Past and Future Trends in Frequency of Heavy Rainfall Events over Pakistan

Farah Ikram, M. Afzaal, Syed Ahsan Ali Bukhari, Burhan Ahmed

Abstract


The South Asian summer monsoon directly affects the lives of almost one-half of the world’s population, with substantial variability in the monsoon season such as fluctuating between wet (heavy precipitation) and dry spells. This study presents the changes in temperature and precipitation over Pakistan under two different RCP scenarios of a statistically downscaled CMIP5 Coupled General Circulation Model. The analyses are extended to seasonal scale with a focus on the Summer Monsoon season and heavy precipitation events. There is a positive change in mean temperature of 2.7°C under RCP 4.5 scenario and 8.3 °C under RCP 8.5 scenario till the end of this century. The seasonal cycle shows that the winters are warming more than summers with an increase in temperature of 6 to 8°C in the 21st century with respect to baseline (1975-2005). The spatial analysis of both RCPs shows a much sharper increase in mean temperature of the northern areas with respect to the southern areas of Pakistan. Whereas, the precipitation maxima in the century show an overall increase of 3 to 4 mm/day at annual time scale. Both scenarios show a shift in the monsoon region towards the northeast along with a dipole like a pattern over the region (increase in JJAS mean precipitation over monsoon belt with a coexisting decreasing trend of up to 2 mm/day over Punjab, some areas of Sindh and Balochistan). Summer mean temperature shows more warming after mid-century in lower areas of Pakistan including Punjab, southern parts of KPK and upper areas of Balochistan. Two indices have been defined for the summer season. First is the frequency of extreme precipitation events based on the thresholds of daily precipitation as 50 mm/day, 100 mm/day and the second are for dry days as < 1 mm/day. Results show a significant increase in a number of dry days over the selected area of Pakistan, 130/year under RCP4.5 scenario and 420/year over under RCP 8.5 Scenario. Spatial analysis of dry days show an increasing trend and their decadal variability in future projections under both RCP scenarios show that frequent dry days extend towards the north. Heavy rainfall events analysis show intense rainfall events over Pakistan being confined to only the key monsoon region and coastal area of Sindh having return periods of 1 to 2 years. The temporal variability of heavy rainfall frequency indicate decreasing trend with an increase in the intensity of rainfall under both RCP scenarios.


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