In normal summer patterns, the research found, there is a 20-day oscillation of the jet stream -- a strong air flow about seven miles high -- that moves north and south, and is tightly linked to normal upwelling activity and the growth of phytoplankton and zooplankton, the basis of the marine food chain.

It's less clear, scientists say, how long-term climate changes, such as El Nino events or global warming, may be affecting the jet stream. But the clear connection between the jet stream and underlying marine productivity is significant in itself, they said.

The study was published this week in the online version of Proceedings of the National Academy of Sciences, a professional journal, by researchers from Oregon State University, the University of North Carolina, and the National Marine Fisheries Service.

"We've known for some time that winds play a fundamental role in controlling upwelling and biological productivity," said Ricardo Letelier, an associate professor of oceanography at OSU. "But now we can better define the larger patterns that force this action and the short-term biological fluctuations that result."

Yvette Spitz, an OSU associate professor of oceanography, also said that in the ocean off central Oregon, there appears to be a very strong and well-defined 18-year cycle of upwelling intensity, and an "upwelling index" in the region is now at almost its highest value since the early 1990s -- a time when, among other things, summer upwelling seems to be delayed longer than usual, but then becomes very intense.

It's possible this is relevant to the recent hypoxic events that have been observed in this region, they said, but more research needs to be done before that linkage can be drawn. There are probably multiple forces that are part of the hypoxia issue.

"The correlation between movements of the jet stream and the underlying biological action in the ocean is really quite strong," Spitz said.

When the system is operating in a healthy and productive pattern, the scientists found, the jet stream oscillates north and south, causing shifts in the wind patterns beneath it, and causing an ebb and flow of nutrient enriched water on the near-shore coast. This forms the basis for one of the world's more productive fisheries.

This situation exists most of the time, although the study documented two years out of 12 when the process broke down.

Other parts of the Pacific Ocean coast off North America and Mexico have coastal upwelling also, the study noted, but it is often less intense and more stable because the jet stream is more distant and has less impact on these areas.

It's not certain what effect global warming or other changes in ocean processes may have on these patterns, the researchers said.