On the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing |
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Authors: | EC Wiebe A J Weaver |
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Institution: | (1) School of Earth and Ocean Sciences, University of Victoria, PO Box 3055, Victoria, B.C., V8W 3P6, Canada, CA |
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Abstract: | An ocean general circulation model coupled to an energy-moisture balance atmosphere model is used to investigate the sensitivity
of global warming experiments to the parametrisation of sub-grid scale ocean mixing. The climate sensitivity of the coupled
model using three different parametrisations of sub-grid scale mixing is 3°C for a doubling of CO2 (6°C for a quadrupling of CO2). This suggests that the ocean has only a weak feedback on global mean surface air temperature although significant regional
differences, notably at high latitudes, exist with different sub-grid scale parametrisations. In the experiment using the
Gent and McWilliams parametrisation for mixing associated with mesoscale eddies, an enhancement of the surface response in
the Southern Ocean is found. This enhancement is largely due to the existence of more realistic sea-ice in the climatological
control integration and the subsequent enhanced ice-albedo feedback upon warming. In accordance with earlier analyses, the
Gent and McWilliams scheme decreases the global efficiency of ocean heat uptake. During the transient phase of all experiments,
the North Atlantic overturning initially weakened but ultimately recovered, surpassing its former strength. This suggests
that in the region around the North Atlantic the ocean acts as a negative feedback on local warming during the transient phase
but a positive feedback at equilibrium. During the transient phase of the experiments with a more sophisticated and realistic
parametrisation of sub-grid scale mixing, warmed Atlantic water was found to penetrate at depth into the Arctic, consistent
with recent observations in the region.
Received: 14 October 1998 / Accepted: 27 April 1999 |
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