Atmospheric Variables 12211 Temperature

Temperature controls evapotranspiration and snow/ice melt processes and has a strong affect on the amount and seasonal distribution of soil moisture and runoff. According to the recent IPCC report global average temperature records show significant warming trends between 1910-1940 and 1960s-recent, which is more significant over the last 50 years [1].

Many researchers analyzed a large number of station records scattered over Turkey to detect overall trends and regional/seasonal characteristics in observed temperature. The review of the studies broadly reveal two distinct periods with opposite behavior: an earlier cooling trend which extends from 1960s to early

1990s and a more recent warming trend which extends from early 1990s to the present day [3-10].

Apart from this overall warming trend, there exist, however, regional and seasonal differences in temperature variations over Turkey. Geographically, this warming trend was found to be more significant in western [11, 12], southern [6, 8, 11] and southeastern [7, 8] regions. Conversely, general cooling trends have been found to prevail in Black Sea Region [6, 11, 12] and Central Anatolia [12]. An exception to this trend was the study of Tecer and Cerit [13] which reported warming trend for Rize station located in eastern Black Sea Region after 1994. A prominent feature in seasonal temperature changes is the widespread and consistent increase in summer temperatures [6, 8, 11, 12] followed by an increase in the number of summer days [5]. For winter season consistent change in temperature records have not been detected. Another pronounced characteristic in temperature time series is the increasing trend in minimum temperatures [4-8, 11] and night-time temperatures [12] which are generally attributed to the urbanization [6-8, 11, 12, 14].

Of course, in addition to the anthropogenic effects, natural weather patterns control temperature changes. Recently, Turkes and Erlat [15] found close associations between the North Atlantic Oscillation (NAO) indices and temperature signals over Turkey. Thus it can be concluded that the temperature patterns/trends prevailing over Turkey is the result of both natural and human-induced forcing on climate. Their relative importance and the extent of anthropogenic influences on natural weather patterns are yet to be understood.

12.2.1.2 Precipitation

Precipitation is the main driver of the hydrologic system over land. Therefore any change in the intensity, frequency and duration of precipitation will have a direct impact on water resources.

Recent climatic studies showed a decreasing precipitation trend in most of the Eastern Mediterranean [16]. The studies that focus on long-term changes in Turkish precipitation generally agree with this decreasing trend [17-19], but regional and seasonal differences exist. Many studies reported general decreasing precipitation trend over western [6, 11, 17, 19, 20] and southern [17-19] parts of Turkey that are dominated by the Mediterranean climate. Some researchers also indicated a decreasing precipitation trend in the southeastern [6, 19] and northwestern [21] parts which are partly influenced by the Mediterranean climate. In contrast, in northern regions and northern parts of Central Anatolia increasing precipitation trend has been reported [6, 11, 17]. In terms of seasonal changes, a decrease in winter precipitation over Turkey [6, 18, 19, 22] has been reported which is more dominant in the western and southwestern parts [11, 19]. The winter precipitation is especially important for Tigris-Euphrates (TE) River flows which are mainly fed by snow melt. A decrease in the amount of snow and an increase in temperature will likely have negative consequences for energy and irrigation projects along the TE river system, in addition to the potential conflicts with the downstream countries.

In spring and fall, an increase in precipitation has been generally reported throughout the country [6, 19]. In the northwest (European) part of Turkey, no coherent behavior in long term precipitation trends have been reported. Focusing on the European part, Aksoy et al. [22] reported insignificant trends at seasonal scale: a decreasing trend in winter and an increasing trend in fall.

Many researchers investigated the link between precipitation trends and weather patterns, e.g NAO. Karabork et al. [23] showed that precipitation and streamflow in winter have significant negative correlations with the NAO Index. Recent consistently more positive phase of the NAO could, to some extent, explain the decreasing trends in precipitation in western parts of Turkey. It is not yet clear whether the global warming is altering NAO and other natural weather patterns.

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