Role of External Forcing and Internal Variability on the Delayed Withdrawal of Indian Summer Monsoon in Recent Decades

Abstract

The timing of withdrawal of the Indian summer monsoon (ISM) experienced a statistically significant delay in recent decades, leading to the lengthening of monsoon rainy season over north-west India. The late withdrawal is aided by the concomitant strengthening of the ISM circulation and precipitation activity at the end of the ISM season, triggered by the increased inter-hemispheric meridional temperature gradient and the increased frequency of cyclonic systems over the region. We find that the well-known decadal phase shift of the Atlantic Multidecadal Oscillation (AMO) and Pacific Decadal Oscillation (PDO) that occurred at the end of the 20 th century is in favour of this strengthening of the ISM precipitation and delaying its retreat phase. By analysing the CESM2 initial condition large ensemble climate model simulation data, we show that both anthropogenic forcing and internal variability, including the combined influence of PDO and AMO, favours a delay in the ISM withdrawal in recent decades. However, the role of internal variability in extending the ISM season is more dominant than the anthropogenic-forced changes of the climate system.