Increased sporadic extremes decrease the intraseasonal variability in the Indian summer monsoon rainfall

Nirupam Karmakar,Arindam Chakraborty,Ravi S Nanjundiah

doi:10.1038/s41598-017-07529-6

Abstract

The Indian summer monsoon (ISM) shows quasi-rhythmic intraseasonal oscillations (ISO) manifested as alternate ‘active’ phases of copious rainfall and quiescent phases of ‘break’. Within these periodic 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 low-frequency ISO (LF-ISO) cycle. However, any physical link between this apparent paradox of increased extreme rainfall events and weakened slower-time-scale components is not yet reported. Here, using observations and numerical model simulations, we show that the occurrence of extreme rainfall events, primarily in the break phase of an LF-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 LF-ISO intensity and mean ISM rainfall.

Highlights

It is inarguably established that the seasonal mean and the nature of rainfall patterns in different spatial and temporal scales are changing[1,2,3,4,5,6,7,8,9,10], and that anthropogenic attribution to these changes is beyond a reasonable doubt[11]
Observational analysis suggests that the number of extreme rainfall events over central India (CI) has significantly increased in the post-1980 (1981–2010; post80) period compared to the pre-1980 (1951–1980; pre80) era during the months from June–September (JJAS), with the dominant change observed in the break phase (Fig. 1a)[5]
Using observational analysis and numerical modeling simulations, our study presents evidences that sporadic extreme rainfall events affect the low-frequency intraseasonal quasi-rhythmic nature of rainfall and seasonal mean during the monsoon over India by changing the vertical shear of zonal winds and the stability of the atmosphere (Fig. 4)

Summary

Introduction

It is inarguably established that the seasonal mean and the nature of rainfall patterns in different spatial and temporal scales are changing[1,2,3,4,5,6,7,8,9,10], and that anthropogenic attribution to these changes is beyond a reasonable doubt[11]. We attempted to mimic the scenario of increased extreme rainfall events over CI using the atmospheric component of a global climate model (GCM) and understand how it affects the seasonal mean and variability of the LF-ISO. Continues for 5 months and ends on October 31st These short simulations were necessary because after the heating was imposed in the model, the HE deviated from the CE and if we integrated long enough, the oscillatory signals might go entirely out-of-phase in two simulations. We present the results obtained from observational analysis and numerical simulations on how change in the number of extreme rainfall events is associated with the changes in LF-ISO and the seasonal mean rainfall

Objectives

Methods

Results

Conclusion