Long-term temperature trends in Climatological Zones of Sri Lanka

Abstract

Ambient temperature in Sri Lanka greatly influences each and every venture throughout the island. The gradual climate change in global scale further worsens the existing situation causing tremendous impacts. Therefore, it became important to understand the vulnerability of the country to the temperature extremes. The primary objective of this study is to assess the presence and significance of linear temperature trends in climatological zones in Sri Lanka, with year as an explanatory variable. This study presents long-term trends in averages and extreme temperatures corresponding to the major climatic zones, that are Dry Zone (DZ), Intermediate Zone (IZ) and Wet Zone (WZ) in Sri Lanka. The data used for this study consist 55-year records of raw (or unadjusted) temperature (C) for the period of 1961-2015. In this regard, daily maximum (Tmax) and minimum (Tmin) ambient air temperature records at 20 synoptic meteorological stations scattered throughout three climatic zones in Sri Lanka were selected for the analysis. The daily average temperature (Tave) was calculated as the mean of daily Tmax and Tmin. The heat-index calculations were performed using 20-year (1996-2015) relative humidity (RH, %) data. The climatic zonal RH average was obtained by grouping stations in respective zones. The slope of the trend lines in the fitted model was tested for the statistical significance using standard methods. Primarily, the trends in average and extreme measures were performed for annual values. For the majority of the measures, the non-parametric Mann-Kendall trend test and Sen-Theil regression was utilized. To test the data with the seasons, modified seasonal Mann-Kendall trend test was applied. The pre-whitening method was used to remove autocorrelation from the time series. For the extreme events, Generalized Linear Modelling (GLM) of the parameters of the Generalized Extreme Value (GEV) distribution was used. The analysis was executed in the R-programming environment, and depending on the need, necessary packages were used. Nearly 30% of the total stations detected for significantly increasing trend in Tmax and Tmin, each. Nuwara Eliya was the only station showed a significantly decreasing Tmax trend. In DZ, the 50% of the stations were detected for significantly increasing Tmax, while it is 20% for Tmin. At least eight stations showed for an increasing trend in Tmax and Tmin in DZ. Though it was not significant, the Mannar station was observed for increasing Tmax and decreasing Tmin trend. There was a decrease in the number of stations, significant for the 2-day, 3-day, 5-day and 7-day recurrence temperature pattern was observed for annual Tave. Being a DZ district, though the Anuradhapura station was detected for the increasing trend during 2-day and 3-day recurrence pattern, it was insignificant during 5-day and 7-day events. Galle was the only WZ station observed for the significance in 3-day, 5-day and 7-day events. All time negative trend was observed in Katugastota which is in WZ followed by Bandarawela in IZ for the time period considered. Badulla was found to show significantly increasing Tave trend for 2-day and 7-day events. The only station with significantly decreasing trend in 3-day recurrence pattern in Tave was Katugastota. It was observed that the slightly higher Tave and heat-index values in DZ than any other climatic zones in Sri Lanka. According to zonal mean heat-index, the least heat stress could be felt in IZ followed by the WZ. It is noteworthy, though WZ of Sri Lanka experience slightly more RH (in average of 73%) than IZ, the heat stress felt is a bit higher. The IZ of Sri Lanka experienced the lowest heat-index value, which implies a cooler climate compared to other parts of the island. The apparent temperature felt in overall is 27 C (real feel like temperature), island-wide. Results showed that there was a much difference in Tmax and Tmin prevailed in three climatic zones of Sri Lanka. The overall trend suggests the warming temperature in Sri Lanka in terms of Tmax and Tmin except for a few stations in WZ and DZ. The findings may help in speculating a big picture of vulnerability to extreme climate in Sri Lanka.