A New Metric for Indian Monsoon Rainfall Extremes*

Abstract

AbstractExtreme monsoon rainfall in India has disastrous consequences, including significant socioeconomic impacts. However, little is known about the overall trends and climate factors associated with extreme rainfall because rainfall greatly varies across India and because few appropriate methods are available to measure extreme rainfall in the context of such heterogeneity. To provide a comprehensive assessment of extreme monsoon rainfall, the authors developed a metric using record rainfall data to measure the changes in the likelihood of extreme high and extreme low rainfall over time; this metric is independent of the characteristics of the underlying rainfall distributions. Hence, the metric is ideally suited to aggregate extreme rainfall information across heterogeneous regions covering India. The authors found that from 1930 to 2013, the likelihood of extreme high and extreme low rainfall increases 2-fold and 4-fold, respectively. These overall trend increases are driven by anomalous increases, particularly in the early 2000s; the likelihood of extreme high and extreme low rainfall increases 5-fold and 18-fold in 2005 and 2002, respectively. These findings imply a broadening of the underlying monsoon rainfall distribution over the past century. The authors also show that the time patterns of the likelihood of extreme rainfall in recent decades are correlated with El Niño–Southern Oscillation, especially when it is in the same phase with the Pacific decadal oscillation and Indian Ocean dipole.