Interdecadal Pacific Oscillation Behind India’s 5-Decade Long Rainfall Decline

• 21 Jul 2020

• In novel approach to understand natural variability factors behind five-decade long rainfall decline till the year 2000 in India, a group of scientists has now examined the differing trends in pre- and post-2000 Indian summer monsoon, its decline and recovery.
• The variability manifested itself in a decline in monsoon rainfall over north central India starting in the 1950s, which persisted for as long as five decades before a reversal from 1999 onwards.
• The study found that neither the five-decade long decline before 2000 nor the subsequent increase can be solely explained as a response to external climate forcing, instead, natural variability factor played the key role.
• External forcing includes changes in greenhouse gases, anthropogenic aerosols,land use, etc.
• Natural variability refers to variations in the mean state due to internal processes within the climate system.
• In addition to anthropogenic climate change, rainfall changes in recent decades are also influenced by natural sea surface temperature oscillation over Pacific basin.
• The prominent natural variability in Pacific sea surface temperature is usually described as the Interdecadal Pacific Oscillation (IPO).
• The scientists found that the differing phases of the IPO played subtle, but crucial supplementary roles in the recent inter-decadal variations of the ISM rainfall.
• Fluctuations in the IPO induced anomalous thermal contrasts between the north and south and changes to ascent and descent throughout the region. These, in turn, resulted in changes to the horizontal advection, from the west and east, of moisture into India.