Frequency distribution of tropospheric triad energy interactions during summer monsoon 1988

—In order to understand the physical mechanism for maintenance of low frequency waves, computations of nonlinear kinetic energy (KE) exchanges into individual triad interactions in frequency domain by use of cross-spectral technique over global tropics (20°S–30°N) at 850 hPa and 200 hPa for June, July and August of 1988 are carried out. The KE exchanges among different frequencies and the interactions between the transient eddies and the time mean flow are found to be an order of magnitude smaller in the lower troposphere than those in the upper troposphere. The results show two distinct spectral peaks of periods 45 and 25 days in nonlinear energy transfer in the lower troposphere. The former is more pronounced than the latter. The role of time mean flow on the low frequency transients is found to be secondary compared to the effect of the leading term due to nonlinear interactions in the lower troposphere. Low frequency waves suffer a net loss of energy in the upper troposphere. In the lower troposphere, north of 20°N low frequency waves lose energy through nonlinear triad interactions, unlike the upper troposphere where gain of energy is noticed. Longitude-frequency distributions suggest that wave-CISK process and strong gradient of SST are the possible mechanisms for the strong energy interactions associated with low frequency waves in the lower troposphere over the west Pacific and east coast of Africa, respectively. The study may aid investigation of the rapid loss of predictability of low frequency modes over the tropics.