Watch the North Pacific waters

The Baltimore Sun

Water temperatures in the North Pacific are changing, and NASA climatologists say the patterns they've seen in recent months may affect temperature and rainfall trends in the United States for decades to come.

They could bring enhanced hurricane activity in the Atlantic, and more mild winters in Central Maryland, like this last one.

They could produce more dry weather for the parched Southwest and Southeast, and cooler temperatures with even more rain than normal in the Northwest, the upper Midwest and the Great Lakes.

Or not.

The problem with predicting the impact on East Coast weather is that the connections between climate cycles in the Pacific and Eastern weather are complicated by many other factors, including yet another cycle of ocean temperature and air pressure called the North Atlantic Oscillation, or NAO.

That said, scientists at NASA's Jet Propulsion Laboratory in California report that the water temperatures of the North Pacific have now entered the "cool phase" of the Pacific Decadal Oscillation, a natural cycle that can last for 20 to 30 years.

The warm and cool phases of the PDO are similar to the warm and cool phases farther south in the tropical Pacific known as El Nino and La Nina. But the PDO cycles last much longer - decades, versus 6 to 18 months for an El Nino or La Nina.

When the two are out of step (say, cool PDO, warm El Nino) their effects on distant weather events are blunted, scientists believe. But when they're in sync (cool PDO, cool La Nina) their effects are combined and amplified.

William Patzert, an oceanographer and climatologist at NASA's Jet propulsion Laboratory in Pasadena, Calif., said a cool PDO may also suppress the "signals" of global warming - rising temperatures and sea levels - at least for a while.

"It can mask the signal for a decade or so. But then, when there's a switch to a positive [warm] PDO, all of a sudden you can get a dramatic rise" in sea levels and temperatures, said Patzert.

The PDO cycle was first noted and named in 1996 by University of Washington fisheries scientists, led by Stephen Hare. He found that warm phases correlated with better salmon fishing in Alaska and depressed activity off the West Coast of the lower 48 states. In cool phases, the correlations reversed.

Climatologists say the cool PDO that appeared last fall combined with a strengthening La Nina and may have reinforced the mild and unusually snow-free winter Baltimore enjoyed this year - a hallmark of La Nina winters.

The season's first storm, on Dec. 5, left only 4.6 inches on the ground at Baltimore-Washington International Airport. The second, on Jan. 17, left just 2.4 inches. The last two, on Feb. 20 and 22, totaled just 1.3 inches - broom snow.

The 8.5-inch total for the season was the skimpiest snow total at the airport since 2001-2002, when only 2.3 inches fell there.

There was so little snow in Baltimore that The Sun put an indefinite hold on a story outlining the dangerous stresses snow shoveling imposes on the human body. The story was filed Dec. 12, ready for publication the next time Baltimore got snow deep enough to shovel. It never did, and the article is still on ice.

The last time the PDO went from warm to cool was in 1947. That lasted until 1976. A warm phase followed and persisted until the mid-1990s, when the pattern became more erratic.

But after a decade of "flipping" and uncertainty about where it was headed, Patzert said, "now it looks like [the PDO] has settled into its cooler negative phase."

The evidence began to emerge last fall in images from the U.S.-French Topex/Poseidon satellite, which uses radar to measure sea-surface heights with astonishing precision.

Because warm water expands, sea levels become a proxy for water temperature. A big, warm El Nino can raise sea levels in the area by nearly a foot above normal Patzert said.

Last fall, the satellite images began to show falling sea surface heights (cooler temperatures) in the horseshoe-shaped pattern characteristic of the PDO. The cooler water prevailed from Alaska, southward along the west coast of North America, then curved westward out to sea. Enclosed in the horseshoe was a large pool of relatively warm water. It's the reverse of the warm-phase pattern.

Scientists don't really understand what drives these phase changes, or why they persist. But ocean temperature measurements and tree ring data extend the record of PDO shifts back 1,500 years.

Patzert said there's no way to know how long the new cool-phase PDO will last. But he expects it to bring cooler temperatures and above-average rainfall in the northwestern United States, the upper Midwest and the Great Lakes. During this past winter, he noted, "the Pacific Northwest really got clobbered, and the upper Midwest had a lot of snow."

It also means dry weather in the Southwest and the Southeast.

"After January in the American Southwest, the rain just turned off," he said, "so we've had record-breaking dry conditions in February, March, April, and now in May. You can see what a powerful pull the Pacific has on these rainfall and temperature patterns."

With a cool PDO underway, he said, "we might be looking at a more frequent drought scenario across the southern tier of the United States."

The East Coast, Patzert said, is "a more difficult call. ... A lot of what you see in the mid-Atlantic and the Northeast has to do with what happens in the North Atlantic, too."

A key ingredient here is the North Atlantic Oscillation, or NAO, said Mike Halpert, deputy director of the National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Springs. A cool NAO corresponds with colder weather in the Northeast and mid-Atlantic states.

But "the NAO is far different than the PDO," Halpert said, with fluctuations measured in weeks and months rather than decades. And, its influence is strongest in winter.

When NAO cool phases have dominated - as they did from the 1950s into the 1970s - winters were colder than average, Halpert said. When the warm phases dominated in the 1980s and 1990s, winters were milder.

But the NAO's high short-term variability limits its usefulness. "It's not a factor we use in seasonal forecasts, except to acknowledge it adds uncertainty," Halpert said.

Using PDO cycles to forecast Atlantic hurricane activity is similarly problematic.

The effects of a La Nina and the cool-phase PDO should theoretically amplify each other to produce warm, dry weather in the Southeast and enhanced hurricane activity in the Atlantic.

But long-term hurricane forecasting is "notoriously bad," Patzert said. "Last year [as the La Nina developed] everybody predicted a big hurricane season, and it was a wimp."

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