Transient Response of the Southern Ocean to Idealized Wind and Thermal Forcing across Different Model Resolutions

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  • 1 Climate Change Research Centre and ARC Centre of Excellence for Climate System Science, University of New South Wales, Sydney, NSW, Australia
  • 2 Climate Change Research Centre and ARC Centre of Excellence for Climate Extremes, University of New South Wales, Sydney, NSW, Australia
  • 3 Research School of Earth Sciences and ARC Centre of Excellence for Climate Extremes, Australian National University, Canberra, Australian Capital Territory, Australia
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Abstract

The Southern Ocean has undergone significant climate-related changes over recent decades, including intensified westerly winds and increased radiative heating. The interplay between wind-driven cooling and radiative warming of the ocean is complex and remains unresolved. In this study, idealized wind and thermal perturbations are analyzed in a global ocean–sea-ice model at two horizontal resolutions: nominally, 1° and 0.1°. The sea surface temperature (SST) response shows a clear transition from a wind-driven cooling phase to a warming phase. This warming transition is largely attributed to meridional and vertical Ekman heat advection, which are both sensitive to model resolution due to the model-dependent components of temperature gradients. At higher model resolution, due to a more accurate representation of near-surface vertical temperature inversion and upward Ekman heat advection around Antarctica, the anomalous SST warming is stronger and develops earlier. The mixed layer depth at mid-latitudes initially increases due to a wind-driven increase in Ekman transport of cold dense surface water northward, but then decreases when the thermal forcing drives enhanced surface stratification; both responses are more sensitive at lower model resolution. With the wind intensification, the residual overturning circulation increases less in the 0.1° case because of the adequately resolved eddy compensation. Ocean heat subduction penetrates along more tilted isopycnals in the 1° case, but it orientates to follow isopycnal layers in the 0.1° case. These findings have implications for understanding the ocean response to the combined effects of Southern Hemispherewesterly wind changes and anthropogenic warming.

Corresponding author: Qian Li, qian.li5@unsw.edu.au

Abstract

The Southern Ocean has undergone significant climate-related changes over recent decades, including intensified westerly winds and increased radiative heating. The interplay between wind-driven cooling and radiative warming of the ocean is complex and remains unresolved. In this study, idealized wind and thermal perturbations are analyzed in a global ocean–sea-ice model at two horizontal resolutions: nominally, 1° and 0.1°. The sea surface temperature (SST) response shows a clear transition from a wind-driven cooling phase to a warming phase. This warming transition is largely attributed to meridional and vertical Ekman heat advection, which are both sensitive to model resolution due to the model-dependent components of temperature gradients. At higher model resolution, due to a more accurate representation of near-surface vertical temperature inversion and upward Ekman heat advection around Antarctica, the anomalous SST warming is stronger and develops earlier. The mixed layer depth at mid-latitudes initially increases due to a wind-driven increase in Ekman transport of cold dense surface water northward, but then decreases when the thermal forcing drives enhanced surface stratification; both responses are more sensitive at lower model resolution. With the wind intensification, the residual overturning circulation increases less in the 0.1° case because of the adequately resolved eddy compensation. Ocean heat subduction penetrates along more tilted isopycnals in the 1° case, but it orientates to follow isopycnal layers in the 0.1° case. These findings have implications for understanding the ocean response to the combined effects of Southern Hemispherewesterly wind changes and anthropogenic warming.

Corresponding author: Qian Li, qian.li5@unsw.edu.au
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