Study of Ocean Upwelling Near California Shows Greater Variability Over Latter Part of 20th Century

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A grove of blue oaks, also called foothill oaks, at Pacheco State Park in California.

FAYETTEVILLE, Ark. – A nearly 600-year reconstruction of climate indicators associated with ocean upwelling along the West Coast of North America indicates that this rising in seawater in the California Current became more variable over the latter part of the 20th century.

Upwelling is the rise of cold waters carrying nutrients from deep in the ocean to warmer zones near the surface. Scientists monitor coastal upwelling because it is an important control on the productivity of critical marine ecosystems. But their efforts have been impeded by insufficient data due to instrumental records that cannot compare recent trends to those from previous centuries.

The study, published Friday in Science, demonstrated that the recent levels of increased variability happened only twice in the past 600 years and are associated with weak upwelling, which has a negative impact on marine productivity.

David Stahle, Distinguished Professor of geosciences at the University of Arkansas, co-authored the study and produced tree-ring chronologies of blue oak trees that depend on precipitation caused by the same atmospheric conditions that cause winter upwelling. Several other scientists contributed to the study, which was led by Bryan Black, assistant professor of marine science at the University of Texas Marine Science Institute.

The research documented long-term changes in the variability of marine upwelling in the California Current system, which during the winter lifts deep, cold and fertile oceanic waters into sunlit surface layers of the ocean. This process fuels vast phytoplankton blooms that provide food for fish, seabirds and marine mammals. The atmospheric conditions that control winter upwelling also control onshore precipitation and tree growth by guiding the storm track into or around the western United States.

In addition to the tree-ring chronologies, the analyses also used direct measurements of phytoplankton productivity and the growth of marine birds and fish. Researchers focused on increments of otolith growth for a species of rockfish from 1948 to 2006, the mean egg-laying dates for Cassin’s auklet, a seabird, and the breeding success of the common murre, another seabird, from 1972 to 2006. The otolith is a small, three-part structure in the inner ear of vertebrates that produces annual growth layers similar to tree rings.

“The annual growth data gleaned from fish, seabirds and blue oak trees are highly correlated and demonstrate a remarkable degree of connectivity across the coastal interface,” Stahle said. “Taken together, the data not only provide a long historical context for interpreting modern variability in observational records, but may also inform decisions about managing coastal ecosystems.”  

The researchers also relied on three indices of physical variables to characterize winter climate caused by the California Current: the Northern Oscillation Index, a new index of climate variability based on differences of sea level pressure in the north Pacific; the Bakun Upwelling Index, the standard measure of the volume of water that upwells along the coast; and sea level measurements at San Francisco.

Their multi-centennial perspective demonstrated increased variability and a higher frequency of weak upwelling over that last half of the 1900s and early part of the 2000s. In other words, winter upwelling appears to have become more volatile during the 20th Century, which may have impacted the marine food chain off the California coast.

For more than 30 years, Stahle has taken core samples from trees and examined the chronology of their rings to reconstruct past climate and help explain the societal impact of drought. Specifically, his research has helped document the climate context of prehistoric and early colonial settlements, including the Aztec civilization in central Mexico and the lost colony of Roanoke Island on the coast of present-day North Carolina. This new study of marine productivity expands on the scientific applications of his research – focusing on the flow of the Sacramento River, San Francisco Bay salinity and ancient blue oak trees in California, which have already been used to develop high-quality reconstructions of precipitation.