Nature of the differences in the intraseasonal variability of the Pacific and Atlantic storm tracks: A diagnostic study

This study quantifies the extent and nature of the differences in the intraseasonal variability of the N. Atlantic and N. Pacific storm tracks in winter. This is done with the use of forty winters of NCEP/NCAR Reanalysis data (1963 to 2003). An index measuring the change in the intensity of a storm track from early/late winter to mid-winter is introduced. A positive (negative) value indicates maximum (minimum) intensity of the storm track in mid-winter, referred to as MWMAX (MWMIN). Definite MWMIN (MWMAX) occurs in 21 (8) of the 40 winters over N. Pacific. In contrast, there are 4 (25) MWMIN (MWMAX) in the 40 winters over N. Atlantic. The time series of the index over Pacific has most power between 5 and 10 years, whereas that over Atlantic has most power between 2 and 6 years.

Composite maps of the background baroclinicity, horizontal deformation and related energy conversion rates are constructed using the data of five pronounced MWMIN winters and five MWMAX winters over each oceanic region. Over Pacific, the increase in the gradient of baroclinicity in the streamwise direction as well as the increase in the horizontal deformation of the time mean jet from early/late winter to mid-winter are distinctly larger in a MWMIN winter than in a MWMAX winter. Associated with these changes, there is a stronger barotropic damping in the January of a MWMIN winter. It greatly reduces the difference of the net conversion rate between mid-winter and early/late winter. On the other hand, the values of net conversion rate in a MWMAX winter over Pacific and Atlantic are quite similar. Even though the barotropic damping increases substantially from early/late winter to mid-winter over Atlantic, it is over-compensated by a larger increase in the baroclinic conversion rate, resulting in MWMAX. These energetic results support the hypothesis that a significant enhancement of barotropic damping relative to the baroclinic growth is a major factor contributing to mid-winter minimum of the Pacific storm track.