Abstract
The global characteristic features of the lower tropospheric disturbances in the tropics are examined using the FGGE level III-b wind data during the northern summer of 1979. The activity of the disturbance with periods shorter than 10 days is greater in the northern equatorial region than in the southern equatorial region. Two spectral peaks at periods of 3-4 days and about 6 days both corresponding to westward moving disturbances with the zonal wavenumbers of 10-12 are pronounced in the zonal means of the time spectra in the northern equatorial region. No dominant spectral peak exists in the southern equatoriai region.
The results of the power spectral analysis at individual grid points reveal large regional difference of the disturbance characteristics showing that the lower tropospheric disturbances. in the tropics are largely affected by the regional conditions.
The 3-4 day period disturbances which correspond to the African wave predominate in a wide area from the central Africa to the central Atlantic and from the equator to about 25°N. The African wave disturbances have maximum amplitudes near the west coast of Africa and decrease their amplitudes in the eastern Atlantic. In the western Atlantic and the Caribbean Sea disturbances with a longer period (-6 days) become dominant.
In the tropical Pacific the largest amplitudes of lower tropospheric disturbances are found from the western Pacific to the East China Sea. The dominant spectral peaks in a period range of 6-10 days exist in a region extending northwestward from the equatorial western Pacific to the East China Sea and this region generally corresponds to typhoon tracks. It is interesting to note that the period of the spectral peak gradually shifts from about 6 days in the equatorial western Pacific to about 10 days in the East China Sea along these tracks. A pronounced spectral peak of about 6 days is found in the eastern Pacific off the coast of Central America. Although the disturbance activity is quite low in the central Pacific as compared with those in the eastern and western Pacific, there exist weak spectral peaks in a 3-4 day period range along the ITCZ in the central Pacific.
Disturbances with large power spectra of the 6-7 day period are found in the Southeast Asia from Indochina to North India and their intensity becomes maximum over the northern Bay of Bengal. These disturbances may correspond to the monsoon disturbances propagating along the monsoon trough region and intensified over the northern Bay of Bengal.
Variations with longer periods (>13.3 days) predominate in the Asian monsoon region extending from the Indian Ocean through Southeast Asia to western Pacific. These variations are mostly due to the 30-50day period oscillations of the Asian monsoon activity which have been recently discovered. Similar long period variations, but with a 13.3 day period, are found in the North America-Atlantic monsoon region.
The results of the spectral analysis of winds are compared with these of the outgoing longwave radiation (OLR) data. The spatial distributions of the time variations of OLR are quite similar to those of winds indicating that the convective activity is strongly coupled with the large-scale lower tropospheric disturbances in the tropics.
