Spatial and seasonal responses of precipitation in the Ganges and Brahmaputra river basins to ENSO and Indian Ocean dipole modes: implications for flooding and drought

We evaluated the spatial and seasonal responses of precipitation in the Ganges and Brahmaputra basins as modulated by the El Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) modes using Global Precipi- tation Climatology Centre (GPCC) full data reanalysis of monthly global land-surface precipitation data from 1901 to 2010 with a spatial resolution of 0.5 0.5 . The GPCC monthly total precipitation climatology targeting the period 1951-2000 was used to compute gridded monthly anomalies for the entire time period. The gridded monthly anomalies were averaged for the years influenced by combinations of climate modes. Occurrences of El Nino alone significantly reduce (88 % of the long-term average (LTA)) precipitation during the monsoon months in the western and southeast- ern Ganges Basin. In contrast, occurrences of La Nina and co-occurrences of La Nina and negative IOD events signif- icantly enhance (110 and 109 % of LTA in the Ganges and Brahmaputra Basin, respectively) precipitation across both basins. When El Nino co-occurs with positive IOD events, the impacts of El Nino on the basins' precipitation dimin- ishes. When there is no active ENSO or IOD events (oc- curring in 41 out of 110 years), precipitation remains below average (95 % of LTA) in the agriculturally intensive areas of Haryana, Uttar Pradesh, Rajasthan, Madhya Pradesh, and Western Nepal in the Ganges Basin, whereas precipitation remains average to above average (104 % of LTA) across the Brahmaputra Basin. This pattern implies that a regular water deficit is likely, especially in the Ganges Basin, with impli- cations for the agriculture sector due to its reliance on con- sistent rainfall for successful production. Historically, major droughts occurred during El Nino and co-occurrences of El Nino and positive IOD events, while major flooding occurred during La Nina and co-occurrences of La Nina and negative IOD events in the basins. This observational analysis will fa- cilitate well-informed decision making in minimizing natural hazard risks and climate impacts on agriculture, and supports development of strategies ensuring optimized use of water resources in best management practice under a changing cli- mate.