The Climatology of Blocking Events in a Perpetual January Simulation of a Spectral General Circulation Model

Maurice L. Blackmon National Center for Atmospheric Research, Boulder, CO 80307

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Steven L. Mullen National Center for Atmospheric Research, Boulder, CO 80307

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Gary T. Bates National Center for Atmospheric Research, Boulder, CO 80307

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Abstract

A variety of statistical comparisons is made between the fluctuations occurring in a 1200-day perpetual January simulation of a spectral general circulation model and those occurring in a 20-winter data set. Attention is focused on the persistent anomalies with lifetimes greater than one week. Over the Atlantic and Pacific oceans, we show that there is generally good agreement between modeled and observed persistent anomalies in their frequency of occurrence and mean lifetime. However, we note a striking deficiency in the simulation of persistent anomalies over the Soviet Union. We focus further on a subset of persistent anomalies blocking highs, and show that the model blocks have vertical structure in agreement with observations. We also show an example of the development of a blocking event which follows an episode of explosive cyclogenesis. Finally, we show an example of the interaction of low-and high-frequency eddies during a period of blocking and at the termination of model blocking.

The results of this study demonstrate that the internal dynamics and physics of the model by themselves are able to generate quite realistic blocking episodes over the wintertime North Atlantic and Pacific oceans. We conclude that blocking is a naturally occurring, internally generated phenomenon of the model and believe that model data can be used as a reasonable proxy for observations to study this phenomenon.

Abstract

A variety of statistical comparisons is made between the fluctuations occurring in a 1200-day perpetual January simulation of a spectral general circulation model and those occurring in a 20-winter data set. Attention is focused on the persistent anomalies with lifetimes greater than one week. Over the Atlantic and Pacific oceans, we show that there is generally good agreement between modeled and observed persistent anomalies in their frequency of occurrence and mean lifetime. However, we note a striking deficiency in the simulation of persistent anomalies over the Soviet Union. We focus further on a subset of persistent anomalies blocking highs, and show that the model blocks have vertical structure in agreement with observations. We also show an example of the development of a blocking event which follows an episode of explosive cyclogenesis. Finally, we show an example of the interaction of low-and high-frequency eddies during a period of blocking and at the termination of model blocking.

The results of this study demonstrate that the internal dynamics and physics of the model by themselves are able to generate quite realistic blocking episodes over the wintertime North Atlantic and Pacific oceans. We conclude that blocking is a naturally occurring, internally generated phenomenon of the model and believe that model data can be used as a reasonable proxy for observations to study this phenomenon.

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