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Nonstationarity impacts on frequency analysis of yearly and seasonal extreme temperature in Turkey

This study investigates the temporal variability in yearly and seasonal extreme temperatures across Turkey using stationary and nonstationary frequency analysis. The analyses are conducted using Generalized Extreme Value (GEV), Gumbel and Normal distributions for minimum and maximum temperatures during historical (1971-2016) and projection period (2051-2100). The future nonstationarity impacts are quantified using a 12-member ensemble of The Coordinated Regional Downscaling Experiment (CORDEX) regional climate models (RCM) based on the worst emission scenario (RCP8.5). The ability to preserve the nonstationarity signals after bias-correction for selected RCMs are also presented. CORDEX ensemble members generally underestimated the temperature across all seven geographical regions of Turkey. The CORDEX-31 (HadGEM2-ES/CCLM) provided the most trustable temperature simulation in each region. GEV and Normal distributions exhibited a closer fit to each other but both distributions showed substantially better fit than Gumbel distribution for temperature extremes. Magnitudes of nonstationarity impacts (30-year return level) show strong spatial and seasonal variability. Notably higher magnitudes are observed for minimum temperature (up to + 10 degrees C) than maximum temperature (up to +4 degrees C). Such positive impacts are more significant particularly in eastern Turkey for yearly and seasonal scales. This effect shows greater regional variability in the historical period but with increased temperature projection it is more homogenous and larger in the future period for each region. In the long term, non-stationarities, particularly in minimum temperatures might contribute to less snowpack, accelerate the time-shifts towards the earlier days of the year in snowmelt runoff peaks of streams, further dwindle the water availability during the summer season.