Analysis of observed variability and trends in numbers of frost days in Turkey for the period 1950-2010

This study examines the climatology of annual frost days, and analyses the size and behaviour of the long-term variability and trends in annual numbers of frost day at the 72 stations over Turkey from 1950 to 2010. The main results are summarized as follows: (1) The annual number of frost days has evidently decreased at most of the stations with some observed regional differences, (2) The decreasing trends are largest over the Eastern Anatolia, the Marmara regions and along the Mediterranean coastline. The meteorological stations located in the continental northeast and the easternmost parts of the Anatolian Peninsula, including Ardahan, Igdir and Van, show a negative linear trend with a rate of four days per decade, (3) As in other regions of the Earth, changes in number of frost days are very likely associated with changes in minimum air temperatures and increasing growing season lengths in Turkey, (4) The decreasing trends in number of frost days also indicated considerable decadal-scale variability. This variability is very likely attributable to the large-scale atmospheric circulation and atmospheric oscillations such as the Arctic Oscillation (AO) or North Atlantic Oscillation (NAO) and the North Sea-Caspian Pattern, (5) Consequently, the long-winter (DJFM) composites of number of frost days were examined for extreme phases of the AO index during the period 19502010 in order to assess the influence of atmospheric oscillations on year-to-year variability in number of frost days. According to the Cramer's tk test, winter number of frost days tended to increase significantly during the high (positive) index AO phase, while they tended to decrease significantly during the low (negative) index AO phase. These relationships are statistically significant at the 1% level at the majority of stations. Copyright (c) 2011 Royal Meteorological Society