Impacts of Broad-Scale Surface Freshening of the Southern Ocean in a Coupled Climate Model

Journal Article · 05 March 2018 · Journal of Climate

DOI: https://doi.org/10.1175/JCLI-D-17-0092.1 · OSTI ID:1423729

Purich, Ariaan ^[1]; England, Matthew H. ^[2]; Cai, Wenju ^[3]; Sullivan, Arnold ^[3]; Durack, Paul J. ^[4]

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1. Commonwealth Scientific and Industrial Research Organization (CSIRO), Aspendale, VIC (Australia). Oceans and Atmosphere; Univ. of New South Wales, Sydney, NSW (Australia). ARC Centre of Excellence for Climate System Science
2. Univ. of New South Wales, Sydney, NSW (Australia). Climate Change Research Centre and ARC Centre of Excellence for Climate System Science
3. Commonwealth Scientific and Industrial Research Organization (CSIRO), Aspendale, VIC (Australia)
4. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Program for Climate Model Diagnosis and Intercomparison

The Southern Ocean surface has freshened in recent decades, increasing water column stability and reducing upwelling of warmer subsurface waters. The majority of CMIP5 models underestimate or fail to capture this historical surface freshening, yet little is known about the impact of this model bias on regional ocean circulation and hydrography. Here experiments are performed using a global coupled climate model with additional freshwater applied to the Southern Ocean to assess the influence of recent surface freshening. The simulations explore the impact of persistent and long-term broad-scale freshening as a result of processes including precipitation minus evaporation changes. Thus, unlike previous studies, the freshening is applied as far north as 55°S, beyond the Antarctic ice margin. It is found that imposing a large-scale surface freshening causes a surface cooling and sea ice increase under preindustrial conditions, because of a reduction in ocean convection and weakened entrainment of warm subsurface waters into the surface ocean. This is consistent with intermodel relationships between CMIP5 models and the simulations, suggesting that models with larger surface freshening also exhibit stronger surface cooling and increased sea ice. Additional experiments are conducted with surface salinity restoration applied to capture observed regional salinity trends. Remarkably, without any mechanical wind trend forcing, these simulations accurately represent the spatial pattern of observed surface temperature and sea ice trends around Antarctica. This study highlights the importance of accurately simulating changes in Southern Ocean salinity to capture changes in ocean circulation, sea surface temperature, and sea ice.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1423729
• Alternate ID(s): OSTI ID: 1491637
• Report Number(s): LLNL-JRNL-723879; 872104
• Journal Information: Journal of Climate, Vol. 31, Issue 7; ISSN 0894-8755
• Publisher: American Meteorological SocietyCopyright Statement
• Country of Publication: United States
• Language: English