A Hydroclimatological Analysis of Precipitation in the Ganges–Brahmaputra–Meghna River Basin

Scott Curtis,Thomas Crawford,Bimal Paul,Mohin Patel,M Miah,Munshi Rahman,M Islam

doi:10.3390/w10101359

Abstract

Understanding seasonal precipitation input into river basins is important for linking large-scale climate drivers with societal water resources and the occurrence of hydrologic hazards such as floods and riverbank erosion. Using satellite data at 0.25-degree resolution, spatial patterns of monsoon (June-July-August-September) precipitation variability between 1983 and 2015 within the Ganges–Brahmaputra–Meghna (GBM) river basin are analyzed with Principal Component (PC) analysis and the first three modes (PC1, PC2 and PC3) are related to global atmospheric-oceanic fields. PC1 explains 88.7% of the variance in monsoonal precipitation and resembles climatology with the center of action over Bangladesh. The eigenvector coefficients show a downward trend consistent with studies reporting a recent decline in monsoon rainfall, but little interannual variability. PC2 explains 2.9% of the variance and shows rainfall maxima to the far western and eastern portions of the basin. PC2 has an apparent decadal cycle and surface and upper-air atmospheric height fields suggest the pattern could be forced by tropical South Atlantic heating and a Rossby wave train stemming from the North Atlantic, consistent with previous studies. Finally, PC3 explains 1.5% of the variance and has high spatial variability. The distribution of precipitation is somewhat zonal, with highest values at the southern border and at the Himalayan ridge. There is strong interannual variability associated with PC3, related to the El Nino/Southern Oscillation (ENSO). Next, we perform a hydroclimatological downscaling, as precipitation attributed to the three PCs was averaged over the Pfafstetter level-04 sub-basins obtained from the World Wildlife Fund (Gland, Switzerland). While PC1 was the principal contributor of rainfall for all sub-basins, PC2 contributed the most to rainfall in the western Ganges sub-basin (4524) and PC3 contributed the most to the rainfall in the northern Brahmaputra (4529). Monsoon rainfall within these two sub-basins were the only ones to show a significant relationship (negative) with ENSO, whereas four of the eight sub-basins had a significant relationship (positive) with sea surface temperature (SST) anomalies in the tropical South Atlantic. This work demonstrates a geographic dependence on climate teleconnections in the GBM that deserves further study.

Highlights

In naturally occurring systems, precipitation that falls within a watershed is a major contributor to its water supply
This study found that indices of the El Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) were related to GBM precipitation anomalies, contributing
The eigenvector coefficient is positive every year, meaning this pattern is a consistently strong feature, with little interannual variability and a downward trend (Figure 2b), which is consistent with other studies that suggest a downward trend in monsoonal rainfall over this time period [5,7]

Summary

Introduction

Precipitation that falls within a watershed is a major contributor to its water supply. Hydroclimatology studies have focused on statistical links between climate teleconnection indices and streamflow, lake and groundwater level variability [1]. A recent review of the hydroclimatology literature “exposed a need to move beyond purely statistical/mechanical treatment of climatic-hydrological variability associations to one where diagnostic analyses are undertaken in order to divulge the underlying climate mechanisms in terms of atmospheric and ocean physics and dynamics that form the cascade of processes that link ocean-atmosphere interactions with the terrestrial branch of the hydrologic cycle” [1] The atmospheric science community tends to focus on climate forcing mechanisms, but generally investigates precipitation within political boundaries [2] or over large latitude-longitude domains [3], without thought to precipitation’s effect on terrestrial hydrology. Until recently have studies limited their climatological analysis to the boundaries of large river basins. This study found that indices of the El Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) were related to GBM precipitation anomalies, contributing

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