Diagnostic and dynamical analyses of two outstanding aspects of storm tracks

Abstract

This paper reports diagnostic and dynamical analyses about two aspects of winter storm tracks in Northern Hemisphere. It is first established with NCEP/NCAR reanalysis data that the Atlantic storm track is distinctly more intense than the Pacific storm track in 30 out of 40 winters even though the Atlantic jet is considerably weaker and that the intensity of the two storm tracks are positively correlated. It is hypothesized that the continent–ocean configuration, in conjunction with the related differential friction, could be a significant contributing factor to such relative intensity. Supporting evidence for this hypothesis is presented in the context of a quasi-geostrophic two-level model analysis. We also establish that the intensity of the Pacific storm track is a minimum in mid-winter (MWMIN) in 21 of 40 winters, whereas in contrast the Atlantic storm track is most intense in mid-winter (MWMAX) in 25 of 40 winters. The structural characteristics of the composite mean flows as well as the energetics in MWMIN and MWMAX are compared. It is hypothesized that a significant enhancement of barotropic damping relative to the baroclinic growth in mid-winter is a major contributing factor to the occurrence of MWMIN. Supporting evidence for this hypothesis is presented from another idealized model analysis. Successful simulation of MWMIN can be made in a global spherical multi-level primitive equation model only if the proxy forcing is enhanced in early winter. However, the model still captures the difference between MWMIN and MWMAX even without using such enhanced forcing. The major caveat of this study is that moist dynamics has not been taken into consideration.