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    Climatological responses of winter precipitation in Turkey to variability of the North Atlantic Oscillation during the period 1930-2001
    (Springer Wien, 2005) Türkes, M; Erlat, E
    Climatological responses of winter (DJFM) precipitation at 78 stations of Turkey to variability of the North Atlantic Oscillation (NAO) were investigated for the period 1930-2001. The analysis was performed with respect to relationships between precipitation and three different NAO indices (NAOIs) and composite precipitation changes corresponding to the extreme phases of the NAOIs, and individual wet conditions and drought events linked to the extreme NAOI events. Main conclusions of the study can be evaluated as follows: (a) The Ponta Delgada-Reykjavik (PD-R) NAOI is superior among the three NAOIs compared, followed by the Lisbon-Stykkisholmur/Reykjavik NAOI, with regards to its ability to control year-to-year variability in winter precipitation series and composite precipitation conditions corresponding to the extreme NAOI phases in Turkey. (b) Variability of winter precipitation at most stations in Turkey is significantly correlated with variability of the three NAOIs. Negative relationships are stronger over the Marmara, the Mediterranean Transition and the Continental Central Anatolia regions, and the Aegean part of the Mediterranean region. (c) Composite precipitation analysis exhibited an apparent opposite anomaly pattern at the majority of stations between the weak and strong phases of the NAOIs. Composite precipitation means corresponding to the weak NAOI phase are explained mostly by wetter than long-term average conditions, whereas composite precipitation responses to the strong NAOI phase mostly produce drier than long-term average conditions. (d) Composite wet (dry) conditions during the weak (strong) phase of the NAOI are significant at about 32% (69%) of 78 stations for the PD-R NAOI, and about 38% (55%) for the L-S(R) NAOI The dry signals from the strong NAO phases are stronger and show a larger spatial coherence over Turkey. The significant signals are evident in the west, centre and south of the country. (g) Widespread severe droughts in 1943, 1957, 1973, 1974, 1983, 1989, 1990, 1992, 1993 and 1994, and widespread strong wet conditions in 1940-1942, 1956, 1963, 1966, 1969 and 1970 were linked to the extreme high- and low-index events of at least two NAOIs, respectively.
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    Öğe
    Influences of the North Atlantic oscillation on precipitation variability and changes in Turkey
    (Soc Italiana Fisica, 2006) Türkes, N; Erlat, E
    The anomalous 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 degrees W and 60 degrees E longitudes and by the 20 degrees N and 70 degrees 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.