LOW-FREQUENCY MODES OF THE TROPICAL RADIATION BUDGET

Low-frequency modes of the monsoon are examined in the context of their radiation balance and diagnosed for the 13-month period between May 1979 and May 1980 using Earth radiation budget and cloud measurements taken by experiments carried on board the Nimbus-7 satellite. Simultaneous observations of the albedo, longwave radiation, absorbed shortwave radiation, and net radiation at the top of the atmosphere (TOA), and the total cloud fraction and cloud-top temperature are considered. The use of broad-band radiation budget measurements permits a description of the observed longwave, shortwave, and net radiative energy exchange by the low-frequency modes. When wavenumber one fields are considered, the entire morphology of the 1979 summer monsoon (pre-onset, onset, break, re-intensification, and withdrawal) can be fully explained in terms of an eastward propagating mode. Ridge passages occurred over the Arabian Sea and India in June prior to onset, during the July break, and during the retreat of the monsoon. Trough passages occurred prior to the onset during a period of increased tropical cyclone activity, at the time of the onset, and immediately following the break. These low-frequency waves can be unambiguously tracked around the world over extended time periods. The latitudinal structure of the waves indicated that a thermally direct Hadley Cell perturbation propagated eastward with the oscillation. These cells were evident from extratropical extensions of the oscillation, each about 180-degrees of longitude out-of-phase with the tropical oscillation. Because the absorbed shortwave and emitted longwave radiative components are in phase and of nearly identical amplitudes, the net radiative effect of the low-frequency mode is small in general. However, in certain latitudinal belts, the passage of the waves induced perturbations in the net radiation. Because longwave cloud-radiative forcing acts in the same direction as latent heat release, it is able to contribute to the diabatic energetics maintaining the structure and propagation of the eastward propagating 30- to 60-day waves. Between trough and ridge, the TOA longwave flux varies in a coherent manner by on the order of 50 to 60 Wm-2.