[Stoch] Improved Climate Simulations through a Stochastic Parameterization of Ocean Eddies

[Paul Williams]

Dear all,

Here are details of our new paper, which is published in the December issue of Journal of Climate:

Improved Climate Simulations through a Stochastic Parameterization of Ocean Eddies

by Williams / Howe / Gregory / Smith / Joshi

http://journals.ametsoc.org/doi/10.1175/JCLI-D-15-0746.1

Abstract:  In climate simulations, the impacts of the subgrid scales on the resolved scales are conventionally 
represented using deterministic closure schemes, which assume that the impacts are uniquely determined by the resolved 
scales. Stochastic parameterization relaxes this assumption, by sampling the subgrid variability in a computationally 
inexpensive manner. This study shows that the simulated climatological state of the ocean is improved in many respects 
by implementing a simple stochastic parameterization of ocean eddies into a coupled atmosphere–ocean general circulation 
model. Simulations from a high-resolution, eddy-permitting ocean model are used to calculate the eddy statistics needed 
to inject realistic stochastic noise into a low-resolution, non-eddy-permitting version of the same model. A suite of 
four stochastic experiments is then run to test the sensitivity of the simulated climate to the noise definition by 
varying the noise amplitude and decorrelation time within reasonable limits. The addition of zero-mean noise to the 
ocean temperature tendency is found to have a nonzero effect on the mean climate. Specifically, in terms of the ocean 
temperature and salinity fields both at the surface and at depth, the noise reduces many of the biases in the 
low-resolution model and causes it to more closely resemble the high-resolution model. The variability of the strength 
of the global ocean thermohaline circulation is also improved. It is concluded that stochastic ocean perturbations can 
yield reductions in climate model error that are comparable to those obtained by refining the resolution, but without 
the increased computational cost. Therefore, stochastic parameterizations of ocean eddies have the potential to 
significantly improve climate simulations.

Best wishes,
Paul

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Dr Paul Williams - Royal Society University Research Fellow.
Department of Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, UK.
Phone: +44 (0)118 378 8424.
Web: www.met.reading.ac.uk/~williams  |  www.twitter.com/DrPaulDWilliams
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