Changing climate, winds bring ‘dead zones’ to Pacific

Published Friday, 9th October 2009

Climate change appears to be changing wind patterns in the US Pacific Northwest, which contributes to the development of ocean “dead zones” in the area, according to researchers at Oregon State University (OSU).

The scientists report the Pacific Ocean off Oregon’s coast again experienced low-oxygen waters near the seafloor this summer, although the winds that fuel annual upwelling abated sufficiently in August and September to avoid severe hypoxia, or oxygen-depleted waters. As a result, this season’s hypoxia area was “about average” in size and duration, compared to recent years.

“We did experience hypoxic conditions for the eighth consecutive year, but unlike 2006 when strong, steady winds led to near zero-oxygen, or anoxic, conditions, we got a break,” said Jack Barth, a professor of oceanography at OSU. “A series of wind reversals late in the summer helped dissipate the low oxygen, in essence allowing the system to ‘flush itself.’ “

Barth said the ability of oceanographers to monitor and measure hypoxic conditions is improving every year and should become even greater when OSU deploys a new network of undersea gliders and cabled moorings off the coast as part of the national Ocean Observatories Initiative, a $386.4 million effort funded by the National Science Foundation to gauge the effects of climate change on the world’s oceans.

Oregon scientists now have 10 times the sensors in the water as they did when hypoxia was first discovered off the central Oregon coast early this decade, Barth noted. The expanded instrumentation is allowing them not only to measure low oxygen, but to understand the underlying mechanisms behind it and how hypoxia manifests itself along the coast.

Unlike hypoxic areas in the Gulf of Mexico, which are caused by agricultural runoff and pollution, the low-oxygen waters of the Pacific Ocean off Oregon are triggered by seasonal upwelling, or the wind-driven mixing of cold, nutrient-rich deep water with surface waters. This fertilisation of the upper water column generates large phytoplankton blooms, and as the plant material dies, it sinks to the bottom and decomposes, lowering the oxygen level of the water just off the seafloor.

While this seasonal upwelling is normal, scientists say, hypoxia hadn’t been observed in near-shore waters prior to 2002. What changed, Barth said, was the pattern of Northwest winds and decreasing oxygen levels in the deep, offshore waters that are upwelled toward the coast.

“Historically, winds would blow at the coast for a week or so, then settle down for several days,” Barth said. “As the winds eased, so did upwelling, and low-oxygen water was washed away — likely off the continental shelf. But in some years, those traditional wind patterns have shifted and now may last 20 to 30 days instead of a week. The system doesn’t have time to cleanse itself.”

Barth said the change in wind patterns and decrease in the oxygen levels in deep offshore waters are consistent with impacts suggested by many climate change models.

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