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The influence of the El Nin??o-Southern Oscillation (ENSO) phenomenon on flooding in California coastal streams is investigated by analyzing the annual peak floods recorded at 38 gauging stations. The state of ENSO prior to and during flooding is characterized by the multivariate ENSO index (MEI), where MFI < -0.5 is defined as the La Nin??a phase and MEI > 0.5 as the El Nin??o phase. Flood magnitude in all 20 streams located south of 35??N has a significant positive correlation (r = 0.3 to 0.6), whereas in 3 of the 4 streams located north of 41??N flood magnitude has a significant negative correlation (r = -0.3 to -0.4), with MEI from -2.2 to + 3.2. Correlations with MEI are uniformly weak and insignificant, however, when the floods are subdivided into El Nin??o and non-El Nin??o phases. A comparison of the geometric mean El Nin??o flood to the geometric mean non-El Nin??o flood determined that the means were statistically different at gauging stations south of 35??N and north of 41??N. For 20 streams located south of 35??N, the geometric mean of annual peak floods recorded at a stream gauge during El Nin??o phases is 2-14 times the geometric mean of annual peak floods recorded during non-El Nin??o phases. Thus, south of 35??N along the California coast, floods are significantly larger during an El Nin??o phase than a non-El Nin??o phase. For the three streams located north of 41??N, the geometric mean of annual peak floods during an El Nin??o phase was less than 70% of the geometric mean of annual peak floods during a non-El Nin??o phase. The relative strength of the El Nin??o phase, however, has, at most, a weak influence on flood magnitude. Flood exceedance probabilities for the El Nin??o and non-El Nin??o periods were calculated for all gauging stations using a three-parameter log gamma distribution. For exceedance probabilities from 0.50 to 0.02, the ratio of the El Nin??o to non-El Nin??o floods varies from greater than 10 near 32??N to less than 0.7 near 42??N. Latitude explains 76%-90% of the observed variation in the relative magnitude of El Nin??o versus non-El Nin??o floods over the range of exceedance probabilities.
Additional Publication Details
Influence of ENSO on flood frequency along the California coast