| Sea level variations of the United States, 1854-1999 - :14663 | National Ocean Service (NOS)
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Sea level variations of the United States, 1854-1999
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Sea level variations of the United States, 1854-1999
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    "In this report, monthly mean sea level (MSL) variations are analyzed for 117 stations of the National Ocean Service's (NOS) National Water Level Observation Network (NWLON) having between 25 and 146 years of data. Monthly MSL data up to the end of 1999 are used to calculate linear trends, and to obtain the average seasonal cycle, the residual time series, and the autoregressive coefficient of the residual with accurate estimates of standard errors. Months with extreme high or low residuals are defined and periods of broad regional correlations between station anomalies are observed. Since the derived trends include the local vertical land motion, they are spatially variable. Calculated MSL trends range from 9.85 mm/yr for Grand Isle, LA to -16.68 mm/yr for Skagway, AK and are consistent with previous station trends published by NOS. The appendices of this report include time series plots for each station of the monthly MSL with the seasonal cycle removed, the seasonal cycle, and the MSL residual after both the seasonal cycle and the trend are removed. The location and timing of any major earthquakes near stations in tectonically-active areas are noted, since an associated vertical offset or a change in MSL trend is possible. An inverse power relationship is derived empirically, relating the standard error for linear trends to the year range of MSL data. An estimated 50 to 60 years of data are required for obtaining linear MSL trends having a 1 mm/yr precision with a 95% statistical confidence interval. For a given length of data, the standard errors for trends at Pacific Ocean and western Gulf of Mexico stations tend to be greater than standard errors for trends at Atlantic coastal stations. MSL trends for the most recent 50-year period of 1950-1999 are compared with trends obtained from each station's entire data set. The trend for the past 50 years is significantly lower at only three out of sixty stations (Eastport, Portland, and Boston). At no station is the 1950-1999 trend significantly higher than the trend obtained from the station's entire data set. In an examination of 50-year MSL trends at sixteen of the longest term stations, it was found that for six Atlantic stations, the periods centered on years from 1930 to 1955 tend to have significantly higher trends than periods centered on years from 1965 to 1975. For San Francisco, trends for all 50-year periods centered from 1890 to 1915 are significantly lower than the overall trend and the trend since the 1906 San Francisco earthquake"--Executive Summary.

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