Statistical Aspects of Lower Atmospheric Disturbances Delineated from Conventional and Satellite Data Over the Tropical Pacific

A statistical study was made on the disturbances in the mean lower atmosphere over the tropical Pacific which appears as a well-defined integrated cloud distribution on the photo-climatological cloud picture. In the northern hemisphere summer, the photo-climatological cloud picture shows two well-defined cloud bands separated by an equatorial cloudless region extending from west to southeast. The monthly surface pressure and wind fields indicate that the northern one of these cloud bands is associated with the Intertropical Convergence Zone (ITCZ) while the southern one is associated with the South Pacific polar trough. These two cloud bands are characterized by marked cyclonic vorticity and convergence, low steadiness of the winds, and high variances of the pressure as well as of the zonal and meridional wind components about their means. The equatorial region appears to be associated with the equatorial trough in the western Pacific and the equatorial ridge in the eastern Pacific. This region is characterized by large negative vorticity and divergence, relatively high steadiness of the winds and relatively low variances of the pressure and winds. The frequenency-spectrum analysis of the local variations in the mean fields over the Pacific indictes that a 4-5 day period is predominant north of the ITCZ and in the South Pacific trough. In the equatorial cludless region extending from west to southeast a 4-5 day period is predominant in the western Pacific while an 8-10 day period is predominant in the eastern Pacific. The space-time spectrum analysis made along latitude circles over the Pacific indicates that near 10°N, in the region of the ITCZ, westward moving easterly waves with wavelengths of 2,500 km and periods of 4-5 days are observed but they are much more predominant at 20°N. Waves of 5,000 km wavelength moving toward the east with a 6-day period are observed at 30°N, which are identified with the. North Pacific subtropical highs moving toward the east with a 5,000 km * This research was partly supported by Japan-U . S Cooperative Science Program under JSPS Grant 4R004 and also by NOAA under NESC Grant E-189-68 (G). 74 T. Izawa Vol. XXIII No. 2 spacing. At 20°S are observed waves of 5,000 km wavelength moving toward the east with a 4-5 day period and with 10 day periods against the basic easterly currents. These may be identified as the South Pacific subtropical highs moving toward the east with a 5,000 km spacing. In the equatorial region between 10° S and 10°N waves with a 6,000 km wavelength are observed moving toward the west with 6 day and 10 day periods. These waves are identified as the equatorial anticycolones moving toward the west with a 6,000 km spacing. Finally, it is observed that the southward momentum transport by the disturbances takes place north of the ITCZ, and the northward. transport takes place south of the ITCZ, thus suggesting the convergence of the easterly momentum by the disturbances along the ITCZ. The wavenumber-frequency co-spectrum analysis shows that at 10°N the northward and southward transports of the easterly momentum take place due to the waves moving toward the west with a 4,000 km wavelength -5.5 day period and with a 2,500 km wavelength —5.5 day period , respectively, the latter corresponding to the easterly waves. The southward transport of the easterly momentum by the easterly waves is most predominant at 20°N.