NINETEENTH CENTURY US WEST COAST SEA LEVEL AND TIDAL PROPERTIES

Portland State University, Portland OR

Investigators

Abstract

Tidal data are the oldest and longest records available from the ocean, but much of the 19th and early 20th century tidal record (along with the weather data collected at tide stations) remains un-digitized and unused. In this project, novel ways of recovering, digitizing, vetting and analyzing 19th century tide data from Astoria, OR (1853-1876) and San Diego, CA (1853-1872) will be pioneered; these are two of the three oldest Pacific Ocean tidal records. The products of this work and related metadata recovery will be used to improve estimates of 19th century mean sea level (MSL) in the north-eastern Pacific, assess the secular change in tidal properties, and estimate long-term changes in storminess, for example, the climate induced, non-tidal variance that occurs due to wind, river flow, and other processes. This project will therefore address important contemporary scientific issues such as the long-term variation and acceleration in MSL, the long-term effects of climate change and local change, and the quality of old data. The data will be improved (by removing errors) and the tabulated data that has already been recovered for Astoria and San Diego will be augmented by obtaining digital copies of selected marigrams (22 m long scrolls that each contain one month of pencil tide tracings) from the US National Archive. Metadata (leveling and datum records, time checks and weather records), will also be recovered and used to correct the raw data and to compare historic mean sea-level with modern measurements. One novel aspect is that recovered tide rolls will be digitized to 6 minute resolution using software, rather than the traditional slow and error prone hand tabulations. Further, innovative analysis techniques will be used to find and correct errors. Using the corrected data sets, MSL variation from San Diego, San Francisco, and Astoria will be assessed and compared. To the extent possible, corrections for barometric pressure, river flow, and isostatic rebound will be applied. The analysis will address whether MSL at San Francisco is representative of the US West Coast, and will, by comparison with the modern record, help constrain long-term trends in MSL variation. Secular shifts in the primary constituents (e.g., M2) and the shallow-water overtides (e.g., M4) will be assessed. Spectral analysis techniques and weather data collected at tide stations will be used to investigate secular changes to the non-tidal variance in the water level signal for San Francisco. Because overtides are produced by bathymetry, river flow, stratification, and other local processes, assessing their change over time can provide insight into locally forced changes to the functioning of estuaries. To the extent possible, the effects of local changes, climate, and river forcing will be separated. Broader Impacts: A better understanding of changes to MSL and storminess helps address important contemporary scientific questions with policy implications. Because the proposal is a proof of concept, the methods to be developed here can in the future be used to recover hundreds of station-years of the 1850-1950 tide data that are currently stored in analog form (usually as marigrams) in archives. The PI is an early career scientist.