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 winter series and to increase in some spring and summer series during 1930–1993. 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 years. Spring rainfall series depicted high-frequency oscillations with cycles 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 positive anomalies from late 1970s to early 1990s, and showed an upward trend in winter and summer. Significant negative correlations were found between geopotential height and rainfall anomalies in winter over most of Turkey. Cycles of 2–2.2 and 3.2–3.8 years in spring rainfalls appeared to be associated with similar oscillations of spring geopotential heights. Cycles of 13 years in both winter geopotential series reflected in a similar cycle of 14 years in annual and winter rainfall. The number of depressions reaching Turkey tended to decrease for about 10 years. Increased frequencies and intensities of dry conditions in the last ca. 20 years may have been related to increased geopotential heights and decreased frequency of depressions over Turkey. Signs of warm minus cold event winter anomalies during various stages of the Southern Oscillation revealed the existence of some coherent regions without significant signals. Most of the selected 48 stations had a positive sign anomaly during year −1 warm and cold events. The cold event rainfall means showed a coherent region of significantly increased rainfall conditions over the central-west and central parts of Turkey. Slightly wetter than normal warm event conditions during year +1 were observed in many stations. Most of warm and cold event responses were characterized by a decreased rainfall. Drier than long-term average conditions were significant at some stations during year +1 cold events. Warm minus cold event differences had an opposite signal between year −1 and year 0 (+1) in many stations. Opposition of composite anomalies was evident in most stations between year −1 and year +1 cold events. © 1998 Royal Meteorological Society