An Observational Study of Easterly Waves over the Eastern Pacific in the Northern Summer Using FGGE Data

Abstract

FGGE revel III-b data provided by the European Centre for Medium-Range Weather Forecasts and the outgoing longwave radiation data measured by satellites are used to investigate observationally relationships between deep cloud activity and large-scale meteorological fields during the Northern Hemisphere summer (May?September) of 1979 over the eastern Pacific. Aside from the summer monsoon area over southern and eastern Asia, the eastern Pacific is the area where strong, deep convection frequently develops in the northern summer. This is also one of the tropical ocean areas that have been least explored meteorologically. A power spectral analysis using the Maximum Entropy Method is made for the meridional wind component at 850 mb at various grid points in the analysis domain for each mouth from May through September. The waves with a period of 4?6 days are not only stronger in July and August than those of other months, but active in the regions of 100°?130°W in the eastern Pacific and 130°?160°E in the western Pacific within the zone of 5°?15°N. These waves possess a wavelength of 3000?3500 km and travel westward with a speed of 5?7 m s?1. Deep convection is found to occur at or slightly behind the wave trough axis. The structure of easterly waves obtained by a composite technique is similar to those of African waves observed in GATE Phase III and easterly waves in the western Pacific determined by Reed and Recker. These similarities include the amplitude of the wave-related meridional wind (4 m s?1) and a cold core of temperature anomaly pattern at low levels. The manner in which the position of the surface confluence line shifts latitudinally with the passage of waves is remarkably similar to that found in GATE Phase III by Chen and Ogura. However, the structure at the upper levels does not show a distinct secondary maxima of the wave-related perturbations, which is a significant feature in the easterly waves observed in other areas. The positions of both the axis of the strongest deep cloud activity, inferred from the satellite IR data, and the surface wind confluence line exhibit seasonal variation in the eastern Pacific. Nonetheless, they are closely collocated. Further, they are located over the area of maximum sea surface temperature (SST), suggesting a close relationship among SST, the ITCZ, and the confluence of the surface wind. At 850 and 700 mb, most of the tropical Pacific is dominated by easterly flow. However, both latitudinal and vertical shears of the easterly flow are much weaker than those observed over the eastern Atlantic and western Africa, suggesting that dry barotropic instability alone cannot account for the formation of easterly waves in the eastern Pacific.