GCM and Observational Diagnoses of the Seasonal and Interannual Variations of the Pacific Storm Track during the Cool Season

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Abstract
Previous work has found that the Pacific storm track intensity during the cool season is negatively correlated with the upper-tropospheric jet strength. In the seasonal march, such a variation manifests itself as the midwinter suppression of the storm track intensity, while in interannual variations the storm track intensity during midwinter is found to be weaker during years in which the Pacific jet is particularly strong. In this paper, GCM simulations and observational data have been analyzed to shed light on the physical mechanisms responsible for such variations. By examining the eddy energy budget and eddy structure, two different mechanisms have been found to contribute to the reduction in storm track activity associated with increases in jet intensity and baroclinicity in midwinter. For the seasonal variations, it was found that the major difference between fall and midwinter lies in the changing role of diabatic heating. During fall and spring, diabatic heating acts to generate eddy potential energy, while during midwinter its effect is to dissipate eddy energy. In the GCM simulations, this changing role is found to be due largely to changes in contributions from condensational heating, but NCEP–NCAR reanalysis data suggest that seasonal variations in surface sensible heat fluxes may also play a role. For the interannual variations, the analyses suggest that the reduction in storm track intensity associated with increasing jet intensity during midwinter is due to changes in eddy structure. Eddies are found to be less efficient in converting mean potential energy to eddy potential energy due to being trapped closer to the tropopause and having relatively weak low-level circulations during the strong jet months. Such a change in eddy structure is also consistent with the finding that the group velocity of eddy propagation is much enhanced during those months. Both the decrease in baroclinic generation and increase in group velocity contribute to the slower downstream growth in eddy energy during the strong jet months despite higher baroclinicity. For both cases, there is no evidence that changes in barotropic dissipation contributes to the suppression of storm track activity.