The role of Antarctic sea ice in abyssal ocean heat uptake

Emily Rose Newsom1, Cecilia M Bitz2, Frank Bryan3, Ryan P Abernathey4 and Peter R Gent3, (1)California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, (2)University of Washington, Seattle, WA, United States, (3)National Center for Atmospheric Research, Boulder, CO, United States, (4)Lamont -Doherty Earth Observatory of Columbia University, Palisades, NY, United States
Abstract:
The deep cell of the Meridional Overturning Circulation is characterized by the downwelling of dense water formed near Antarctica, its slow transit through much of the abyssal global ocean, and its eventual return to the ocean surface. The dynamics governing this circulation depend on small-scale processes; however, they may mediate global climate over long timescales via their influence on deep ocean heat content. This presents a challenge for global climate models (GCM’s), which are typically run at relatively coarse resolution. Indeed, large biases in abyssal ocean properties are endemic to GCMs, revealing a potential bias in long-term climate perturbation studies.

We examine the impact of model resolution on the behavior of the deep cell in response to a doubling of atmospheric carbon dioxide, in the Community Climate System Model (CCSM) version 3.5. This fully coupled GCM was run at two resolutions in the ocean and sea ice components: low resolution (LR: 1.0 degrees) and high resolution (HR: 0.1 degrees). In the latter, eddies are predominantly resolved and sea ice processes are resolved on finer scales.

Before perturbing the climate, the deep cell is stronger and extends further northward into the abyssal ocean at HR. We show that its descending branch is sustained by surface water mass transformation in the Antarctic marginal seas, which is more vigorous due to the resolution of more discrete and localized sea ice polynas at HR. These features enable more extreme loss through the sea ice pack in HR. Perhaps surprisingly, differences in interior water mass transformation between model versions deriving from differing eddy behavior and diapycnal mixing patterns have secondary effects.

These distinct dynamics lead to resolution-dependent responses to surface warming. After carbon dioxide doubling, the deep cell circulation slows significantly, irrespective of resolution. However, this response is stronger and reaches deeper into the abyssal ocean at HR due to a dramatic reduction in surface water mass transformation. The varied circulation response subsequently impacts the pattern of abyssal ocean heat uptake, which is five times greater below 3500m at HR.

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