18th Conference on Climate Variability and Change


Transient response of an atmospheric general circulation model to sea ice and SST anomalies in the North Atlantic


Clara Deser, NCAR, Boulder, CO; and R. Tomas

The transient atmospheric response to realistic patterns of anomalous sea ice cover and sea surface temperature (SST) over the Arctic/North Atlantic is investigated by means of a large ensemble of experiments with an atmospheric general circulation model, the NCAR Community Climate Model version 3 (CCM3). Each ensemble member consists of a pair of CCM3 integrations,one with climatological sea ice and SST distributions and one with perturbed ice and SST starting from the same initial conditions (different initial conditions are used for each ensemble member). The transient response is investigated by examining the ensemble mean response for each day of the 152-day integration that begins on December 1 and ends on April 30. The intial response, which develops within the first week, exhibits a baroclinic vertical structure localized to the vicinity of the anomalous forcing. Subsequently, the response develops a hemispheric equivalent-barotropic structure that resembles the northern annular mode which is the leading structure of internal variability in the model. This equilibrium response is considerably stronger than the initial response. The initial response is forced directly by anomalous heating, while the equilibrium response, which takes several weeks to develop, is maintained by transient eddy heat and momentum flux convergences. Despite differences in their spatial pattern and strength, the sea ice and SST anomalies elicit similar equilibrium atmospheric circulation responses although their initial heating-forced responses are distinctive.

Poster Session 3, Climate Modeling and Diagnostic Studies
Thursday, 2 February 2006, 9:45 AM-11:00 AM, Exhibit Hall A2

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