Influence of ENSO on Flood Frequency along the California Coast

Abstract

The influence of the El Niño?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 MEI < ?0.5 is defined as the La Niña phase and MEI > 0.5 as the El Niño 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 Niño and non?El Niño phases. A comparison of the geometric mean El Niño flood to the geometric mean non?El Niño 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 Niño phases is 2?14 times the geometric mean of annual peak floods recorded during non?El Niño phases. Thus, south of 35°N along the California coast, floods are significantly larger during an El Niño phase than a non?El Niño phase. For the three streams located north of 41°N, the geometric mean of annual peak floods during an El Niño phase was less than 70% of the geometric mean of annual peak floods during a non?El Niño phase. The relative strength of the El Niño phase, however, has, at most, a weak influence on flood magnitude. Flood exceedance probabilities for the El Niño and non?El Niño 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 Niño to non?El Niño 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 Niño versus non?El Niño floods over the range of exceedance probabilities.