A Study of the Relationship Between the Intraseasonal Variability and Extreme Rainfall Events in the Indian Summer Monsoon Rainfall in a GCM

One of the most widely-studied topics regarding the Indian summer monsoon is its intraseasonal variability (ISV), often marked by active and break spells of rainfall. In a recent study, using observational data, we have shown that the relative strength of the northward propagating low-frequency ISV mode has a significant decreasing trend during the past six decades (Karmakar et al., 2015). This is associated with a significant decreasing trend in the percentage of extreme events during the active phase of the monsoon, which is compensated by a significant increasing trend in the percentage of extreme events in break phase. In this study, using numerical model simulations, we investigated if the weakening of the association between ISV and extremes is a cause of the reduction in the low-frequency ISV intensity. We used the atmospheric component of the Community Earth System Model (CESM) version 1.2 (CAM5 forced with monthly varying climatological sea surface temperature) to understand the changing association between ISV and extremes. We prescribed heating in the atmosphere during the break phase in the monsoon months mimicking the conditions for extreme rainfall events as observed over central India during June–September. We found that the increased extreme events in the model decreased the intensity of the low-frequency ISV mode over the central Indian region, the northern Bay of Bengal, and the Arabian Sea. This reduction is associated with a reduction of rainfall in active phase and increase in the length of break phase. A reduction in the seasonal mean over India is also observed. These changes in the model precipitation patterns are associated with a significant reduction in the strength of the vertical easterly shear over the northern Indian region during break–active transition phase. This basically weakens the conditions for the growth of Rossby wave instability, thereby elongating break phase and reducing the rainfall intensity in the following active phase. Results found in this modeling study match with those in observations. This study establishes the fact that more extreme rainfall events during break phase will affect the rhythmic nature of the rainfall variability by reducing the rainfall in following active phase.