A New Paradigm for Active-Break Cycle in the Indian Summer Monsoon

A multi-channel singular spectrum analysis of 121 years of daily rainfall over India and 43 years of outgoing long wave radiation and horizontal wind has revealed two dominant intraseasonal oscillations (ISOs) that describe the space-time variability of monsoon rainfall. The two nonlinear oscillations are not perfectly periodic but exhibit broad-band spectra centered at 45 days and 28 days. These modes have coherent and near-regular spatial structure and temporal variations. This study posits that the two nonlinear oscillations provide a consistent and comprehensive framework to study the variability and predictability of intraseasonal rainfall variations. The active and break cycles of monsoon rainfall over India have been traditionally defined by using an arbitrarily chosen duration of persistence of arbitrarily chosen amount of rainfall averaged over an arbitrarily chosen area. This study shows that the phases of the two objectively derived modes of variability provide an objective method for defining the active and break cycles. The two ISOs are further shown to make negligible contribution to the seasonal mean rainfall and its interannual variability. This study proposes that the investigations of the origins of these nonlinear modes, and modulations of their amplitudes and phases, provide a new paradigm for future research on validation and predictability of intraseasonal variations in climate models.