| Details: |
The Indian summer monsoon (ISM) shows quasi-rhythmic intraseasonal oscillations (ISO) manifested as alternate ‘active’ phases of copious rainfall and quiescent phases of ‘break’. ISO is marked by northward propagation of convection from the equatorial region to the foothills of the Himalayas in every 30–60 days time period during May—October. It is shown that ISO plays a significant role in modulating not only the seasonal cycle of rainfall (onset/demise of the ISM), but also the genesis and structure of synoptic scale low pressure systems. It is also found that active and break phases are related to each other and a strong break condition over central India leads to a stronger and slower northward propagation of convection from the equatorial region (strong active).
Within these quasi-periodic ISO phases, the daily rainfall shows large variability and exhibits spatiotemporally sporadic extreme rainfall events. The recent decades have witnessed a significant increase in the number of these extreme rainfall events, especially in the quiescent phases. This increase is accompanied by a decreasing trend in the mean monsoon rainfall and a weakening variance of its ISO cycle. There is a need to understand the physical link between this apparent paradox of increased extreme rainfall events and weakened ISO. Using observations and numerical model simulations, we have shown that the occurrence of extreme rainfall events, primarily in the break phase of an ISO cycle, reduce the intensity of the following active phase by stabilizing the atmosphere. We found that extreme events in a monsoon break leads to a reduction in the vertical shear of zonal winds and an increase in the static stability of the atmosphere in the following break-to-active transition and active phases. These conditions oppose the initiation and development of an active phase and lessen its intensity. This reduces the ISO intensity and mean ISM rainfall. This study has direct implications towards short to medium range prediction of rainfall over India, and practical applications on agriculture and water resource management. |