A 15-year climatology of synoptical disturbance over tropical northwestern Pacific during summer

Abstract

This work investigated the sumertime northwestward propagating waves over the tropical western North Pacific and their relationship with tropical cyclone activities. The 15-season (May-October 1974-1988) Navy tropical global band data were separated into three groups according to the degree of organization in the multiple-set canonical correlation (MCC) modes that were computed from the surface v wind. Groups A (6 seasons) and B (5 seasons) showed orderly large-scale propagating patterns at the surface and 700 hPa with an average period of 8 days and a wavelength of about 2500-3000 km. The structure is robust whether the MCC analysis was performed on each of the seasons individually or on groups of years. A strong in-phase relationship between the low-level disturbance cyclonic cells and tropical cyclone centers was found. Composite analysis of all cases in these two groups whose MCC mode 1 amplitude exceeded 1.5 m/s showed organized northwestward propagating divergence/convergence patterns at 200 hPa. Group C (4 seasons) shows the least organized structure, with a large difference between MCC modes obtained from the entire group and those from each individual season. The possibility that group C contains irregular tropical cyclone tracks was discussed.