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

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.

Indian summer monsoon: Extreme events, historical changes, and role of anthropogenic forcings

“…The Indian monsoon underwent oscillating changes at multidecadal scales since 1871, which has been reported in the literature (Goswami, Madhusoodanan, Neema, & Sengupta, ; Kodra, Ghosh, & Ganguly, ). The overall declining trend of summer monsoon rainfall concurrent with an increasing trend of extremes over central India, the core region of maximum convergence, is one of the most widely studied changes (e.g., Ajayamohan et al, ; Goswami, Venugopal, et al, ; Karmakar et al, ; C. K. B. Krishnamurthy et al, ; Pai et al, ; Rajeevan et al, , ; Roxy et al, ; Sagar et al, ; C. Singh, ; D. Singh, Tsiang, et al, ). However; there exist inconsistencies across different dataset in computing the trends during the last few decades, particularly on sub‐regional scales.…”

Large‐Scale Extreme Rainfall‐Producing Synoptic Systems of the Indian Summer Monsoon

“…Understanding the physical processes that lead to extreme rainfall events (EREs) is a challenging multiscale problem that has received wide attention in the literature. The literature on EREs includes aspects like flash floods (Bosart & Sanders, 1981; Maddox et al, 1978, 1979), convective systems (Doswell III et al, 1996; Schumacher & Johnson, 2005), the observed trends (Goswami et al, 2006; Karmakar et al, 2017; Nikumbh et al, 2019; Rajeevan et al, 2008), and their future projections (Emori & Brown, 2005; Muller et al, 2011; O'Gorman, 2015; Trenberth, 1999). A multitude of extreme precipitation events, however, remained unpredicted (Coumou & Rahmstorf, 2012).…”

The Relation of Intraseasonal Variations With Local Onset and Demise of the Indian Summer Monsoon

“…However, demise dates are strongly associated with the HF‐ISO phases across entire India and demise of the ISM occurs mostly in phase 7 of HF‐ISO except some grid points over northeast India (Figure h). Onset of monsoon rainfall occurring in unfavorable phases of LF‐ISO over the eastern India coincide with the fact that most of the low‐pressure systems that develop over the BoB, in unfavorable LF‐ISO phases in many cases, propagate toward this region and cause substantial amount of rainfall during May–early June (Goswami et al, ; Karmakar et al, , ). This may mark the onset of the ISM rainfall over these regions just before the low‐level westerlies strongly establishes over the domain.…”

“…The extraction of ISO modes in the rainfall data is done using multichannel singular spectral analysis (MSSA; Ghil et al, 2002;Plaut & Vautard, 1994). This data-adaptive filtering technique is quite successful in extracting oscillatory signals from a short and noisy time series and has been used in many studies recently to understand the behavior of Indian monsoon (Karmakar et al, 2017a(Karmakar et al, , 2017bKarmakar & Krishnamurti, 2018;Krishnamurthy & Shukla, 2007;Moron et al, 2012). We removed the daily climatology at each grid point from the 104-year daily data.…”