Why are the Indian monsoon transients short-lived and less intensified during droughts vis-a-vis good monsoon years? An inspection through scale interactive energy exchanges in frequency domain

摘要

The interannual variability of Indian summer monsoon rainfall (ISMR) is largely dictated by the continental tropical convergence zone (CTCZ), characterized by the frequent cyclogenetic tendency associated with the transitory low pressure systems (LPS(s) or monsoon transients) such as lows, depressions, cyclonic storms etc. A good monsoon is characterized by more intense and sustainable transients having longer westward propagation in comparison with the LPS(s) during weak monsoon. Now the questions arise from the energetics aspects-how does the energy favour the CTCZ to be more cyclogenetic during the good monsoon years? From where the monsoon transients are gaining energy to intensify and to sustain for longer durations during good monsoons compared with those in weak monsoons? The non-linear scale interactions may possibly be the dynamical reason for the intensification and maintenance of LPS(s), but, the quantification of non-linear interactions among the seasonal mean, low frequency oscillations, and synoptic scales are not revealed so far in previous studies. First time, the scale interactive energy exchanges in frequency domain have been explored to unravel the causes of weak LPS(s) during droughts vis-a-vis good monsoon years. It may be inferred from this study that the seasonal mean may play a crucial role for the generation of low and high frequency oscillations through mean-wave interaction whereas the wave-wave interactions have a significant contribution in intensification as well as the maintenance of monsoon transients. The dominant transfer of kinetic energy from the low frequency oscillations to synoptic scale observed over the large region of CTCZ favours the synoptic systems to intensify, survive for longer duration over that region before dissipation causing CTCZ to be more cyclogenetic during the good monsoon period with respect to those in weak monsoons attributing to the rainfall variability over the Indian landmass in contrasting monsoon seasons.