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    Influence of the Arctic Oscillation on the variability of winter mean temperatures in Turkey
    (Springer Wien, 2008) Tuerkes, M.; Erlat, E.
    Observed changes and variability in winter (DJF) mean temperature series at the 70 stations of Turkey and the circulation types at 500-hPa geopotential height level were investigated to explain atmospheric controls of the temperature variations during the extreme and normal phases of the Arctic Oscillation Index (AOI). The analysis was performed with respect to relationships between winter (monthly and seasonal) mean temperatures and the AOI and the composite winter (monthly and seasonal) temperature changes corresponding to the various phases of the AOI. Main conclusions and evaluations of the study are as follows: (1) Negative relationships are found between year-to-year variability of normalized winter (seasonal and monthly) temperatures and the AOI winter (seasonal and monthly) indices at all stations. Correlation coefficients are statistically significant at the 1% level for the majority of stations. (2) The height of the 500-hPa level circulation is anomalously high over middle of the North Africa, the Middle East and the Caspian Sea during the negative phase of the AOI, and anomalously low over the northeast Atlantic-Europe regions. This 500-hPa level AO pattern forms the spatially coherent warm signals in Turkey by increasing the westerly and southwesterly circulation carrying Atlantic and northernmost African warm air into the eastern Mediterranean and Turkey. (3) During the positive phase of the AOI the 500-hPa level is anomalously high over the northeast Atlantic and Europe including Turkey, while it is anomalously low over the region of the Icelandic Low. This pattern leads to intrusions of cold polar air southward into the Balkans, Turkey and the eastern Mediterranean by increasing northerly and northeasterly circulation and therefore, spatially coherent cold signals over Turkey.
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    Öğe
    Influences of the North Atlantic oscillation on precipitation variability and changes in Turkey(*)
    (Soc Italiana Fisica, 2006) Turkes, M.; Erlat, E.
    The anoialous circulations at 500-hPa geopotential level during the extreme North Atlantic Oscillation Index (NAOI) phases were investigated in order to explain atmospheric causes of the changes in precipitation of the 78 stations of Turkey during the extreme NAOI phases. We arranged and analysed the 500-hPa height data of the 231 grid points for a large region delimited by the 40(circle)W and 60(circle)E longitudes and by the 20 N-circle and 70(circle)N latitudes. The main conclusions of the study are as follows: 1) Annual, winter, spring, autumn and partly summer composite precipitation means are mostly characterised by wetter than long-term average conditions during the negative NAOI phase, whereas the positive NAOI responses mostly exhibit drier than long-term average conditions annually and in all seasons except summer. 2) Spatially coherent and statistically significant changes in the precipitation amounts during the extreme NAOI phases are more apparent in the west and mid Turkey. 3) The 500-hPa circulation corresponding to the negative NAOI phase brings above long-term average precipitation to Turkey in winter, spring and autumn and annually, associated with the NAO pattern in which the 500-hPa geopotential level is anomalously high in the area of the Icelandic Low and anomalously low across the regions of the Azores High and the Europe in general. 4) Contrary, the NAO pattern over the North Atlantic and the Europe is responsible for the drier than long-term average precipitation conditions in Turkey during the positive NAOI phase, when the 500-hPa geopotential level is anomalously low over the area of the Icelandic Low and the anomalously high across the subtropical and mid-latitude north-east Atlantic and the Europe regions.