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Abstract:
The variability of winter mean precipitation as observed at 14 Romanian rain gauge stations From 1901-1988 is examined. Pettitt's statistic is used to detect changes of regimes in the time series. Almost all stations exhibit a systematic decrease (''downward shift'') at about 1969. Furthermore upward shifts are identified for the southwestern stations at about 1933, and a downward shift in the mid 1920's in the northwest. An upward shift at about 1919 for the Bucharest station is likely determined by the urbanisation effect. These systematic changes are shown to be real and not an artifact due to inhomogeneities in the precipitation data in a two-step procedure. First, the precipitation field and the European-scale sea-level air-pressure field are related to each other through a Canonical Correlation Analysis (CCA). Two relevant pairs of characteristic patterns are found. In a second step, the CCA-coefficients of these two pairs are studied with Pettitt's statistic. In both pairs of time series, simultaneous change points are found in the precipitation and in the pressure-related coefficients. The 1933 and 1969 change paints are related to a change of the southwesterly flow which brings moist Mediterranean air to Romania. The mid-1920s change point is triggered by changes in the frequency or intensity of the northwesterly circulation. As a byproduct, we found that Pettitt's statistic is sensitive to the presence of trends and serial correlation so that its use for statistical hypothesis testing is limited. Therefore, we have used Pettitt's statistic only as an explanatory tool.
