FY 1990

Long-term trends and interannual variations of sea level in the Pacific northwestern region of the United States

Mofjeld, H.O.

In Fisheries, Global Ocean Studies, Marine Policy and Education, Oceans '89 Proceedings, Vol. 1, IEEE Publication Number 89CH2780-5, Oceanographic Studies, Marine Technology Society, Seattle, WA, 18–21 September 1989, 228–230 (1989)

Long-term observations of sea level at sites in the inland and coastal waters of Washington State provide a useful case study of how sea level trends relative to the land can vary in magnitude and even in sign within the same geographical region. Within 200 km, increases in relative sea level occur at Seattle (1.9 mm/yr) in Puget Sound and Friday Harbor (1.0 mm/yr) to the north, while a decrease occurs at Neah Bay (–1.6 mm/yr) near the coast. These trends are comparable in magnitude with the average (1.5 mm/yr) for the coast of the United States. Tectonic processes seem to account for the variations in trend. The smaller trend at Friday Harbor is consistent with more intense glacial rebound due to heavier ice loading during the last ice age, while decreasing relative sea level at Neah Bay is consistent with uplift due to subduction of the Juan de Fuca Plate under the North American Plate. Superimposed on the trends are interannual variations in sea level associated with El Niño events (e.g., 1914–15, 1940–41 and 1982–83) in which higher oceanic temperatures increased sea level (up to 30 cm for the 1982–83 winter period). Interannual variations in atmospheric pressure and wind forcing also cause large variations in sea level from year to year, predominantly in winter.