Analysis of General Circulation Model Sea-Surface Temperature Anomaly Simulations Using a Linear Model. Part I: Forced Solutions

Grant Branstator National Center for Atmospheric Research, Boulder, CO 80307

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Abstract

Experiments are presented which indicate that many features of the response of a general circulation model to sea-surface temperature anomalies in the equatorial Pacific east of the dateline can be reproduced with a linear nondivergent barotropic vorticity-conserving model. The midlatitude response to anomalous forcing is especially well reproduced by the simple model if it is linearized about the general circulation model’s wavy control climatology. Diagnosis of the linear solutions using kinetic energy and enstrophy budget, as well as indicators of group velocity, indicates that basic state–perturbation interaction supplies nearly as much energy to the perturbation flow as anomalous forcing does.

Further experiments show that the linear model is incapable of reproducing the finding of Geisler et al. that the structure of the general circulation model's midlatitude response is insensitive to the longitudinal position of the forcing anomaly. However, a Green’s function analysis of the linear model points out that the midlatitude pattern which dominates the general circulation model experiments is very easily forced by anomalies over the East Indies. Thus it may be that anomalous precipitation in that region, caused by a weakening of the Walker circulation, is the primary impetus for the midlatitude flow anomalies.

Abstract

Experiments are presented which indicate that many features of the response of a general circulation model to sea-surface temperature anomalies in the equatorial Pacific east of the dateline can be reproduced with a linear nondivergent barotropic vorticity-conserving model. The midlatitude response to anomalous forcing is especially well reproduced by the simple model if it is linearized about the general circulation model’s wavy control climatology. Diagnosis of the linear solutions using kinetic energy and enstrophy budget, as well as indicators of group velocity, indicates that basic state–perturbation interaction supplies nearly as much energy to the perturbation flow as anomalous forcing does.

Further experiments show that the linear model is incapable of reproducing the finding of Geisler et al. that the structure of the general circulation model's midlatitude response is insensitive to the longitudinal position of the forcing anomaly. However, a Green’s function analysis of the linear model points out that the midlatitude pattern which dominates the general circulation model experiments is very easily forced by anomalies over the East Indies. Thus it may be that anomalous precipitation in that region, caused by a weakening of the Walker circulation, is the primary impetus for the midlatitude flow anomalies.

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