Edward C. Wiebe - Abstracts
On the sensitivity of global warming experiments to the parameterisation of sub-grid scale ocean mixing
Wiebe, E.C. and A.J. Weaver
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 parameterisation of sub-grid scale ocean mixing.
The climate sensitivity of the coupled model using three different
parameterisations of sub-grid scale mixing is 3 degrees Celsius
for a doubling of CO2 (6 degrees Celsius 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 parameterisations.
In the experiment using the Gent and McWilliams parameterisation 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 parameterisation of sub-grid scale mixing, warmed
Atlantic water was found to penetrate at depth into the Arctic, consistent
with recent observations in the region.
Climate Change and Sub-Grid-Scale Mixing in Coupled Model
M. Sc. Thesis Abstract
Global warming experiments are performed using three different versions of
an ocean model, coupled to an energy-moisture balance atmosphere model
and a thermodynamic ice model. The equilibrium climate sensitivity of the
model is 3 degrees Celcius for doubled CO2
(6 degrees C for quadrupled CO2) in all
experiments. This suggests that the ocean has only a weak feedback on the
global mean air temperature. There are, however, significant regional
differences, notably at high latitudes, between models with different
sub-grid-scale parameterisations. For example, the addition of the Gent
and McWilliams parameterisation, of the effects of mesoscale eddies
on mixing, results in an enhancement of the surface response in the
Southern Ocean. This is largely the consequence of more realistic
sea ice in the climatological control integration and subsequent
enhancement of the ice-albedo feedback upon warming.
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 at first acts as a negative feedback on local
warming, but contributes a positive feedback at equilibrium.
In the transient phase of the experiments with a more sophisticated
and realistic parameterisation of sub-grid- scale mixing, warmed
Atlantic water was found to penetrate at depth into the Arctic,
consistent with recent observations in the region.
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