Seasonal and regional variability of wet and dry spell characteristics over Turkey

Abstract

Understanding the variations and leading atmospheric mechanisms causing wet and dry episodes are very important for the water management strategies. For this purpose, this study investigates the spatiotemporal variation and background environmental conditions for wet and dry spell lengths in Turkey. With this aim, Mann-Kendall rank statistic test and fitted ordinary least squares regression method are implemented to the 92 meteorology stations, which are homogenously distributed over Turkey, for the period 1966–2018. In terms of atmospheric circulation mechanisms, synoptic composites of NCEP/NCAR Reanalysis data (sea level pressure, air temperature at 850-hPa level, geopotential height at 500-hPa level) and sea surface temperature from NOAA High Resolution data are applied to the five years of highest frequency long dry/wet spells. According to the results, both the increasing frequencies of wet spells and their highest contribution to the precipitation are mainly found in the eastern Black Sea Region (BSR) of Turkey during all seasons, especially in spring. In this sub-region, statistically significant increasing (decreasing) springtime wet (dry) spell lengths in the neighboring five stations (i.e. Ardahan, Artvin, Bayburt, Hopa, Rize) indicated that the majority of wet spells (almost 30%) appear to occur between 3 and 5 days and the maximum number of the wet spell days are shown in the last two decades of the eastern BSR. Local quasi-stationary surface high located over eastern Black Sea (1016-hPa core) and associated weak northerly winds transfer relatively warm moist air from the sea surface (11.8 °C in average). This air mass meets with cold- land and low level (6 °C at 850 hPa) air masses, developing instability conditions and resulting in low precipitation rates in wet spells for the eastern BSR. On the other hand, by the extension of the south-west Asian Monsoon to the inner parts of Turkey, relatively hot dry air is transferred to the eastern BSR via southerly winds. As a result of this long-staying Asiatic monsoon characteristics, more dry days are shown in the region due to insufficient moisture and associated lack of precipitation.