Northwestward-Propagating Wave Patterns over the Tropical Western North Pacific during Summer

Abstract

The possible relationship between northwestward-propagating wave disturbances and tropical cyclones over the tropical western North Pacific during summer is studied using data assimilated by the navy's global model during May?September 1989?91. A multiple-set canonical correlation (MCC) analysis is applied to the 850-hPa meridional (v) component over a core domain covering the western Pacific. The analysis seeks the maximal geometrically averaged correlation between 12 consecutive twice-daily fields. Two MCC components, with a 90° phase difference and comparable variances that combine to nearly one-third of the total variance, describe the northwestward-propagating pattern with a period near 8?9 days. Upstream of this steady northwestward-propagating pattern there is a weaker, westward propagation along 5°N that may be traced back to 170°E. The surface pressure cell advancing east of the Philippines is consistent with low-level winds for a circulation in gradient wind balance. It has a zonal wavelength near 28° longitude, a northeast?southwest meridional tilt, a slightly forward tilt from 850 to 300 hPa, and a phase reversal above 200 hPa. The warm core extends from 925 to 200 hPa over the surface low with maximum at 200 hPa. Although there is a positive correlation, the low-level moisture structure is different from the surface pressure and v850. A poleward moisture flux is clearly seen around the leading cell, but in the adjacent cell (with opposite polarity) to the southeast, 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 dominates. Both the structure and a comparison of named storm center locations against the various phases of the MCC modes suggest that the disturbance cyclonic cells during periods of high wave amplitudes are associated with tropical cyclone occurrences. During such periods either the wave disturbances modulate the sensitivity of the tropical atmosphere to the various physical mechanisms associated with tropical cyclone occurrences, or the presence of tropical cyclones modulate the amplitude of the wave disturbances.