Sea Level Trends: Sea Level Variations of the United States Derived from National Water Level Observation Network Stations

Water level records are a combination of the fluctuations of the ocean and the vertical land motion at the location of the station. Monthly mean sea level (MSL) variations were analyzed for the stations of the National Ocean Service's (NOS) National Water Level Observation Network (NWLON) having at least 30 years of data. The sea level variations determined are the relative sea level trend, the average seasonal cycle, and the interannual variability at each station. Since the derived trends include the local vertical land motion, they are spatially variable. The relative sea level trend plots include the time series for each station of the monthly MSL with the seasonal cycle removed, a 5-month average, and the linear trend with its 95% confidence interval; the average seasonal cycle; the interannual variation of MSL. The plots are updated monthly and the long-term trend is recalculated every year. 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.

The mission of the National Ocean Service, Center for Operational Oceanographic Products and Services is to provide the national infrastructure, and the scientific and technical expertise to monitor, assess, and distribute tide, current, water level, and other coastal oceanographic products and services necessary to support NOAA's mission.

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Recorded water levels are a combination of changes in the sea level and the vertical land motion at the location of the gauge. Therefore, the trends derived are relative MSL trends and can be considered valid only for a region near the gauge with similar vertical land motion. Calculation of absolute MSL trends requires the accurate determination of vertical land motion at the gauges and was beyond the scope of this project.

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The 95% confidence intervals of the MSL trends for all water level stations are listed in the tables of US and global trends. The range of confidence intervals are inversely proportional to the length of the data series. A minimum of 30 years is required for the data time period to obtain a 95% confidence interval of +/- 1.5 mm/yr. The widest confidence intervals are for the stations with the shortest time periods of data. Since most of the U.S. East Coast stations have been in operation for many years, the trends are well determined showing narrow confidence intervals. The U.S. West Coast and Alaska station trends have a greater variability due to the variety of tectonic activity affecting vertical land motion at different stations. Some of the newer West Coast and Alaskan stations have wide confidence intervals due to shorter data sets. Several stations have separate trends determined for pre- and post- earthquake time periods. A group of stations in western Gulf Coast show large positive trends. The stations are undergoing rapid land subsidence due to sediment loading and compaction of the Mississippi River delta and/or oil and gas extraction.

For most stations, the plotted values are relative to the 1983-2001 mean sea level datum. Some stations with rapid positive and negative trends are on a 5-year epoch, and their values are plotted relative to the station's most recent 5-year epoch.

NOS has collected water level data for a span of more than 30 years at many US coastal locations. Although there may be long data gaps at some of the locations, the existing data can provide good estimates of linear MSL trends since vertical datums have been carefully maintained through periodic leveling to stable benchmarks with respect to the adjacent landmass. There are a wide range of error bars associated with the trends. Most of this variation can be attributed to the length of the data time series. In order to compare linear MSL trends between stations over different time periods, linear mean sea level trends were calculated for overlapping 50-year segments, for the stations with the longest data records. The differences between stations are mostly due to differences in vertical land motion.

The 95% confidence intervals of the MSL trends for all water level stations are listed in the tables of US and global trends. The range of confidence intervals are inversely proportional to the length of the data series. A minimum of 30 years is required for the data time period to obtain a 95% confidence interval of +/- 1.5 mm/yr. The widest confidence intervals are for the stations with the shortest time periods of data. Since most of the U.S. East Coast stations have been in operation for many years, the trends are well determined showing narrow confidence intervals. The U.S. West Coast and Alaska station trends have a greater variability due to the variety of tectonic activity affecting vertical land motion at different stations. Some of the newer West Coast and Alaskan stations have wide confidence intervals due to shorter data sets. Several stations have separate trends determined for pre- and post- earthquake time periods. A group of stations in western Gulf Coast show large positive trends. The stations are undergoing rapid land subsidence due to sediment loading and compaction of the Mississippi River delta and/or oil and gas extraction.