The Recent Decline and Recovery of Indian Summer Monsoon Rainfall: Relative Roles of External Forcing and Internal Variability

Abstract

The Indian summer monsoon (ISM) rainfall affects a large population in South Asia. Observations show a decline in ISM rainfall from 1950 to 1999 and a recovery from 1999 to 2013. While the decline has been attributed to global warming, aerosol effects, deforestation, and a negative-to-positive phase transition of the interdecadal Pacific oscillation (IPO), the cause for the recovery remains largely unclear. Through analyses of a 57-member perturbed-parameter ensemble of model simulations, this study shows that the externally forced rainfall trend is relatively weak and is overwhelmed by large internal variability during both 1950-99 and 1999-2013. The IPO is identified as the internal mode that helps modulate the recent decline and recovery of the ISM rainfall. The IPO induces ISM rainfall changes through moisture convergence anomalies associated with an anomalous Walker circulation and meridional tropospheric temperature gradients and the resultant anomalous convection and zonal moisture advection. The negative-to-positive IPO phase transition from 1950 to 1999 reduces what would have been an externally forced weak upward rainfall trend of 0.01 to 20.15 mm day 21 decade 21 during that period, while the rainfall trend from 1999 to 2013 increases from the forced value of 0.42 to 0.68 mm day 21 decade 21 associated with a positive-to-negative IPO phase transition. Such a significant modulation of the historical ISM rainfall trends by the IPO is confirmed by another 100member ensemble of simulations using perturbed initial conditions. Our findings highlight that the interplay between the effects of external forcing and the IPO needs be considered for climate adaptation and mitigation strategies in South Asia.

A very likely weakening of Pacific Walker Circulation in constrained near-future projections

“…2d ). We find that the composite differences resemble the IPO-like pattern in both observation 39 and the internal IPO mode of MPI-GE (Supplementary Fig. 1a ), implying that IPO is the dominant internal mode in regulating the PWC variabilities.…”

“…By adding an adjustment term to the original PWC trend following refs. 39 , 56 , the adjusted linear trend of the PWC is given by where the adjusted PWC trend of member [ ] is the sum of the forced PWC trend [ ] plus the internal component of the PWC trend [ ] with an adjustment term [ ]. The internal adjusted PWC trend is expressed as where where is the adjustment term and takes the observed IPO phase transitions into consideration.…”

Asymmetric response of the Indian summer monsoon to positive and negative phases of major tropical climate patterns

“…The Indian summer monsoon exhibits strong interannual variability from bi-annual to multi-decadal time scales 1 – 5 . These variations are due to external forcings as well as internal variability of the non-linear system 6 , 7 . Tropical teleconnections such as the El Nino Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), the tropical Atlantic variability (ATL), and the preceding winter ENSO (W-ENSO) influences ISM rainfall.…”

The dynamic and thermodynamic processes dominating the reduction of global land monsoon precipitation driven by anthropogenic aerosols emission

“…The changes of global monsoon are modulated by several factors. Different factors including the greenhouse gases (GHGs) (Kitoh et al, 2013;Song et al, 2014;Chen and Zhou, 2015), anthropogenic aerosols (AAs) (Held et al, 2005;Lau et al, 2006;Meehl et al, 2008;Bollasina et al, 2011;Qian et al, 2011;Jiang et al, 2013Jiang et al, , 2015Guo et al, 2013;Wu et al, 2013;Polson et al, 2014;Song et al, 2014;Li et al, 2016;Zhang et al, 2018), and natural internal variability of the climate system, such as the Inter-decadal Pacific Oscillation (IPO) (Zhu and Yang, 2003;Yang et al, 2004;Zhou et al, 2008b;Li et al, 2010;Wang et al, 2012;Huang et al, 2020), have been proposed to explain the observed reduction in monsoon precipitation. The impact of individual factors is regional dependent, including the sign and magnitude of impact (Polson et al, 2014;Pascale et al, 2017).…”