Nature of the Differences in the Intraseasonal Variability of the Pacific and Atlantic Storm Tracks: A Diagnostic Study

On the basis of an intraseasonal variability index of storm track evaluated for 40 winters (1963–64 through 2003–04) of NCEP–NCAR reanalysis data, it is found that well-defined midwinter minimum [MWMIN; (midwinter maximum MWMAX)] occurs in 21 (8) winters over the North Pacific. In contrast, MWMIN (MWMAX) occurs in 4 (25) of the 40 winters over the North Atlantic. The power spectrum of such an index for the Pacific has a broad peak between 5 and 10 yr, whereas the spectrum of the index for the Atlantic has comparable power in two spectral bands: 2–2.8 and 3.5–8 yr. Over the North Pacific, the increase in the zonal asymmetry of the background baroclinicity as well as in the corresponding horizontal deformation of the time-mean jet from early/late winter to midwinter is distinctly larger in an MWMIN winter. Associated with these changes, there is a distinctly stronger barotropic damping rate in the January of an MWMIN winter. The increase in the net conversion rate of eddy kinetic energy from early/late winter to midwinter is much larger in an MWMAX winter than that in an MWMIN winter. Even though there is a modest increase in the barotropic damping from early/late winter to midwinter over the North Atlantic, it is overcompensated by a larger increase in the baroclinic conversion rate. That would result in MWMAX. These results are empirical evidences in support of a hypothesis that a significant enhancement of the barotropic damping relative to the baroclinic growth from early/late winter to midwinter is a major contributing factor to MWMIN of the Pacific storm track.