INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 23: 1771–1796 (2003) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/joc.962 PRECIPITATION CHANGES AND VARIABILITY IN TURKEY LINKED TO THE NORTH ATLANTIC OSCILLATION DURING THE PERIOD 1930–2000 MURAT T ¨ URKE ¸ S a, * and ECMEL ERLAT b a Department of Research, Turkish State Meteorological Service, Ankara, Turkey b Department of Geography, University of Aegean, Bornova — ˙ Izmir, Turkey Received 18 November 2002 Revised 18 August 2003 Accepted 18 August 2003 ABSTRACT Relationships between the variability of the North Atlantic oscillation (NAO) indices and the normalized precipitation at 78 stations in Turkey, and the influences of the extreme NAO index (NAOI) episodes and the year-to-year and longer time-scale variations in the NAO on the precipitation conditions were investigated. The results of the study have led to the following main conclusions and evaluations for Turkey: (i) There is a negative relationship between interannual variability of the Turkish precipitation series and the NAO indices. Negative relationships that are particularly strong in winter and partly in autumn are detected to be weaker in spring and almost non-existent in summer. Correlation coefficients are significant at 61 stations in winter, whereas they are significant at 23 and eight stations in autumn and spring respectively. (ii) Composite precipitation means corresponding to the extreme NAOI phases mostly exhibit an apparent opposite anomaly pattern, except in summer, between the negative and positive NAOI phases. (iii) Annual, winter, spring, autumn and partly the summer composite precipitation means are mostly characterized by wetter than the long-term average conditions during the negative NAOI phase, whereas the positive NAOI responses mostly exhibit drier than the long-term average conditions annually and in all seasons except summer. (iv) Spatially coherent and statistically significant changes in the precipitation amounts during the extreme NAOI phases are more apparent in the west and mid Turkey. (v) Low-frequency fluctuations in the circulation over the Atlantic have been closely linked to the coherent large-scale precipitation anomalies that have persisted, particularly in winter, over Turkey since the early 1960s. (vi) There is a great resemblance between the spatial distribution and magnitude of the negative correlation coefficients and the spatial distribution patterns and severity of the wet (dry) conditions with negative (positive) NAOI phase, annually and in the winter and autumn seasons. (vii) The coherent regions characterized with significant correlation coefficients coincide perfectly with the coherent regions characterized by the extreme NAOI signals. These clear associations increase our confidence with the results. Copyright  2003 Royal Meteorological Society. KEY WORDS: Turkey; precipitation; the North Atlantic oscillation; the NAO index; extreme NAOI phases; composite precipitation anomaly and mean; correlation and Cramer’s t k test 1. INTRODUCTION The atmospheric circulation and its year-to-year and decadal variations are accepted as the main determinants and/or controls on the Earth’s climate and the large-scale climatic variability. Variations in many climatic variables are strongly related through large-scale features of the atmospheric circulation, as well as through interactions involving the land and ocean surfaces (Nicholls et al., 1996). The well-known El Ni˜ no–southern oscillation (ENSO) and the North Atlantic oscillation (NAO) are probably the most-studied examples of such large-scale regional features. The NAO has been considered as one of the most important teleconnection patterns: it significantly affects Atlantic weather patterns and produces regional climatic anomalies associated with itself, particularly in Europe and the Mediterranean Basin. The NAO can be defined as a large-scale swaying of atmospheric * Correspondence to: Murat T¨ urke¸ s, Department of Research, Turkish State Meterological Service, PO Box 401, Ankara, Turkey. Copyright  2003 Royal Meteorological Society