ENSO connection with monsoon rainfall over Bangladesh

Abstract

This paper investigates the variability and trends of summer monsoon rainfall over Bangladesh and its relation with ENSO (El Nino Southern Oscillation). The data of rainfall for 30 selected rain gauge stations of Bangladesh have been used over a period of 47 years (1961–2007) collected from Bangladesh Meteorological Department (BMD) and ENSO data have been downloaded from the website of Golden Gate Weather Services, Canada. The data are analyzed using different statistical parameters. The major findings of the study are that the mean monthly rainfall of Bangladesh shows a unimodal pattern throughout the year with highest in July (20%) whereas lowest in January (0.36%). Most of the rainfall occurs in the monsoon season (70.69%). However, the pre-monsoon and post-monsoon rainfall accounts 18.94% and 8.94% respectively. Moreover, the winter is very dry and has 1.53% of the annual rainfall. The coefficient of variation is high in the winter and low in the monsoon season. All Bangladesh mean summer monsoon rainfall is 1616 mm (70.6%) where annual country average rainfall is 2286 mm, and standard deviation and coefficient of variance is 208.9mm and 11.76% respectively. Summer monsoon rainfall widely varies over the geographical areas with lowest in central-western part conversely highest in southeastern part with moderate in northeastern part of the country. The short term variability of the scale 2–4 years is highly dominant. The plots using 5-year moving average data smooth out the high frequency oscillations and the variability of longer time scale are better discerned. The decadal scale variation with time period around 10–11 years and 14–16 years are found to be prominent in the filtered series. The higher time scales of around 20 years are also evident in the pre-monsoon and monsoon rainfall. The trend analysis results show that winter has the trend value of 0.56 mm/year against an annual rainfall of 34.9 mm. The relatively high trends of rainfall are obtained from pre-monsoon and post monsoon (2.14 and 2.12 mm/year). In case of ENSO connection, it is found that there have strong impact of ENSO on the monsoon system of subcontinent scale, though the relationship is weak in case of Bangladesh.