Global Distribution of Vertically-Averaged Meridional Momentum Transport Statistics for January: A Comparison between Observations and General Circulation Model Simulations

David D. Houghton Department of Meteorology. University of Wisconsin, Madison 53706

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Robert M. Chervin National Center for Atmospheric Research, Boulder. CO 80307

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Abstract

Vertically-averaged meridional transports of westerly momentum are analyzed in sampled ensembles of January simulations of an NCAR GCM and an equivalent ensemble of five years of observational January data according to a simple time-domain decomposition. Ensemble averages and standard deviations are compared in terms of both zonally-averaged and grid-point presentations for the steady and transient flux components highlighting the relative characteristics of the fundamental time-domain elements. Results from 5 and 2.5° horizontal resolution versions of the model demonstrate the impact of truncation error on model simulations of these flux statistics.

Comparing grid point measures constitutes a more stringent model performance evaluation since regional differences between observed and simulated transports often are found to he considerably larger than zonally-averaged differences. Such regional considerations also reveal substantial differences between model and observations in the location and orientation of transport maxima and minima. Typically the transient flux component is smaller in the model simulations than in the observations although there are some regional exceptions. The steady flux component, however, is generally larger in the model simulations (particularly the 2.5° version) than in the observations and is affected more than the transient component by resolution changes. Analysis of the estimated standard deviations of the flux components shows that the model's inherent variability is typically at least a factor of two lower than the observed interannual variability with substantial regional differences.

Abstract

Vertically-averaged meridional transports of westerly momentum are analyzed in sampled ensembles of January simulations of an NCAR GCM and an equivalent ensemble of five years of observational January data according to a simple time-domain decomposition. Ensemble averages and standard deviations are compared in terms of both zonally-averaged and grid-point presentations for the steady and transient flux components highlighting the relative characteristics of the fundamental time-domain elements. Results from 5 and 2.5° horizontal resolution versions of the model demonstrate the impact of truncation error on model simulations of these flux statistics.

Comparing grid point measures constitutes a more stringent model performance evaluation since regional differences between observed and simulated transports often are found to he considerably larger than zonally-averaged differences. Such regional considerations also reveal substantial differences between model and observations in the location and orientation of transport maxima and minima. Typically the transient flux component is smaller in the model simulations than in the observations although there are some regional exceptions. The steady flux component, however, is generally larger in the model simulations (particularly the 2.5° version) than in the observations and is affected more than the transient component by resolution changes. Analysis of the estimated standard deviations of the flux components shows that the model's inherent variability is typically at least a factor of two lower than the observed interannual variability with substantial regional differences.

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