(Portland, Ore.) - Tides in the eastern Pacific Ocean, from Alaska to Mexico, have increased in amplitude, with the range between highest and lowest tides increasing, and global climate change the likely culprit.
Though global sea level rise has been well documented, the astronomically driven tides have long been considered to be constant – “the music of the spheres.” But a new study by David A. Jay, professor of Civil and Environmental Engineering at Portland State University reveals evidence to the contrary.
Using historic tidal data from 34 sites along the Pacific coast of North and Central America, Jay looked at hourly recordings of water level spanning 44 to 109 years. From 18˚N in central Mexico to 62˚N in Alaska, the historic record showed a growing disparity between daily high and low tides, with both the once-daily and twice-daily tides increasing by an average of 2.2 percent/century. Astoria, Ore., showed one of the most pronounced changes, with tide range increasing at a rate of 0.8 feet/century since 1925. The study also found significant increases in the tidal amplitudes at San Francisco, in Queen Charlotte Islands of British Columbia, and in southeastern Alaska.
Reasons for large-scale evolution of tides—and implications of these findings— remain unclear. Changes in either the salinity and temperature structure of the ocean or in the average ocean winds field at certain critical latitudes might alter the location of amphidromic points in the deep ocean around which tidal waves rotate. Either of these mechanisms, if involved, would be symptomatic of global climate change, says Jay. Smaller-scale changes in internal tides and harbor development over the past century likely affected individual station readings, but do not explain the larger pattern.
The growth in tidal amplitudes may accelerate coastal erosion, especially in areas along the Oregon coast where sea level is rising and storm waves are growing larger. Tides also play an important role in oceanic processes related to healthy ecosystems (primary production, turbulent mixing, upwelling of nutrients). The role larger tides may play in these complex processes, and what affect the increased tides may have on nutrient supplies and fisheries, requires further study.
With 30 years of research on Columbia River hydrology and ecosystems, Jay first discovered the tidal change almost by accident in 2003, training a high school student on how to run a tidal analysis program. When the student’s results indicated an increase in tidal amplitude near Astoria, Jay had him check his work again, and then suggested analyzing data from a San Francisco site. There too, tides were increasing. That began a five-year project to collect and analyze historic tidal readings throughout the Eastern Pacific.
Jay’s study, “Evolution of tidal amplitudes in the eastern Pacific Ocean,” was published February 19, 2009, in Geophysical Research Letters, and was funded by the National Science Foundation, with additional support from Bonneville Power Administration, National Oceanic and Atmospheric Administration’s National Marine Fisheries Service, and the U.S. Army Corps of Engineers, Portland District.
Professor Jay and colleagues hope to secure funding for a second phase of the project, analyzing and modeling tidal changes in the Pacific Ocean as a whole to better understand the mechanisms at work, the causes for, and implications, of these changes.
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By: David Santen, Office of University Communications, Portland State University
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Source: David A. Jay, Department of Civil and Environmental Engineering, Maseeh College of Engineering and Computer Science, Portland State University
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