Role of triad kinetic energy interactions for maintenance of upper tropospheric low frequency waves during summer monsoon 1988

In order to determine nonlinear energy exchanges into individual triad interactions in the frequency domain, spectral formulas are derived by use of the cross-spectral technique. First time attempt has been made to understand the problem of maintenance of low frequency waves for tropical weather system by using this technique. The TOGA basic level III daily wind analyses on a 2.5 degree square grid around the global zone from 20°S–30°N at 200 hPa for 92-day periods covering June, July and August of 1988 are used. Kinetic energy is gained at high frequencies and lost at low frequencies. In the planetary scale dynamics over tropics, barotropic nonlinear energy transfer plays a negative role. Low frequency wave of period 45 day loses maximum amount of energy when it interacted with frequencies of periods 92 day and 30 day. 45 day cycle is also the main source of energy for other frequencies. Distrubances of period 15 day gain maximum amount of energy. The major contribution comes from the triad interaction of the frequencies 〈7,1,6〉. North of 20°N low frequency waves of period 30 to 92-day gain energy through nonlinear triad interaction with the maximum gain at 22.5°N. The study may help to investigate the rapid loss of predictability of low frequency modes over tropics.