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The North Atlantic Oscillation in General Circulation Models

PI: Kenneth R. Sperber
Institution: Lawrence Livermore National Laboratory / PCMDI
Additional Investigators: Peter J. Gleckler, James S. Boyle
The simulation of the North Atlantic Oscillation (NAO) will be evaluated in control and climate change simulations in anticipation of the IPCC Fourth Assessment Report. The observed NAO is defined using monthly sea-level pressure anomalies over the Atlantic sector from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis. Sea-level pressure from the models is projected onto the observed modes to ensure that a standardized analysis is performed by comparing to a common metric. The principal components obtained from each of the models will be regressed against other variables, including but not limited to, sea-level pressure, surface air temperature, sea-surface temperature, and precipitation to evaluate the space-time variability of the NAO in the models. Additionally, the NAO influence on the extratropical storm tracks will be investigated. The available climate change scenario runs will be analyzed to assess anthropogenically forced perturbations to the NAO in space and time.

From a previous analysis of CMIP2+ control integrations we found that the North Atlantic Oscillation was well represented (AchutaRao et al. 2004, An Appraisal of Coupled Climate Models, PCMDI Technical Report, UCRL-TR-202550). The spatial pattern of the large-scale surface air temperature response to the sea-level pressure perturbation over the Atlantic is akin to that observed, with the spatial error indicating the model response is not as strong as observed. This shortcoming was systematic across the models analyzed, and it was suggested that improvement in the extratropical flow will improve this aspect of the NAO response. Our previous analysis of the CMIP2+ models provides a benchmark against which the new IPCC runs can be compared to determine whether the simulation of the NAO has improved during the course of model development.

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