Abstract
Spectrum analysis is applied to satellite observed cloud data over the tropical region of the northern Pacific during the period from October, 1968, through September, 1969. Power spectra of the zonally averaged cloud amount between 160E and 120W are first computed along every 2.5 degree latitude band from 30N to the equator. They reveal three prononuced peaks in the period ranges of 16.7-25.0 days, 7.1-10.0 days, and 3.3-5.0 days. The shorter period fluctuation (3.3-5.0 days), which appears to be a feature of the low latitudes (5-10N) only, is less significant than the others with longer time periods. The pronounced spectral peaks at 7.1-10.0 day period are encountered in low latitudes near 5 to 10N. The spectra with a period range of 16.7-25.0 days reveal a significant feature of two maxima in the substropics near 20N and the tropics near 5-10N, separated by a minimum near 15N.
Spectra at individual localities are computed at every 2.5 degrees latitude and longitude square mesh. They also exhibit three pronounced peaks at the period ranges of 16.7-25.0 days, 7.1-10.0 days, and 3.3-5.0 days, respectively. From the analysis of the coherence and phase difference with respect to several reference points, one can estimate the wavelength of disturbances associated with major spectral peaks. The wavelength estimated for disturbances with the 16.7-day period is about 7, 000km (10, 000km) at tropical latitudes near 7.5-10N, and about 15, 000km (13, 000km) at subtropical latitudes near 22.5-25N during the winter (summer) months. The wavelength for the 10-day period disturbances is measured only at 7.5-10N, as the fluctuation with this period is not primarily important in more northerly latitudes. It is 7, 000km for the winter months, and 12, 000km for the summer months. As for the 4.5-5.0-day disturbances, the estimated wavelength is about 4, 000km at 7.5-10N during the winter months, while it is unreliable for the summer months because of somewhat poor coherence.
