New research presents evidence that waves in the North Pacific Ocean -- particularly in southern California -- have increased substantially in size and intensity over the past half century as a result of stronger wind and storm activity.
The findings also suggest that the approach direction of winter ocean swells impacting southern California has rotated from more northwesterly directions to more westerly directions over the years. This change reduces the natural sheltering effects of the Channel Islands and Point Conception, resulting in more direct effectson the coast.
Winter storms have been getting stronger and the storm track has been shifting southward -- both changes have important effects on the wave environment in southern California," said lead author Nick Graham of Scripps Institution of Oceanography at the University of California, San Diego, and the Hydrologic Research Center. "This information could be important for studying changes in the coastal environment and for coastal management and planning."
The paper, co-authored by Henry Diaz of the National Oceanic and Atmospheric Administration (NOAA) Climate Diagnostics Center, appears in a recent issue of the Bulletin of the American Meteorological Society.
Graham said that extreme-wave studies with colleagues since the 1970s repeatedly suggested that waves in the North Pacific have been increasing in size and strength.
But observational methods and data necessary to provide rigorous support for such an assumption were limited. That was until the National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR) developed a "reanalysis" dataset that provided, among other data, a consistent record of sea level pressure and surface wind data. This allowed accurate reconstructions of winter storm activity and ocean waves from 1948 though 1998.
Graham took that crucial database and cross-referenced it with in-the-field data from ocean-station vessels, radiosonde reports, data buoys, and observing ships.
Together, the results portrayed a "surprisingly regular" and statistically significant pattern of increasingly vigorous storm-driven wave activity across much of the North Pacific since the mid-twentieth century.
The backward-looking wave analysis, or "hindcast," shows that the wave climate over many parts of the North Pacific has become rougher since 1948, with extreme wave heights increasing approximately 1 to 2 meters, or roughly 20 to 30 percent.
Especially significant changes have occurred in southern California.
"Over the past 50 years the hindcast results indicated that winter waves have gotten 35 percent larger in southern California, which is significant," said Graham.
The report says the change may be attributed directly to increases in upper-level winds. These stronger "jet stream" westerly winds provide an environment more favorable for the formation and intensification of strong winter storms.
What then, is behind the changes in upper-level winds?
Although Graham and Diaz do not arrive at a conclusion, they argue that those increases may be attributed to one or a combination of factors, including changes in sea surface temperatures in the tropics. These changes may be related to human-produced alterations caused by greenhouse warming, natural climate variations, or both.
As a byproduct of the investigation, Graham found that the wave direction in southern California has shifted since 1975. Instead of approaching from a more northwesterly direction, the waves are now coming directly from the west. The sheltering previously afforded by natural barriers has diminished.
"These are profound long-term changes and they have important implications from economic and beach-management perspectives," said Graham. "They play into questions about coastal construction, coastal erosion, and debates on coastal protection strategies. We need to account for these significant changes in wave climate and its behavior in our coastal planning."
The study was funded by the NOAA Climate Diagnostics Center, the Director's Office at Scripps Institution of Oceanography, Pacific Weather Analysis, and the Hydrologic Research Center.