INFLUENCE OF GEOPOTENTIAL HEIGHTS, CYCLONE FREQUENCY AND SOUTHERN OSCILLATION ON RAINFALL VARIATIONS IN TURKEY

Normalized rainfall of Turkey tended to decrease in many annual and wi nter series and to increase in some spring and summer series during 19 30-1993. Low-frequency fluctuation of regional winter rainfall series was generally dominated by cycles of 3-3.2, 6-7, 7-8.4, and 14-21 year s. Spring rainfall series depicted high-frequency oscillations with cy cles of 2-2.2 years, and longer cycles of 4.2-4.7 years. Mean 700 and 500 hPa geopotential heights over Turkey generally experienced positiv e anomalies from late 1970s to early 1990s, and showed an upward trend in winter and summer. Significant negative correlations were found be tween geopotential height and rainfall anomalies in winter over most o f Turkey. Cycles of 2-2.2 and 3.2-3.8 years in spring rainfalls appear ed to be associated with similar oscillations of spring geopotential h eights. Cycles of 13 years in both winter geopotential series reflecte d in a similar cycle of 14 years in annual and winter rainfall. The nu mber of depressions reaching Turkey tended to decrease for about 10 ye ars. Increased frequencies and intensities of dry conditions in the la st ca. 20 years may have been related to increased geopotential height s and decreased frequency of depressions over Turkey. Signs of warm mi nus cold event winter anomalies during various stages of the Southern Oscillation revealed the existence of some coherent regions without si gnificant signals. Most of the selected 48 stations had a positive sig n anomaly during year -1 warm and cold events. The cold event rainfall means showed a coherent region of significantly increased rainfall co nditions over the central-west and central parts of Turkey. Slightly w etter than normal warm event conditions during year +1 were observed i n many stations. Most of warm and cold event responses were characteri zed by a decreased rainfall. Drier than long-term average conditions w ere significant at some stations during year +1 cold events. Warm minu s cold event differences had an opposite signal between year -1 and ye ar 0 (+ 1) in many stations. Opposition of composite anomalies was evi dent in most of stations between year -1 and year +1 cold events. (C) 1998 Royal Meteorological Society.