NORTHWESTWARD-PROPAGATING WAVE PATTERNS OVER THE TROPICAL WESTERN NORTH PACIFIC DURING SUMMER

The possible relationship between northwestward-propagating wave distu rbances and tropical cyclones over the tropical western North Pacific during summer is studied using data assimilated by the navy's global m odel during May-September 1989-91. A multiple-set canonical correlatio n (MCC) analysis is applied to the 850-hPa meridional (upsilon) compon ent over a core domain covering the western Pacific. The analysis seek s the maximal geometrically averaged correlation between 12 consecutiv e twice-daily fields. Two MCC components, with a 90 degrees phase diff erence and comparable variances that combine to nearly one-third of th e total variance, describe the northwestward-propagating pattern with a period near 8-9 days. Upstream of this steady northwestward-propagat ing pattern there is a weaker, westward propagation along 5 degrees N that may be traced back to 170 degrees E. The surface pressure cell ad vancing east of the Philippines is consistent with low-level winds for a circulation in gradient wind balance. It has a zonal wavelength nea r 28 degrees longitude, a northeast-southwest meridional tilt, a sligh tly forward tilt from 850 to 300 hPa, and a phase reversal above 200 h Pa. The warm core extends from 925 to 200 hPa over the surface low wit h maximum at 200 hPa. Although there is a positive correlation, the lo w-level moisture structure is different from the surface pressure and upsilon(850). A poleward moisture flux is clearly seen around the lead ing cell, but in the adjacent cell (with opposite polarity) to the sou theast, moisture is nearly out of phase with pressure. This asymmetric moisture distribution is similar to that normally found in a tropical cyclone and its associated anticyclone where widespread subsidence do minates. Both the structure and a comparison of named storm center loc ations against the various phases of the MCC modes suggest that the di sturbance cyclonic cells during periods of high wave amplitudes are as sociated with tropical cyclone occurrences. During such periods either the wave disturbances modulate the sensitivity of the tropical atmosp here to the various physical mechanisms associated with tropical cyclo ne occurrences, or the presence of tropical cyclones modulate the ampl itude of the wave disturbances.