocean and atmosphere. The Southern Ocean is dominated by the Antarctic Circumpolar Current (ACC); immediately north of the ACC, large air–sea heat Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00382-020-05473-2) contains supplementary material, which is available to authorized users. * R. Justin Small [email protected] 1
Climate and Global Dynamics Division, National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO 80305, USA
Department of Applied Mathematics, University of Colorado, Boulder, USA
2
fluxes drive Subantarctic Mode Water formation (McCartney 1982; Hanawa and Talley 2001;
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