Spatial organization in Europe of decadal and interdecadal fluctuations in annual rainfall

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AbstractThe spatial patterns of decadal and bidecadal fluctuations in annual rainfall in Europe are identified. Filtering of time series of the anomaly in annual rainfall is carried out using the Morlet wavelet technique. Reconstruction is achieved by summing the contributions from bands of wavelet time scales; the decadal band and the bidecadal band are composed of contributions from the bands of the (10 to 17 year) and (17 to 27 year) time scales respectively. Results indicate that (1) the spatial organization of the decadal and bidecadal components of annual rainfall are standing‐wave‐like organized patterns. Three standing decadal fluctuations zonally aligned formed the spatial pattern from 1900 until 1931; thereafter the pattern changed into a NW–SE orientation. The decadal band shows an average 12 year period. (2) The spatial organization of the bidecadal component was composed of three standing fluctuations from 1903 to 1986. After 1987, two standing bidecadal fluctuations were located over Europe. The orientation of the bidecadal fluctuations changed during the period under study. Until 1913 the spatial pattern of the bidecadal component was zonally aligned. Since 1913 and until 1986 the three bidecadal fluctuations composing the spatial pattern were aligned SW–NE; starting in 1987, the spatial pattern is composed of two standing fluctuations zonally aligned. The bidecadal spatial pattern shows an average period of 20 to 22 years in length. (3) At the decadal and bidecadal time scales, the first principal component of the spatial field of anomaly in annual rainfall and the North Atlantic oscillation (NAO) index are connected. The upper positive third (lower negative third) of values of the first principal component are indicative of an extensive area with positive (negative) anomalies in annual rainfall. (4) At the decadal time scale the relative phase between the first principal component and the NAO index changes through the period under study; these changes define three regimes. (i) During the regime covering the period 1900 (start of period under study) to about 1945, at the time of peak values in decadal NAO index there takes place a transition between extremes (a neutral state) of the decadal rainfall spatial pattern (first principal component takes small absolute values). In addition, for a positive (negative) peak value of NAO index the spatial pattern of annual rainfall evolves toward an area of predominantly positive (negative) anomalies. (ii) The second regime starts about 1946 and continues to the early 1980s. At the time of the negative (positive) peak in decadal NAO there is a prevailing spatial pattern of positive (negative) anomalies of decadal rainfall. (iii) The third regime starts around the late 1970s and continues to the end of the period under study (in 1996). There is a change of relative phase within this period in the late 1980s. In this regime a spatial pattern of prevailing positive or negative anomaly of decadal rainfall takes place during values of decadal NAO close to zero. (5) At the bidecadal time scale the relative phase between the first principal component and the NAO index remains almost constant through the period under study. The first principal component of the transformed bidecadal component of annual rainfall anomalies attains its positive (negative) peak about 3 years before the bidecadal component of NAO reaches its negative (positive) peak. Copyright © 2002 Royal Meteorological Society.