North Atlantic Oscillation: Driving Climate
For centuries, a massive atmospheric system has regularly altered weather patterns, fishery production and animal migrations across the North Atlantic Ocean. At last, Earth scientists and climate modelers are beginning to understand how--and when - the North Atlantic Oscillation happens.
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Drought in the Middle East. Canadian cod fisheries gone bust. Spine-buckling cold snaps in New England. Brown tides staining Long Island coasts.
Name any debacle of nature, and someone will likely blame it on that bad seed of climatology: El Niño. But there’s another large-scale climate pattern that’s been overlooked. It’s El Niño’s fickle Nordic sister, the North Atlantic Oscillation. First noticed by Norse seafarers many centuries ago, the NAO is the most significant driver of climate variability in the middle and high latitudes of the Northern Hemisphere. It regularly stirs up trouble in Europe, Canada, and the eastern US, and is the leading suspect in the four natural mishaps above. At this point, we ask: Can the NAO get a little respect? And a better stage name?
Historically speaking, as a climate pattern the NAO tends to flip-flop between two phases over the course of years. These phases are dubbed positive and negative, reflecting numerical differences in regional air pressure that drive each phase. Each of the seesawing phases can afflict land, ocean, and air with alternating drama. One winter, for example, the NAO may be cooking French Riviera beaches with a dry heat wave. The next winter, it’s soaking them with storms.
Many mysteries remain about the NAO. But researchers do have a decent grasp on what it can do to everyday Northern Hemisphere folks. Have you, or people you know, had any of the experiences below in the last decade? If so, then you may have been graced by the NAO’s recent inclination towards a positive phase.
Seen fewer snow days?
Mild winters for the US east coast and Northern Europe, with warmer temperatures, less snow, and fewer nor'easters, are a favorite gift of positive NAO phases. They’ve even been linked with warmer winters in cities much farther-flung across the Northern Hemisphere, including Seattle, Dallas, Paris, and Tokyo.
Considered cod a rare delicacy?
Canadian Atlantic cod landings have been slumping for decades, sinking from 2 million tons in the 60s to just 35,718 tons in 2002. Repeated positive NAO phases have been offered as a cause, driving colder waters into the Labrador Sea near Newfoundland. Extra-chilly water slows cod growth and reproduction rates, resulting in runty fishand the official closure of three essential cod fishing zones in 2003.
Hosted a visitor from Norway?
If so, maybe it’s because they’ve had more cash for a transatlantic vacation, thanks to lower energy bills during a positive NAO year. Positive-phase winters soak Norway with wetter weather, bringing abundant streamflow for hydropower plants come spring. That’s a surplus of energyand pocket change.
Eaten very few Long Island bay scallops?
The scallop fisheries on eastern Long Island coasts remain devastated after repeated “brown tides,” or algal blooms, in the 80s and 90s. Brown-tide algae starve bay scallops by paralyzing their feeding mechanisms. Some scientists suggest that yearly variations in the NAO phase trigger surges of nutrients on which brown-tide algae flourish.
Known a soldier working on water access in Iraq?
Positive NAO phases spin fewer storms into Turkey, parching the headwaters of the Tigris-Euphrates River system. Since the runoff supplies fresh water to ⅔ of the Arabic-speaking population of the Middle East, downstream neighbors Syria and Iraq are directly affected by reduced flow.
Filled up your car?
There’s a chance the gas you’ll pump in coming years will originate from an oil rig redesigned to meet the demands of the NAO. Storminess from repeated positive phases has boosted wave heights by more than 10% in the North Atlantic over the last 50 years. That’s bad news for oil-rig operators in the North Sea between Scotland and Norwaya high, unruly wave overtaking a deck can topple a platform. To brace for these more likely events, oil companies are strengthening and raising the decks of new rigs there.
Sipped a mediocre Portuguese wine?
Positive NAOs are stingy on rainfall during crucial spring growing months in Portugal. That makes for a worse grape harvest come August, resulting in unexceptional local wineand disappointed vintners.
And the Weirdest NAO Effect Award goes to…
Eaten some cheap, delicious British bread?
Positive winter NAO signs cause less rainfall the following summer in England and Wales. The dry weather results in very robust wheat grains, which apparently bake into the best-quality bread.
So what happens in a negative phase of the oscillating NAO? Directly the opposite of the above. You’d expect to get severe winter weather along the US east coast, more cod, a cash-strapped Norwegian, a wetter Middle East, smaller Atlantic waves, a great glass of Portuguese wineand lest we forget, crumbly, ill-slicing English bread.
If it seems like you haven’t experienced many negative-phase impacts recently, you’re on the money. That’s because during the past 32 years, the NAO’s normally capricious self has favored the positive phase over the negative one, 25 to 7! This has inspired some head-scratching among researchers. Is this stick-to-itiveness natural for the NAO? Or is it unprecedented, and connected somehow to global warming? To this end, scientists in the last decade have initiated an onslaught of NAO investigations to get a better sense of its mechanisms, effects, and natural cycle over many hundreds of years. The ultimate goal? To build an accurate predictive computer model of the seemingly-erratic NAO, which could be used to powerful cultural and economic ends.
For the French, the winter of 1879 was terrible. In fact, it was cold enough to make a 25-year-old fledgling researcher at France’s Central Bureau of Meteorology wonder why. So Léon-Phillipe Teisserenc de Bort decided to investigate common threads between it and other anomalous winters in Europe’s recent memory. And indeed he found a shared link, which he coined “centres d’action” large regions of air pressure hovering over distinct spots on the globe. Today these centers, located above Iceland and the Azores Islands off the coast of Portugal, are still where “ d’action” is that drives the North Atlantic Oscillation, or NAO.
The NAO can be defined most simply as the relationship between the center of low atmospheric pressure over Iceland and the center of high pressure above the Azores. We’ve discovered in the century since Teisserenc de Bort that the centers tend to vary in intensity relative to one another. They do this year-round, but between December and March their swayings seesaw dramatically. Then, when the winter Icelandic low is very low, the Azores high prefers to be very high. That leaves a large pressure difference between the two centerswhat we call a positive-phase NAO. Conversely, when the winter low is greater than usual, the high tends to be lower than usual. The pressure difference is slim, thus, a negative-phase NAO. The phases modulate major air currents and storm tracks over the Atlantic, which ultimately propel large-scale climate patterns over much of the Northern Hemisphere.
Each NAO phase spins its particular brand of atmospheric tumult, affecting temperature, precipitation, cloudiness, and windiness in different regionssometimes to drastic ends. Since the phases can fluctuate over the course of weeks, months, and years, researchers like to track them with a rollicking plot of average pressure difference over time called the NAO index.
Jim Hurrell, an atmospheric scientist, is one such researcher. He’s a director at the National Center for Atmospheric Research in Boulder, CO, and the December-March index he compiles is used worldwide by those keeping tabs on the NAO’s erratic behavior.
Hurrell asserts that although its sign can flip-flop on a weekly basis, the NAO can’t be blamed for weekly weather per se: one smarting winter storm, for example, or a rainy spell. “Rather, the NAO conditions the atmosphere so that it’s much more likely you’re going to have such events,” he explains. A key distinction here is the difference between weather and climate. Average the weather in one area for decades and you’ve got a natural regional climate, which is what NAO ultimately influences. (Climate can denote a collection of weather from a time period as short as a season or as long as a century. The industry standard is 30 years.) “Really, the NAO is something that appears when you look at longer time scales,” says Hurrell.
A Turbulent Tale
So how does the NAO do its thing? Hurrell points to the particular positioning of the low and the high pressure centers. The high happens to be directly south of the low. Through a sequence of events, this placement is responsible for steering the westerly winds that send distinct weather patterns to different regions during a positive or negative NAO.
The first step is the fact that in the Northern Hemisphere, air tends to move counter-clockwise around a low-pressure center like that over Iceland. Conversely, air moves clockwise around the Azores high. Picture this air flow as two giant atmospheric gears whirring over the Atlantic, spinning their wheels in figure-8 formation. “The flow around these two centers meets at about 40°-45° north latitude,” says Hurrell. The giant jet stream of wind that naturally flows west to east in the midlatitudes gets channeled straight through the meeting point by these cranking cogs of current.
In a positive NAO phase, the high and low are particularly robust. That makes the gears rev their speed, whipping jet-stream-borne storms on a brisk northeastern path. “The storms pick up warmth and moisture from the Atlantic Ocean, carry it over to Northern Europe, and drop it there as rainfall,” Hurrell says. In fact, his research has shown that the warm air can kick up winter Northern European surface temperatures as much as 3°C during a positive NAO.
The remainder of the air currents swirling around the low and the high act as traffic cops, steering more weather signatures of a positive NAO phase. The northwest edge of the counter-clockwise gear around the Icelandic low hurtles dry, cold air out of the Arctic to the south, freezing Northern Canada and Greenland. Meanwhile, warm air from the Caribbean gets cycled around the Azores high, blessing the US east coast with warmer winters.
In a negative phase, however, the low and high relax. The gears of current slow down, weakening the drive of the westerly jet stream. The reverse effect then occurs: That slack allows storms to drift more naturally toward the Mediterranean and points south, releasing the rain and usual warm weather there instead. The Northwesterly Arctic air is similarly less directed, now allowed to descend further into the northeast US and Northern Europe, bringing with it cold snaps and blustery winters. Northern Canadian and Greenlandic winters are relatively mild.
What a Ride It’s Been
Keep in mind that we must contend with all that climatic turmoil only during an intensely positive or negative NAO winter. Hurrell cautions that distinct NAO indexes with extreme, telltale activity aren’t always the norm. “In any given winter, or any given month, it may not look like NAO at all,” he says.
“NAO-blah” is precisely what’s been happening in recent memory. The NAO index for the winter of 2003-2004 clocked in at a statistically-zero -.07. The winter of 2002-2003 was a tepid +.2. Such wimpy indexes have made recent northeast US winters seem, well, like typical northeast US winterswith snow, sure; but no real juicy drama to report.
Still, the trend of the NAO since 1972 has been definitively, positively positive. That includes more peak positive years, like the highest on record, 1989 (+5.08). Prior to that, in the 50s and 60s, the Northern Hemisphere idled in negative phases. (So entertain your elders’ complaints about the hard, cold winters they suffered back then!) These recent long-term trends are unprecedented in the instrumental NAO index, which has been plotted back to 1821.
To be certain these decadal trends are truly a break from tradition, scientists must trace the NAO’s path much, much farther back than 183 years. In fact, a common goal of the recent fervent work on the NAO is to analyze past events to determine if our current positive trend is being goaded by an outside influencenamely, global warming.
Understanding the NAO’s historical behavior is also enormously important for climate prediction. Knowing what the NAO has done in the past (and why) might allow scientists to do the seemingly impossible: forecast what it will do in forthcoming seasons or decades. For more on how scientists are investigating these concepts, click on NAO Data Hunting (And Gathering) or Forecasting the Unpredictable.
While converting Greenlanders in the 1770s, Danish Christian missionary Hans Egede Saabye noticed something. In his diary, he penned: “In Greenland, all winters are severe, yet they are not alike… when the winter in Denmark was severe, as we perceive it, the winter in Greenland in its manner was mild, and conversely.”
Sounds like the North Atlantic Oscillation to us. But little did Saabye know that beyond local winter temperature twists, the NAO flexes serious climatic and ecological muscle over much of the Northern Hemisphere. When the NAO perturbs wintertime temperatures on land and sea, storm activity, precipitation, or ocean currents, snowball effects can ensue: Ecosystems can get knocked off-kilter. Marine-life food supplies can fluctuate wildly. Populations of land mammals can sink or soar.
Uncovering the NAO’s true chain-reaction impacts hinges on one factor--high-quality scientific data taken over long periods of time. And the climatic and biological data that have been hoarded for decades are, as of late, finally hitting NAO pay dirt.
Trawling for Information
“It’s very important to collect data over time for climate,” says NOAA marine forecaster Scott Prosise. “That way, we can establish a baseline against which everything can be measured.” Prosise talks shop while on deck of the Oleander, a commercial container ship. As it does every week, the 386-foot-long vessel is chugging the 650 miles from Port Newark, NJ to Bermuda, shuttling groceries, clothing, and other consumer goods to the island. Prosise is one of many scientific volunteers invited aboard by the ship’s owners, the Bermuda Container Line, to log sea surface temperatures, wind speed, salinity, current velocities, plankton levels, and other data en route in the North Atlantic.
Such information, collected for decades by merchant vessels and Navy and Coast Guard fleets, have formed the backbone of marine climate datasets. These collections of numbers over time have enabled researchers to flesh out how the NAO is influencing the oceans and vice versa.
Prosise aims what appears to be a large water pistol toward the waves. He tugs on the launcher’s holding pin, releasing a 4-inch metal probe of an expendable bathythermograph into the water. As it sinks to the seafloor, the bathythermograph continuously measures changing sea temperatures at successive depths, transmitting the data via a wire connected to a computer on deck.
This constantly moving marine layer cake of cold and warm depths has been found to change dramatically under NAO influence. Such changes have notable repercussions, like the increased frequency of US-east-coast-bound hurricanes during a positive year (fueled by warmer currents between Bermuda and Florida) and the devastation of Canadian cod stocks (thanks to a colder Labrador Sea).
Atlantic datasets have even proven that the NAO has the power to bully the Gulf Stream itself: Since positive phases shift the westerly storm track northward, the winds help nudge the major Atlantic current accordingly, about 100 km to the north. Such shifts string along Atlantic tuna fishermen, who seek good catches at the northern edge of the warm Gulf Stream.
Still, single boats like the Oleander crisscrossing the wide ocean freeway can’t provide the breadth of monitoring needed for a nationwide marine dataset of the highest caliber. The prototype? A team of 10 “smart” buoys bobbing innocuously about the Gulf of Maine. Called GoMOOS, the 3-year-old Gulf of Maine Ocean Observing System is setting a new standard for incredibly thorough, 24/7, freely available measurements of currents, temperature, salinity, dissolved oxygen, phytoplankton biomass, and more.
Researchers have determined that a positive NAO channels warm, salty water into the Gulf of Maine (spurred, actually, by the northerly shift of the warm Gulf Stream). In negative NAO years, cold, fresher water floods the Gulf. These two conditions can alternately affect local ecosystems. As GoMOOS’ datasets accumulate over decades, they’ll enable science teams to more clearly understand the NAO’s impacts on the Gulf, including plankton abundance (and its effect on the 300 surviving North Atlantic right whales that eat the plankton), red tides and other algal blooms, lobster reproduction, and other phenomena.
Wish You Were Here
How long a dataset do NAO researchers need, really--years, decades, or hundreds of years? Though project-dependent, generally the longer the time frame of study, the more clearly patterns will emerge. And for those striving to put the NAO’s current variability in a context of its historical, “natural” index, the best dataset is the longest one you can find.
Easier said than done. People simply weren’t sticking barometers in Iceland and the Portugal area and reading them regularly before about 1821. In the absence of sea level pressure data, which directly calculates the index, scientists must take poetic license--they extrapolate winter indices for previous centuries of the NAO from other available information. Called proxies, such data “libraries” are found in natural objects that bear climate imprints from times past.
One researcher — dendroclimatologist Ed Cook — uses tree rings as his proxy. Together with other proxies like Greenland ice cores, he’s managed to reconstruct a virtual winter NAO index to the year 1400. Cook , who calls himself “Dr. Dendro” and looks suspiciously Paul Bunyan-esque, is the director of Columbia University’s Tree Ring Lab at their Lamont-Doherty Earth Observatory. He has traveled to key North American and European forests, coring thousands of geriatric trunks with an arborist’s version of a hypodermic needle. Cook and his colleagues then analyze the cores’ ring widths for clues about previous rainfall and temperature patterns in the area. A narrow ring reflects a year in which climate was poor for growth. A wide ring records a year of favorable climate conditions.
Since each ring represents a single growth year, Cook can peg long-gone climates with eerie accuracy. “We can say, for example, what the climate was like for the Battle of Hastings in the year 1066, simply from ring growth contained in British oaks that grew back then,” he marvels.
So how do you get a 600-year record from a 300-year old tree? “We find pieces of remnant wood from the same species nearby,” says Cook. By matching a telltale ring series for several years in both the live and dead tree, dendroclimatologists can overlap the patterns to create an extended paleoclimate history back 10,000 years for some areas of the world!
The reconstructed NAO index from proxies is still not complete to make a definitive claim about the NAO’s natural variability patterns for the long haul. But according to Cook, the indication so far is that the positive trend we’ve been experiencing since the early 70s is rare, but not necessarily unheard of, within the past 600 years. “That makes it harder to argue that the NAO is being uniquely forced into a brand new mode of variability that has never been seen before,” he says. That said, as is often the case for many arguments about global warming, the jury’s still out.
The North Atlantic Oscillation’s positive-phase trend since the early 70s, with its mild winters over the northeast US and Northern Europe, has added fuel to the already potent debate about global climate change. Undeniable is that Northern Hemispheric temperatures are now at their warmest. And for the past 40 years, the rate of warming has been particularly brisk.
Scientists attribute a sizeable amount of Northern Hemisphere surface temperature increase to the NAO's mechanism. But they are concerned about global warming’s potential influence on this smaller-scale climate pattern. Many researchers agree that the accumulation of greenhouse gases is influencing, to some extent, the NAO's recent positive trend. “The precise mechanisms by which these changes occur is what’s up for debate,” says atmospheric scientist Jim Hurrell, a director at the National Center for Atmospheric Research.
Scientists aren’t completely clear what prompts the NAO’s seasonal and decadal trends. But several research hotspots are offering tantalizing clues. The hopes are that one of these avenues will eventually allow forecasting of future NAO phases, or reveal a “smoking gun” that explains global warming’s link to the NAO.
To answer the big-picture questions about the NAO, climate scientists require computer models to tease apart random occurrences from true causes and effects. “When you want to study the complex interaction between the ocean, the atmosphere, and the land, it’s not possible to build a physical model,” says physical oceanographer Martin Visbeck of the Lamont-Doherty Earth Observatory at Columbia University. “We have to build computer analogs that obey the same physical laws that the climate does.” Visbeck has developed such a model used extensively to investigate the interplay between NAO and the ocean.
If you’ve watched a predicted storm barrel across a video map on a weather forecast, you’ve seen a visual representation of a computer climate model. Visbeck describes the underlying equations as a mathematical version of a “Lego world”: a globe sectioned into countless interlocking bricks. Just as on Earth, each brick in the model obeys its own laws as determined by its spatial position and geophysical characteristics. Each computerized brick interacts with its neighbors in numerically describable ways. “There’s a chain reaction,” says Visbeck. “For example, if it’s warm in one box, then we know the next box downwind is going to warm up.”
Once the computer program is assembled, scientists can enter data—for example, temperatures—into this dynamic Lego machine and watch what comes out; how the model’s version of the atmosphere, oceans, or land responds to those temperatures for a particular time period. To test a model’s efficacy, it’s often run in reverse to simulate the climate for a number of prior years. The results are then back-checked against actual observations. If the model simulation seems solid, it might be able to be run forward to accurately forecast future events.
Visbeck singles out three recent research areas where computer models have shed brighter light on the cause of the NAO’s seasonal and decadal variability. While the most obvious route to understanding causality seems like it’d be to uncover what drives the changes in the Icelandic and Azores pressure systems, that’s not where researchers have been looking.
“It’s hard to make a forecast for the pressure differences,” explains Visbeck. That’s because pressure systems are largely a product of highly mercurial atmospheric change. The atmosphere, as Visbeck explains, has a fairly lousy “memory.” Atmospheric events—air temperatures, currents, and the like—change their values and directions often, rarely repeating patterns. (Scientists call this concept “chaos.”)
Scientists have turned instead to more sluggish systems, ones that project patterns for the longer term, for links to NAO activity. Ultimately, these systems can drive atmospheric change, and thus have a better shot at being predictors.
The first example comes from a team at the Met Office, the UK’s meteorological service. The Met Office predicts the sign of the approaching winter NAO solely from ocean surface temperatures of the North Atlantic in the preceding May. By running their model of this relationship back fifty years, the team has correctly "back-casted" the phase of the actual winter NAO two times out of three. By operating their model forward, Met Office is attempting to predict the upcoming winter NAO. Their forecast for winter 2004-5? You guessed it—positive.
What’s the connection between ocean temperatures and this climate pattern? The NAO is intimately linked with the storm track over the North Atlantic, which affects how warm the water gets. By summer, the storms peter out, but the ocean can sustain its May temperature into the winter. By then, the seawater temperatures are influencing air pressures above—the starting point for the next NAO.
While the North Atlantic can stoke some seasonal atmospheric change, tropical seas, with their extra dose of solar warming—exert much more pronounced, long-term effects on global climate than northerly oceans. One example is El Niño: it originates in the tropics but affects midlatitude weather.
Jim Hurrell knows this about the tropics. That’s why, in 2003, he plugged winter sea surface temperatures of the Indian Ocean into a global climate model. In response, the model reproduced the weather hallmarks of a positive NAO phase.
Hurrell’s research reflects a second breakthrough on what mechanisms may be governing NAO variability. Since 1950, average winter temperatures of the Indian Ocean have heated up about 0.6 of a degree, with an associated increase in rainfall. Hurrell says that this tropical change is likely a direct consequence of global warming.
Hurrell suspects that the mechanism linking the Indian Ocean with the far-off Atlantic has to do with tropical waters’ effect on the west-to-east path of storms in the North Pacific. “This storm track goes around the world, so it has a ‘downstream’ effect over the North Atlantic.” As this link gets investigated further, it may be able to be used to generate long-range NAO forecasts.
A third area of big-picture NAO research is looking at what’s affecting the atmosphere from the top down rather than the bottom up. It’s fairly well accepted these days that greenhouse gases are changing the chemistry of the stratosphere, the atmospheric layer above the troposphere. The troposphere is the layer in which we live and where the NAO goes about its business.
Scientists have proposed that the reduction of ozone and the buildup of greenhouse gases in the stratosphere have livened up the circulation in this sensitive atmospheric layer in recent years, such that its westerly flow has significantly strengthened. Some teams’ findings suggest that an intensified stratospheric circulation can provoke the troposphere below, nudging it toward a more positive NAO index. Although this coupling is highly debated, it brings up intriguing ideas: that if we could predict our anthropogenic influence on stratospheric chemistry over the next several decades, we might be able to likewise forecast the NAO’s behavior. And if we temper our influence, we could alleviate our potential effect on the NAO.
If scientists develop accurate NAO forecasts, business owners, government officials, and everyday citizens will benefit: Growing seasons and crop yields could be anticipated, ecosystems could be protected, and water and energy resources could be better managed and financed. Visbeck points to his work with several Norwegian energy companies as an example. Because hydroelectric energy prices directly correlate to the NAO phase that season (high NAO = more water flow = cheaper electricity), predicting the index would help the companies plan for their purchasing of surplus energy if needed.
“It's very rewarding for a scientist to be able to say what the future might hold,” says Visbeck. “But more importantly, predictability allows for more insightful stewardship of our planet, of our resources, of our whole population.”