RISKS OF PRE-MONSOON EXTREME RAINFALL EVENTS OF BANGLADESH: IS ANTHROPOGENIC CLIMATE CHANGE PLAYING A ROLE?

摘要

Northeastern Bangladesh (NEB; 90.5°-92.5°E, 24°-25.5°N) has unique lowland areas known as haors. The "Boro" paddy (rice) crop is cultivated in these haors in dry winter season (December-February) and harvested during April-May. Boro accounted for-55% of national rice production per annum during 2011-16 and NEB contributed to -15% of the total Boro rice production over this period (FAO 2017). NEB receives highest annual mean rainfall of >4,300 mm, in contrast to western Bangladesh, which receives1,400 mm (Shahid 2010). Topographic uplifting of the southwesterly flow by the Meghalaya Plateau and other surrounding mountains triggers heavy convective rainfall at NEB in the pre-monsoon season (Murata et al. 2008,2011; Sanderson and Ahmed 1979; Shahid 2010). Pre-monsoon rainfall during March-April facilitates growth of Boro but events >150 mm of total 6-day rainfall can cause early flash floods and damage crops. On 27 March 2017, extreme rainfall over NEB triggered the earliest flash flood since 2000 (Ahmed et al. 2017). Subsequently, -850,000 households were affected and -220,000 ha of nearly harvestable Boro were damaged. Crop failure in 2017 contributed to a record 30% rice price hike comparedto 2016 (FAO 2017). Analyzing NOAAs Climate Prediction Center (CPC) observation data, we find that during 16 March-15 April 2017, 6-day total rainfall over NEB amounted to 225 mm (i.e., 33.33% higher than the flash flood triggering threshold of 150 mm).The highest positive rainfall anomaly (up to 100 mm day"1; relative to 1979-2017) during 16 March-15 April 2017 is found at NEB (see red box in Fig. la). To determine how rare this event is, we apply a generalized Pareto (GP) distribution fit to CPC dataexceeding 90th percentile values, and find a return time of -70 years (Fig. Id).