Sea Level Rise and Quasi-Periodicity in Storminess Along the West Coast

Error-corrected hourly tide gauge data from 1858 to 1999 at San Francisco (SFO) suggests a trend for greater coastal impacts during winter months as a result of a combination of both an increased rate in sea level rise and recently increased storminess. Changes in storminess are determined from non-tide residuals, while sea level rise impacts are estimated from tide gauge anomalies. Assuming that non-tidal forcing varies smoothly across the tide gauge spectrum, non-tide water level estimates are obtained by linearly interpolating Fourier spectral estimates across the tidal bands, with the variance of the interpolated estimates determined from spectral level variation on both sides of the band. Tide gauge anomalies are determined from the difference between the raw tide gauge data and mean of the monthly means. Thus, tide gauge anomalies can include significant contributions resulting from El Niño related thermal expansion along the West Coast as well as long term sea level rise, while non-tide residuals exclude water level variation at time-scales greater than 90 days and are more closely associated with storminess. Tide gauge anomalies show an increasing trend in both the number of hours and occurrence of extreme water levels (above the 98^th percentile) beginning about 1930, inferred to be primarily the result of an increase in the rate of sea level rise. Five-year moving variance analyses show quasi-periodic decadal-scale variability from 1858 onward for both non-tide residuals and tide gauge anomalies, with variance peaks generally centered near extreme ENSO episodes. The range in non-tide variation suggests that storminess during ENSO episodes has not increased substantially since 1858 (except for perhaps the last 10 yrs). Measures of non-tide variability indicate that storminess comparable to or exceeding the great El Niño's of 1982-83 and 1997-98 occurred during decadal-scale periods centered near 1878 and 1916, suggesting that any climate change that may have occurred has not caused significant changes in storm trends during the last 140 years that are visible in the sea level record at SFO. The California Department of Boating and Waterways supported this research.