A Diagnostic Study of the Time-Averaged Budget of Atmospheric Zonal Momentum over North America

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  • 1 Geophysical Fluid Dynamics Program, Princeton University, NJ 08540
  • | 2 Geophysical Fluid Dynamics Laboratory/NOAA, Princeton University, NJ 08540
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Abstract

The terms in the time-mean zonal momentum equation which depend only on the large-scale motions are evaluated for the North American continent during the winter season. The computations are based on two different sets of upper air atmospheric circulation statistics between 850 and 200 mb. The first (GFDL) data set consists of global objective horizontal analyses of monthly circulation statistics evaluated at individual stations; the second (NMC) data set was compiled by processing twice-daily synoptic analyses on a hemispheric grid. The mean residual forces needed for balance are discussed assuming that they represent the effects of horizontal and vertical subgrid-scale processes.

The results derived from the two largely independent data sets are similar. Zonal momentum is produced in the free atmosphere over North America by a local, thermally direct meridional circulation, with mean poleward ageostrophic flow above 700 mb. This production is partially counterbalanced by a net export of zonal momentum from the region. The flux divergence associated with large-scale vertical transports appears to be insignificant.

A residual force of considerable magnitude is needed for balance. In the lower troposphere, this force has an accelerating effect on the zonal flow almost everywhere over the eastern portion of the North American continent, and a decelerating effect over the western mountainous region where, however, the results are less certain. In the upper troposphere, the residual force over the entire continent has a net decelerating effect, but significant geographical differences, appear to occur.

A tentative interpretation of the results for the residual force is offered.

Abstract

The terms in the time-mean zonal momentum equation which depend only on the large-scale motions are evaluated for the North American continent during the winter season. The computations are based on two different sets of upper air atmospheric circulation statistics between 850 and 200 mb. The first (GFDL) data set consists of global objective horizontal analyses of monthly circulation statistics evaluated at individual stations; the second (NMC) data set was compiled by processing twice-daily synoptic analyses on a hemispheric grid. The mean residual forces needed for balance are discussed assuming that they represent the effects of horizontal and vertical subgrid-scale processes.

The results derived from the two largely independent data sets are similar. Zonal momentum is produced in the free atmosphere over North America by a local, thermally direct meridional circulation, with mean poleward ageostrophic flow above 700 mb. This production is partially counterbalanced by a net export of zonal momentum from the region. The flux divergence associated with large-scale vertical transports appears to be insignificant.

A residual force of considerable magnitude is needed for balance. In the lower troposphere, this force has an accelerating effect on the zonal flow almost everywhere over the eastern portion of the North American continent, and a decelerating effect over the western mountainous region where, however, the results are less certain. In the upper troposphere, the residual force over the entire continent has a net decelerating effect, but significant geographical differences, appear to occur.

A tentative interpretation of the results for the residual force is offered.

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