On the Processes of Transitions between Circulation Regimes during Winter over the Pacific and North America

Abstract

A statistical investigation is made into the characteristics of different circulation regimes during winter over the Pacific and North America and processes of transitions between these regimes.
Using 5-day mean 500mb height data during the winters of 40 years (1946-1985), a principal component analysis was performed. Taking account of component stores, persistent circulation regimes of four types (P1+, P1-, P2+ and P2-) were derived. Investigation reveals that P1+ and P1-regimes have no relation to the Southern Oscilation Index (SOI) and are formed and maintained by the stationary wave activity sources along the jet in mid-latitudes. On the other hand, P2+ and P2 regimes have a close relation to the SOI and are consistent with the stationary wave propagation from low latitudes over the western Pacific to North America.
On the basis of a composite analysis, it is shown that wave trains appear from over the Atlantic to Eurasia, preceeding the occurrences of circulation regimes. However, features of the wave train preceeding the occurrences of P1+ and P1- are different from those preceeding P2+ and P2-. In addition, two wave train indices (la and lb) are proposed as measures for detecting transitions between these regimes.