Transient Eddy Forcing of the Rotational Flow during Northern Winter

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

The total direct transient eddy forcing of the vorticity balance in the upper troposphere during northern winter is examined using 11 years of 2 to 8-day bandpassed global analyses. Most examinations of the importance of high-frequency eddy vorticity fluxes to the maintenance of either the climatological standing waves or low-frequency anomalous flows have focused on only the horizontal blow or the rotational component of the horizontal transient flow. The latter simplification has been shown to be questionable for planetary scales. The vorticity flux by the transient divergent flow produces a forcing of the mean streamfunction that is of comparable magnitude with the equivalent rotational term. However, the streamfunction forcing by the sum of the transient vertical advection and twisting terms largely balances the fearing by the vorticity flux convergence associated with the divergent flow. The result is that the convergence of the eddy vorticity flux by the total transient flow is not a good approximation to the total forcing of the long-term mean streamfunction by the high-frequency eddies. These results are quantified for the mean northern winter season November–March 1980/81–1990/91.

The respective roles of each transient eddy term in the vorticity equation in maintaining two large-scale, low-frequency anomalous flows are also examined. One case involves a pronounced circulation anomaly that persisted for more than a decade over the North Pacific, and the second case relates to the maintenance of extremes of the North Atlantic Oscillation. In both cases, transient vorticity fluxes systematically reinforce and help to maintain the upper-tropospheric streamfunction anomalies. Just as for the climatological standing waves, a consideration of the total transient eddy forcing of the mean anomalous streamfunction yields a different interpretation than if only the convergence of the vorticity flux is considered.

Abstract

The total direct transient eddy forcing of the vorticity balance in the upper troposphere during northern winter is examined using 11 years of 2 to 8-day bandpassed global analyses. Most examinations of the importance of high-frequency eddy vorticity fluxes to the maintenance of either the climatological standing waves or low-frequency anomalous flows have focused on only the horizontal blow or the rotational component of the horizontal transient flow. The latter simplification has been shown to be questionable for planetary scales. The vorticity flux by the transient divergent flow produces a forcing of the mean streamfunction that is of comparable magnitude with the equivalent rotational term. However, the streamfunction forcing by the sum of the transient vertical advection and twisting terms largely balances the fearing by the vorticity flux convergence associated with the divergent flow. The result is that the convergence of the eddy vorticity flux by the total transient flow is not a good approximation to the total forcing of the long-term mean streamfunction by the high-frequency eddies. These results are quantified for the mean northern winter season November–March 1980/81–1990/91.

The respective roles of each transient eddy term in the vorticity equation in maintaining two large-scale, low-frequency anomalous flows are also examined. One case involves a pronounced circulation anomaly that persisted for more than a decade over the North Pacific, and the second case relates to the maintenance of extremes of the North Atlantic Oscillation. In both cases, transient vorticity fluxes systematically reinforce and help to maintain the upper-tropospheric streamfunction anomalies. Just as for the climatological standing waves, a consideration of the total transient eddy forcing of the mean anomalous streamfunction yields a different interpretation than if only the convergence of the vorticity flux is considered.

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