Although the planet as a whole has been warming the past decade was the warmest since instrumental records began in the 19th century natural climate variability still steals the show from time to time, causing some regions to buck the global trends. The Bering Sea where temperatures have been on a roller coaster ride in recent years offers an example of what regional variability looks like up close.
The Bering Sea, west of Alaska, is one of those regions of the globe that has experienced some of its coldest temperatures on record during the past four years — a downturn after experiencing the area’s warmest six years on record between 2000 and 2005. As depicted on Discovery Channel’s popular show “The Deadliest Catch,” the stormy region’s fisheries feed a large portion of North America, with lucrative crab, pollock, halibut, salmon, and cod industries.
The Pacific Decadal Oscillation, or PDO, affects sea surface temperatures and wind flow in the North Pacific. This graphic from NASA shows sea surface temperature departures from average, as well as wind anomalies (arrows), for different phases of the PDO. Credit: NASA JPL.
As part of a $52 million project funded by the National Science Foundation, more than one hundred scientists have been studying how climate change affects this important ecosystem. The story of why the Bering Sea has recently become colder helps illustrate an aspect of climate science that frequently results in misunderstandings between scientists and the public: not every part of the world will warm due to increasing amounts of greenhouse gases in the atmosphere, or warm at the same rate.
Until about four years ago, the Bering Sea was warming up like most of the world’s oceans. “It was extremely warm there not that long ago, and now we’ve got a few years that are really cold in the context of a climate still really warm globally,” says Nathan Mantua, a climate scientist at the University of Washington.
“The reason you can have these shifts regionally is because atmospheric circulation changes, and sometimes you get dramatic regional changes that go against global trends.”
Research has shown that the Pacific Decadal Oscillatio, a long-term atmospheric circulation pattern, is largely responsible for the Bering Sea’s recent temperature flip-flop, though other factors also play a role. Most people are familiar with the El Niño/Southern Oscillation (ENSO), which includes El Niño and La Niña, but the PDO differs in two ways. First, it typically operates on longer time scales, shifting every 20 to 30 years, rather than six to 18 months with El Niño. The PDO has its primary effects on the North Pacific region with secondary effects in the tropics, while El Niño is the opposite.
The PDO was first described by fisheries scientist Steven Hare in 1996, when he, Mantua, and three other University of Washington scientists linked its oscillations to variability in Coho and Chinook salmon production in the Pacific Northwest. With a one to three year lag, during warm phases of the PDO, salmon in the Pacific Northwest tend to fare poorly, and the trend reverses when the PDO shifts to its cool phase. Interestingly, the PDO appears to have the reverse effects on Alaskan salmon production so when the Pacific Northwest salmon boomed recently, previously thriving Alaskan runs declined across the state.
While the third-highest Chinook salmon returns were recorded in Oregon and Washington in 2010, the Commerce Department issued a “fishery failure” determination for the Yukon River Chinook salmon in Alaska. The declaration paved the way for federal assistance to Alaskan communities that suffered economic damages from the lackluster commercial fishing season. This year, the Washington Department of Fish and Wildlife is predicting the fifth-largest Columbia River fall Chinook salmon return since at least 1948.
The PDO shifted into a warm phase in 1977, widely documented in the scientific literature as a “regime shift” that affected everything from weather patterns to fish in the sea. Around 1998, the PDO shifted back to a cool phase, which begs the question, if the PDO shifted into a cool phase, why did the Bering Sea warm right around that same time? This is where the story gets a little messy.
Time series of shifts in the phase of the Pacific Decadal Oscillation (PDO), from 1925 to 2010. Red bars indicate positive (warm) years; blue bars indicate negative (cool) years. Credit: NOAA PMEL.
Firstly, scientists say the PDO does not have uniform effects across the Pacific. The warm phase is associated with warm temperatures along the western coast of North America, but cool ocean temperatures in the center of the North Pacific, and vice versa for a cool phase. Secondly, within the predominant longer-scale oscillations, the PDO has shorter one to five-year oscillations. And lastly, just as large-scale, long-duration climatic patterns like the PDO affect weather patterns, ocean and air temperatures, shorter-term, lesser-known influences also affect year-to-year patterns in the Bering Sea.
In this case, research has shown that winter weather plays a dominant role, specifically wind direction.
“It’s almost as simple as which way the wind blows,” says Mantua. Every summer, the Bering is ice-free, but as colder weather arrives, ice forms at the border between the sub-Arctic and the Arctic. How far south it extends into the Bering Sea each year depends in large part on how far the winter winds push it. Scientists call this “sea ice advection.”
“More wind out of the north brings especially cold air that drives the [winter sea] ice to lower latitudes,” says Mantua. “If the winds are mostly out of south, they’re bringing mild air from lower latitudes to higher latitude parts of the Arctic.” A study led by Jinlun Zhang of the University of Washington recently confirmed that the PDO, combined with winter sea ice advection, might explain much of the variation in the Bering’s water temperatures.
Winter sea ice extent, in turn, affects the entire ecosystem. More winter sea ice means a large “cold pool” exists during spring and summer and a frigid footprint left behind by the melted sea ice. In warm winters with little sea ice, as in 2000 through 2005, the cold pool can be virtually nonexistent. This affects everything from the tiniest zooplankton to marine mammals, fish, and seabirds.
“The next question is, what is it about climate that causes changes in wind?” asks Mantua. “The surface wind variability is strongly influenced by the pressure difference between the high pressure area in Eastern Russia and the Aleutian Low. The PDO is well correlated with the Aleutian Low, but not the Russian High. So the PDO is a significant part of the story, but not the only part of the story.”
Scientists do not yet have any reliable way of predicting how the PDO will change in the future, as they do with the better-studied El Niño. In the meantime, year-round ice cover in the Arctic continues to decline every year, and global average land and sea surface temperatures continue to rise.
And even as some folks get lost amidst these erratic global climate and weather patterns, the reality is that the interaction between manmade climate change and natural cycles like the PDO is complex. “I think a lot of people are confused by it. They think, ‘how can you have these fierce winters in the Eastern U.S. or Western Europe if we’re facing global warming?’” poses Mantua. “The simple answer is that there’s a lot of regional variation that may be completely independent of global warming.”
In other words, the simple answer is there are no simple answers.
Below, view a slideshow of researchers studying the Bering Sea from on board the RV Thompson last summer.
Wendee Holtcamp, a Houston-based science writer, spent a month on board the RV Thompson in the Bering Sea last summer, blogging and reporting for Nature. She also wrote about the Bering Sea Project for BioScience. Homepage photo from flickr/naturemandala