Divergent-rotational Nonlinear Energy Conversions in Wavenumber-frequency Domain During Summer Monsoon

This work deals with computational modelling designed to understand the dynamical mechanism of low frequency monsoonal transients that results from nonlinear divergent–rotational (x–c) kinetic energy (KE) conversions due to the effects of Coriolis force, vorticity and divergence during the summer monsoon 1988 over the latitudinal belt 20°S–30°N at 850 hPa and 200 hPa. The results show two distinct spectral peaks spanning 30–45 days and 18–25 days in the energy conversions from the transient divergent motions to rotational motions. Due to the latitudinal variation of the earth’s rotational effects, the conversion from the transient divergent to rotational motions, associated mainly with wavenumbers 1 and 2, tend to be more pronounced to the north of 15°N on the 30–45-day and 18–25-day time scales in the upper and lower tropospheres, respectively. The contribution of the stationary waves to maintenance of the low frequency rotational flow due to the effect of divergence through barotropic instability is significant at the upper troposphere. Divergent to rotational KE conversion by wave–wave interaction due to divergence is identified as an important mechanism for maintenance of low frequency oscillations in the lower troposphere. The upper tropospheric planetary scale divergent motions associated with 30–45-day oscillation gain substantial energy through nonlinear x–c interaction due to vorticity.