Nakamura (1992) showed that the storm track activity over the Pacific was maximized in November and March/April, while there is a relative minimum in January, when the jet intensity and local baroclinicity are maximized. Such variations have also been found in GCM simulations using the ECHAM3 GCM (Christoph et al. 1997) as well as the GFDL GCM (Zhang and Held 1998). While several mechanisms -- including suppression due to excessively strong advection in January, changes in 'seeding', effects of moisture, etc. -- have been suggested to explain this mid- winter suppression, no conclusive explanation has yet been found. In this study, experiments were performed using the GFDL climate model as well as idealized model to gain further understanding of this phenomenon.
The energy and wave activity budget as simulated by the GFDL GCM has been analyzed. The results suggest that the minimum in January as simulated by the GCM is neither due to a minimum in upstream wave source, nor to a strong divergence of energy or wave activity fluxes due to the intense jet in mid-winter. Instead, the diagnoses suggest that in the fall, diabatic heating acts to generate eddy potential energy, while in mid-winter its effect is dissipative. Hence the mid-winter suppression appears to be due to differences in the moist dynamics in fall/spring and winter. Analyses of the individual components of diabatic heating suggest that the difference is due to the larger supply of moisture in fall/spring compared to mid-winter due to the temperature difference, such that heating due to condensation per unit temperature perturbation (and vertical velocity) is significantly stronger in fall/spring than in mid-winter, and this leads to stronger generation of eddy available potential energy in fall/spring which is sufficient to account for the faster growth of energy in fall/spring than in mid-winter over the entrance of the Pacific storm track region. Experiments were conductied with an idealized nonlinear dry model, with the basic state forced to those resembling October and January basic states. Experiments including a simple parameterization of large scale condensation were able to reproduce the mid-winter suppression, while those excluding the effects of moist heating found stronger Pacific storm tracks for the January basic state. These results from the GCM as well as idealized experiments suggest that interactions of large scale condensation heating with dynamics cannot be ignored in our quest to understand observed eddy statistics.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics