Download full issue - PanamJAS

262FERNANDO E. HIRATA ET ALLIFigure 6. a) Cross wavelet analysis between ML water level and SOI time series (upper left). b) Wavelet coherencebetween ML water level and SOI time series (upper right). c) Cross wavelet analysis between ML water level and NAOindex time series (lower left). d) Wavelet coherence between ML water level and NAO index time series (lower right).The 95% significance against red noise is shown as a thick contour. Relative phase relationship is shown by arrows (inphasepointing right, anti-phase pointing left and ML water level leading by 90º pointing straight down).South American rivers together (Dettinger et al.2000). Zhang et al. (1997) also pointed out that theinterdecadal pattern of the Pacific Ocean presented achange around 1957-58. According to them, thischange was analogous to the one in 1976-77 but hasreceived less attention because its subsequent warmphase was shorter. The mid-1950's shift of ML waterlevel regime may be related to these basin-widechanges in the Pacific described by Zhang et al.(1997).Shifts in variance detected in the previoussection support the idea of a regime from theearly 1920’s until the late 1930’s. The waveletcoherence in Figure 6d suggests a co-variation ofthe water level and SOI from the 1930’s to thelate 1970’s. Allan (2000) reported more robustENSO periods in the 1910’s, 1950’s, 1970’s and1980’s and less energetic periods between 1920’sand 1940’s and during the 1960’s. Again, shifts invariance seem to agree with this argument untilthe 1960’s. Then, the variance decreases in the1963 shift and remains leveled off until the end ofthe record. This discrepancy may also be relatedto the positive shift in the mean that occurred in1958.Zhang et al. (1997) argued that theinterdecadal behavior of Pacific anomalies weredifferent in 1942-43 when compared to thevariations that took place in 1957-58 or 1976-77.The high-variance water level regime that began justafter the great drought of 1943-45 and lasted until1963 may reflect a transition period towards a newbasic climatic state (considering the observed lowvariance of the two major regimes of ML waterlevel). Although the nature of the early 1940’sclimate anomalies is not known, their strong signalin ML water level record and the Pacific anomaliesreported by Zhang et al. (1997) suggest a worldwideclimatic disturbance as proposed by Brönnimann(2005).Pan-American Journal of Aquatic Sciences (2010), 5(2): 254-266