The Strong Control of Sea Ice Dynamics on Lower Cell Circulation Changes

Abstract:

The Lower Cell of the Meridional Overturning Circulation (MOC) is characterized by the formation of dense Antarctic Bottom Water, which descends into the abyssal ocean and is sequestered from the atmosphere for hundreds to thousands of years before outcropping again at the Southern Ocean surface. Thus, the dynamics, and changing dynamics, of the Lower Cell may mediate the heat content of the deep ocean and play a key role in determining the long-term rate of atmospheric warming.

We examine its dynamics in the Community Climate System Model version 3.5 (CCSM 3.5), a fully-coupled global climate model which was run at two resolutions in the ocean and sea ice components: standard resolution (1.0 degrees) and high resolution (0.1 degrees). In the latter, eddies are explicitly resolved throughout most of the global ocean, and fine-scale sea ice processes are improved. We consider the response of the Lower Cell to two perturbations relevant to projected future climate change: an isolated wind-only experiment, in which westerly wind stress alone is increased, and a full “global warming” experiment, in which carbon dioxide was ramped until doubling. We show that in the mean state, this cell is primarily sustained by surface water mass transformation, itself enabled by heat loss from waters within the sea ice pack. Further, this circulation is notably stronger at fine resolution because of the improved simulation of sea ice dynamics.

The response of the Lower Cell to each perturbation is strikingly distinct. Following the intensification of westerly wind stress, overturning in the Lower Cell strengthens by as much as 40%. However, in response to carbon dioxide doubling, the circulation is weakened by up to 60%. In both cases, the sensitivity of this circulation to forcing primarily stems from the strong control that sea ice properties exert on ocean heat loss.

The varied response of the Lower Cell circulation to forcing subsequently impacts global ocean heat content. In the wind perturbation experiment, circulation changes cool the deep ocean, compensating for ocean warming induced by increases in Upper Cell strength. In contrast, in the comprehensive global warming perturbation, a weakening of the circulation compounds far-field ocean warming, particularly in the deep and abyssal ocean, and more so at high resolution.