Stratosphere targets deep sea to shape climate

Thomas Reichler, a University of Utah atmospheric scientist, led a new study showing that changes in winds 15- to 30-miles high in the stratosphere can influence seawater circulation a mile or more deep in the ocean. He says this effect should be taken into account in forecasting climate change distinct from global warming. -  Lee J. Siegel, University of Utah.
Thomas Reichler, a University of Utah atmospheric scientist, led a new study showing that changes in winds 15- to 30-miles high in the stratosphere can influence seawater circulation a mile or more deep in the ocean. He says this effect should be taken into account in forecasting climate change distinct from global warming. – Lee J. Siegel, University of Utah.

A University of Utah study suggests something amazing: Periodic changes in winds 15 to 30 miles high in the stratosphere influence the seas by striking a vulnerable “Achilles heel” in the North Atlantic and changing mile-deep ocean circulation patterns, which in turn affect Earth’s climate.

“We found evidence that what happens in the stratosphere matters for the ocean circulation and therefore for climate,” says Thomas Reichler, senior author of the study published online Sunday, Sept. 23 in the journal Nature Geoscience.

Scientists already knew that events in the stratosphere, 6 miles to 30 miles above Earth, affect what happens below in the troposphere, the part of the atmosphere from Earth’s surface up to 6 miles or about 32,800 feet. Weather occurs in the troposphere.

Researchers also knew that global circulation patterns in the oceans – patterns caused mostly by variations in water temperature and saltiness – affect global climate.

“It is not new that the stratosphere impacts the troposphere,” says Reichler, an associate professor of atmospheric sciences at the University of Utah. “It also is not new that the troposphere impacts the ocean. But now we actually demonstrated an entire link between the stratosphere, the troposphere and the ocean.”

Funded by the University of Utah, Reichler conducted the study with University of Utah atmospheric sciences doctoral student Junsu Kim, and with atmospheric scientist Elisa Manzini and oceanographer Jürgen Kröger, both with the Max Planck Institute for Meteorology in Hamburg, Germany.

Stratospheric Winds and Sea Circulation Show Similar Rhythms


Reichler and colleagues used weather observations and 4,000 years worth of supercomputer simulations of weather to show a surprising association between decade-scale, periodic changes in stratospheric wind patterns known as the polar vortex, and similar rhythmic changes in deep-sea circulation patterns. The changes are:

– “Stratospheric sudden warming” events occur when temperatures rise and 80-mph “polar vortex” winds encircling the Artic suddenly weaken or even change direction. These winds extend from 15 miles elevation in the stratosphere up beyond the top of the stratosphere at 30 miles. The changes last for up to 60 days, allowing time for their effects to propagate down through the atmosphere to the ocean.

– Changes in the speed of the Atlantic circulation pattern – known as Atlantic Meridional Overturning Circulation – that influences the world’s oceans because it acts like a conveyor belt moving water around the planet.

Sometimes, both events happen several years in a row in one decade, and then none occur in the next decade. So incorporating this decade-scale effect of the stratosphere on the sea into supercomputer climate simulations or “models” is important in forecasting decade-to-decade climate changes that are distinct from global warming, Reichler says.

“If we as humans modify the stratosphere, it may – through the chain of events we demonstrate in this study – also impact the ocean circulation,” he says. “Good examples of how we modify the stratosphere are the ozone hole and also fossil-fuel burning that adds carbon dioxide to the stratosphere. These changes to the stratosphere can alter the ocean, and any change to the ocean is extremely important to global climate.”

A Vulnerable Soft Spot in the North Atlantic


“The North Atlantic is particularly important for global ocean circulation, and therefore for climate worldwide,” Reichler says. “In a region south of Greenland, which is called the downwelling region, water can get cold and salty enough – and thus dense enough – so the water starts sinking.”

It is Earth’s most important region of seawater downwelling, he adds. That sinking of cold, salty water “drives the three-dimensional oceanic conveyor belt circulation. What happens in the Atlantic also affects the other oceans.”

Reichler continues: “This area where downwelling occurs is quite susceptible to cooling or warming from the troposphere. If the water is close to becoming heavy enough to sink, then even small additional amounts of heating or cooling from the atmosphere may be imported to the ocean and either trigger downwelling events or delay them.”

Because of that sensitivity, Reichler calls the sea south of Greenland “the Achilles heel of the North Atlantic.”

From Stratosphere to the Sea


In winter, the stratospheric Arctic polar vortex whirls counterclockwise around the North Pole, with the strongest, 80-mph winds at about 60 degrees north latitude. They are stronger than jet stream winds, which are less than 70 mph in the troposphere below.But every two years on average, the stratospheric air suddenly is disrupted and the vortex gets warmer and weaker, and sometimes even shifts direction to clockwise.

“These are catastrophic rearrangements of circulation in the stratosphere,” and the weaker or reversed polar vortex persists up to two months, Reichler says. “Breakdown of the polar vortex can affect circulation in the troposphere all the way down to the surface.”

Reichler’s study ventured into new territory by asking if changes in stratospheric polar vortex winds impart heat or cold to the sea, and how that affects the sea.

It already was known that that these stratospheric wind changes affect the North Atlantic Oscillation – a pattern of low atmospheric pressure centered over Greenland and high pressure over the Azores to the south. The pattern can reverse or oscillate.

Because the oscillating pressure patterns are located above the ocean downwelling area near Greenland, the question is whether that pattern affects the downwelling and, in turn, the global oceanic circulation conveyor belt.

The study’s computer simulations show a decadal on-off pattern of correlated changes in the polar vortex, atmospheric pressure oscillations over the North Atlantic and changes in sea circulation more than one mile beneath the waves. Observations are consistent with the pattern revealed in computer simulations.

Observations and Simulations of the Stratosphere-to-Sea Link


In the 1980s and 2000s, a series of stratospheric sudden warming events weakened polar vortex winds. During the 1990s, the polar vortex remained strong.

Reichler and colleagues used published worldwide ocean observations from a dozen research groups to reconstruct behavior of the conveyor belt ocean circulation during the same 30-year period.

“The weakening and strengthening of the stratospheric circulation seems to correspond with changes in ocean circulation in the North Atlantic,” Reichler says.

To reduce uncertainties about the observations, the researchers used computers to simulate 4,000 years worth of atmosphere and ocean circulation.

“The computer model showed that when we have a series of these polar vortex changes, the ocean circulation is susceptible to those stratospheric events,” Reichler says.

To further verify the findings, the researchers combined 18 atmosphere and ocean models into one big simulation, and “we see very similar outcomes.”

The study suggests there is “a significant stratospheric impact on the ocean,” the researchers write. “Recurring stratospheric vortex events create long-lived perturbations at the ocean surface, which penetrate into the deeper ocean and trigger multidecadal variability in its circulation. This leads to the remarkable fact that signals that emanate from the stratosphere cross the entire atmosphere-ocean system.”

Stratosphere targets deep sea to shape climate

Thomas Reichler, a University of Utah atmospheric scientist, led a new study showing that changes in winds 15- to 30-miles high in the stratosphere can influence seawater circulation a mile or more deep in the ocean. He says this effect should be taken into account in forecasting climate change distinct from global warming. -  Lee J. Siegel, University of Utah.
Thomas Reichler, a University of Utah atmospheric scientist, led a new study showing that changes in winds 15- to 30-miles high in the stratosphere can influence seawater circulation a mile or more deep in the ocean. He says this effect should be taken into account in forecasting climate change distinct from global warming. – Lee J. Siegel, University of Utah.

A University of Utah study suggests something amazing: Periodic changes in winds 15 to 30 miles high in the stratosphere influence the seas by striking a vulnerable “Achilles heel” in the North Atlantic and changing mile-deep ocean circulation patterns, which in turn affect Earth’s climate.

“We found evidence that what happens in the stratosphere matters for the ocean circulation and therefore for climate,” says Thomas Reichler, senior author of the study published online Sunday, Sept. 23 in the journal Nature Geoscience.

Scientists already knew that events in the stratosphere, 6 miles to 30 miles above Earth, affect what happens below in the troposphere, the part of the atmosphere from Earth’s surface up to 6 miles or about 32,800 feet. Weather occurs in the troposphere.

Researchers also knew that global circulation patterns in the oceans – patterns caused mostly by variations in water temperature and saltiness – affect global climate.

“It is not new that the stratosphere impacts the troposphere,” says Reichler, an associate professor of atmospheric sciences at the University of Utah. “It also is not new that the troposphere impacts the ocean. But now we actually demonstrated an entire link between the stratosphere, the troposphere and the ocean.”

Funded by the University of Utah, Reichler conducted the study with University of Utah atmospheric sciences doctoral student Junsu Kim, and with atmospheric scientist Elisa Manzini and oceanographer Jürgen Kröger, both with the Max Planck Institute for Meteorology in Hamburg, Germany.

Stratospheric Winds and Sea Circulation Show Similar Rhythms


Reichler and colleagues used weather observations and 4,000 years worth of supercomputer simulations of weather to show a surprising association between decade-scale, periodic changes in stratospheric wind patterns known as the polar vortex, and similar rhythmic changes in deep-sea circulation patterns. The changes are:

– “Stratospheric sudden warming” events occur when temperatures rise and 80-mph “polar vortex” winds encircling the Artic suddenly weaken or even change direction. These winds extend from 15 miles elevation in the stratosphere up beyond the top of the stratosphere at 30 miles. The changes last for up to 60 days, allowing time for their effects to propagate down through the atmosphere to the ocean.

– Changes in the speed of the Atlantic circulation pattern – known as Atlantic Meridional Overturning Circulation – that influences the world’s oceans because it acts like a conveyor belt moving water around the planet.

Sometimes, both events happen several years in a row in one decade, and then none occur in the next decade. So incorporating this decade-scale effect of the stratosphere on the sea into supercomputer climate simulations or “models” is important in forecasting decade-to-decade climate changes that are distinct from global warming, Reichler says.

“If we as humans modify the stratosphere, it may – through the chain of events we demonstrate in this study – also impact the ocean circulation,” he says. “Good examples of how we modify the stratosphere are the ozone hole and also fossil-fuel burning that adds carbon dioxide to the stratosphere. These changes to the stratosphere can alter the ocean, and any change to the ocean is extremely important to global climate.”

A Vulnerable Soft Spot in the North Atlantic


“The North Atlantic is particularly important for global ocean circulation, and therefore for climate worldwide,” Reichler says. “In a region south of Greenland, which is called the downwelling region, water can get cold and salty enough – and thus dense enough – so the water starts sinking.”

It is Earth’s most important region of seawater downwelling, he adds. That sinking of cold, salty water “drives the three-dimensional oceanic conveyor belt circulation. What happens in the Atlantic also affects the other oceans.”

Reichler continues: “This area where downwelling occurs is quite susceptible to cooling or warming from the troposphere. If the water is close to becoming heavy enough to sink, then even small additional amounts of heating or cooling from the atmosphere may be imported to the ocean and either trigger downwelling events or delay them.”

Because of that sensitivity, Reichler calls the sea south of Greenland “the Achilles heel of the North Atlantic.”

From Stratosphere to the Sea


In winter, the stratospheric Arctic polar vortex whirls counterclockwise around the North Pole, with the strongest, 80-mph winds at about 60 degrees north latitude. They are stronger than jet stream winds, which are less than 70 mph in the troposphere below.But every two years on average, the stratospheric air suddenly is disrupted and the vortex gets warmer and weaker, and sometimes even shifts direction to clockwise.

“These are catastrophic rearrangements of circulation in the stratosphere,” and the weaker or reversed polar vortex persists up to two months, Reichler says. “Breakdown of the polar vortex can affect circulation in the troposphere all the way down to the surface.”

Reichler’s study ventured into new territory by asking if changes in stratospheric polar vortex winds impart heat or cold to the sea, and how that affects the sea.

It already was known that that these stratospheric wind changes affect the North Atlantic Oscillation – a pattern of low atmospheric pressure centered over Greenland and high pressure over the Azores to the south. The pattern can reverse or oscillate.

Because the oscillating pressure patterns are located above the ocean downwelling area near Greenland, the question is whether that pattern affects the downwelling and, in turn, the global oceanic circulation conveyor belt.

The study’s computer simulations show a decadal on-off pattern of correlated changes in the polar vortex, atmospheric pressure oscillations over the North Atlantic and changes in sea circulation more than one mile beneath the waves. Observations are consistent with the pattern revealed in computer simulations.

Observations and Simulations of the Stratosphere-to-Sea Link


In the 1980s and 2000s, a series of stratospheric sudden warming events weakened polar vortex winds. During the 1990s, the polar vortex remained strong.

Reichler and colleagues used published worldwide ocean observations from a dozen research groups to reconstruct behavior of the conveyor belt ocean circulation during the same 30-year period.

“The weakening and strengthening of the stratospheric circulation seems to correspond with changes in ocean circulation in the North Atlantic,” Reichler says.

To reduce uncertainties about the observations, the researchers used computers to simulate 4,000 years worth of atmosphere and ocean circulation.

“The computer model showed that when we have a series of these polar vortex changes, the ocean circulation is susceptible to those stratospheric events,” Reichler says.

To further verify the findings, the researchers combined 18 atmosphere and ocean models into one big simulation, and “we see very similar outcomes.”

The study suggests there is “a significant stratospheric impact on the ocean,” the researchers write. “Recurring stratospheric vortex events create long-lived perturbations at the ocean surface, which penetrate into the deeper ocean and trigger multidecadal variability in its circulation. This leads to the remarkable fact that signals that emanate from the stratosphere cross the entire atmosphere-ocean system.”

The depths of winter: How much snow is in fact on the ground?

Stellar dendrites are tree-like snow crystals that have branches upon branches. -  Kenneth Libbrecht, Caltech
Stellar dendrites are tree-like snow crystals that have branches upon branches. – Kenneth Libbrecht, Caltech

Equipped with specialized lasers and GPS technology, scientists are working to address a critical wintertime weather challenge: how to accurately measure the amount of snow on the ground.

Transportation crews, water managers and others who make vital safety decisions need precise measurements of how snow depth varies across wide areas.

But traditional measuring devices such as snow gauges and yardsticks are often inadequate for capturing snow totals that may vary even within a single field or neighborhood.

Now scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and at other institutions are finding that prototype devices that use light pulses, satellite signals and other technologies offer the potential to almost instantly measure large areas of snow.

In time, such devices might provide a global picture of snow depth.

“We’ve been measuring rain accurately for centuries, but snow is much harder because of the way it’s affected by wind and sun and other factors,” says NCAR researcher Ethan Gutmann.

“It looks like new technology, however, will finally give us the ability to say exactly how much snow is on the ground.”

NCAR is conducting the effort with several collaborating organizations, including the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado Boulder.

The work is supported by NCAR’s sponsor, the National Science Foundation (NSF).

“Snow represents both a hazard and a water resource in the western states,” says Thomas Torgersen, NSF program director for hydrologic sciences. “Both require detailed assessments of snow amounts and depth. This technology will provide new and important guidance.”

Emergency managers rely on snowfall measurements when mobilizing snow plows or deciding whether to shut down highways and airports during major storms.

They also use snow totals when determining whether a region qualifies for disaster assistance.

In mountainous areas, officials need accurate reports of snowpack depth to assess the threat of avalanches or floods, and to anticipate the amount of water available from spring and summer runoff.

But traditional approaches to measuring snow can greatly underreport or overreport snow totals, especially in severe conditions.

Snow gauges may miss almost a third of the snow in a windy storm, even when they are protected by specialized fencing designed to cut down on the wind’s effects.

Snow probes or yardsticks can reveal snow depth within limited areas. But such tools require numerous in-person measurements at different locations, a method that may not keep up with totals during heavy snowfalls.

Weather experts also sometimes monitor the amount of snow that collects on flat, white pieces of wood known as snow boards, but this is a time-intensive approach that requires people to check the boards and clear them off every few hours.

The nation’s two largest volunteer efforts–the National Weather Service’s Cooperative Observer Program, and the Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS)–each involve thousands of participants nationwide using snow boards, but their reports are usually filed just once a day.

More recently, ultrasonic devices have been deployed in some of the world’s most wintry regions.

Much like radar, these devices measure the length of time needed for a pulse of ultrasonic energy to bounce off the surface of the snow and return to the transmitter.

However, the signal may be affected by shifting atmospheric conditions, including temperature, humidity and winds.

The specialized laser instruments under development at NCAR can correct for such problems.

Once set up at a location, they can automatically measure snow depth across large areas. Unlike ultrasonic instruments, lasers rely on light pulses that are not affected by atmospheric conditions.

New tests by Gutmann indicate that a laser instrument installed high above treeline in the Rocky Mountains west of Boulder can measure 10 feet or more of snow with an accuracy as fine as half an inch or better.

In a little more than an hour, the instrument measures snow at more than 1,000 points across an area almost the size of a football field to produce a three-dimensional image of the snowpack and its variations in depth.

Gutmann’s next step will be to build and test a laser instrument that can measure snow over several square miles. Tracking such a large area would require a new instrument capable of taking more than 12,000 measurements per second.

“If we’re successful, these types of instruments will reveal a continually-updated picture of snow across an entire basin,” he says.

One limitation for the lasers, however, is that light pulses cannot penetrate through objects such as trees and buildings.

This could require development of networks of low-cost laser installations that would each record snow depths within a confined area.

Alternatively, future satellites equipped with such lasers might be capable of mapping the entire world from above.

Gutmann and Kristine Larson, a scientist at the University of Colorado, are also exploring how to use GPS sensors for snowfall measurements.

GPS sensors record satellite signals that reach them directly and signals that bounce off the ground.

When there is snow on the ground, the GPS signal bounces off the snow with a different frequency than when it bounces off bare soil, enabling scientists to determine how high the surface of the snow is above the ground.

Such units could be a cost-effective way of measuring snow totals; meteorologists could tap into the existing global network of ground-based GPS receivers.

However, researchers are seeking to fully understand how the density of the snow and the roughness of its surface alter GPS signals.

“Our hope is to develop a set of high-tech tools that will enable officials to continually monitor snow depth, even during an intense storm,” Larson says.

“While we still have our work cut out for us, the technology is very promising.”

Northern Eurasian snowpack could be a predictor of winter weather in US, team from UGA reports

Every winter, weather forecasters talk about the snow cover in the northern U.S. and into Canada as a factor in how deep the deep-freeze will be in the states. A new study by researchers at the University of Georgia indicates they may be looking, at least partially, in the wrong place.

It turns out that snow piling up over a band of frozen tundra from Siberia to far-northern Europe may have as much effect on the climate of the U.S. as the much-better-known El Niño and La Niña.

The new work, just published in the International Journal of Climatology, reports that to understand how cold (or warm) the winter season will be in the U.S., researchers and weather forecasters should also take a closer look at snowpack in northern Eurasia laid down the previous October and November.

“To date, there had been no thorough examination of how snow cover from various regions of Eurasia influences North American winter temperatures,” said climatologist Thomas Mote of UGA’s department of geography and leader of the research. “The goal of this research was to determine whether there is a significant relationship between autumn snow extent in specific regions of Eurasia and temperatures across North America during the subsequent winter.”

Co-author of the paper was Emily Kutney, a former graduate student in Mote’s lab who has since earned her master’s degree and left UGA.

While other scientists have postulated that snow cover on the Eurasian landmass has a strong effect on winters in North America, the new study is the first to narrow down the location of the area that causes the most direct effect on U.S. winters-an area in northwest Eurasia that includes part of Siberia-though the entire effective area extends as far west as northern Scandinavia.

“One difficulty in comparing previous studies is that they have used multiple definitions of Eurasian snow cover,” said Mote. “Our work looked at the role of various key areas of Eurasian snow cover on atmospheric circulation, including the systems called the Arctic Oscillation and the Pacific/North American teleconnection.”

The findings have new significance for seasonal climate outlooks, which predict whether upcoming seasons will be colder or warmer, or wetter or drier than normal. Years with extensive autumn snow in northwest Eurasia were associated with subsequent winter temperatures as much as seven degrees (Fahrenheit) lower near the center of North America. This difference is roughly the same as a one-month shift in climate.

Such information can be crucial for everything from agricultural to daily life in areas that normally have brutal winters. The crucial time to look at the snow cover in Eurasia is during October and November in order to understand the upcoming winters in North America, said Mote.

Even more complexity enters the system of interrelated climate phenomena when looking at the possibility that sea ice in the Atlantic and Arctic Oceans might affect Eurasian snow cover and thus winters in North America.

“It’s interesting, because it implies to us that the potential impact of this new idea could be as large or larger than El Niño and La Niña events,” said Mote.

The new study is more about seasonal climate predictions than short-term modeling for weather.

Mote also led a team that reported in 2008 a dramatic rise in the rate of melt in the ice sheet of Greenland. He and colleagues found that it was 60 percent higher in 2007 than ever before recorded. Mote used a nearly 40-year record of satellite data to discover the dramatic melting.

Warm water causes extra-cold winters in northeastern North America and northeastern Asia

This map shows sea‑surface temperatures averaged over eight days in September 2001, as measured by NASA's Terra satellite. Dark red represents warm water (32 degrees Celsius) and purple is cold (‑2 degrees Celsius). The Gulf Stream can be seen as the orange strip extending from the eastern U.S. toward the Atlantic. -  Ronald Vogel, SAIC for NASA GSFC
This map shows sea‑surface temperatures averaged over eight days in September 2001, as measured by NASA’s Terra satellite. Dark red represents warm water (32 degrees Celsius) and purple is cold (‑2 degrees Celsius). The Gulf Stream can be seen as the orange strip extending from the eastern U.S. toward the Atlantic. – Ronald Vogel, SAIC for NASA GSFC

If you’re sitting on a bench in New York City’s Central Park in winter, you’re probably freezing. After all, the average temperature in January is 32 degrees Fahrenheit. But if you were just across the pond in Porto, Portugal, which shares New York’s latitude, you’d be much warmer-the average temperature is a balmy 48 degrees Fahrenheit.

Throughout northern Europe, average winter temperatures are at least 10 degrees Fahrenheit warmer than similar latitudes on the northeastern coast of the United States and the eastern coast of Canada. The same phenomenon happens over the Pacific, where winters on the northeastern coast of Asia are colder than in the Pacific Northwest.

Researchers at the California Institute of Technology (Caltech) have now found a mechanism that helps explain these chillier winters-and the culprit is warm water off the eastern coasts of these continents.

“These warm ocean waters off the eastern coast actually make it cold in winter-it’s counterintuitive,” says Tapio Schneider, the Frank J. Gilloon Professor of Environmental Science and Engineering.

Schneider and Yohai Kaspi, a postdoctoral fellow at Caltech, describe their work in a paper published in the March 31 issue of the journal Nature.

Using computer simulations of the atmosphere, the researchers found that the warm water off an eastern coast will heat the air above it and lead to the formation of atmospheric waves, drawing cold air from the northern polar region. The cold air forms a plume just to the west of the warm water. In the case of the Atlantic Ocean, this means the frigid air ends up right over the northeastern United States and eastern Canada.

For decades, the conventional explanation for the cross-oceanic temperature difference was that the Gulf Stream delivers warm water from the Gulf of Mexico to northern Europe. But in 2002, research showed that ocean currents aren’t capable of transporting that much heat, instead contributing only up to 10 percent of the warming.

Kaspi’s and Schneider’s work reveals a mechanism that helps create a temperature contrast not by warming Europe, but by cooling the eastern United States. Surprisingly, it’s the Gulf Stream that causes this cooling.

In the northern hemisphere, the subtropical ocean currents circulate in a clockwise direction, bringing an influx of warm water from low latitudes into the western part of the ocean. These warm waters heat the air above it.

“It’s not that the warm Gulf Stream waters substantially heat up Europe,” Kaspi says. “But the existence of the Gulf Stream near the U.S. coast is causing the cooling of the northeastern United States.”

The researchers’ computer model simulates a simplified, ocean-covered Earth with a warm region to mimic the coastal reservoir of warm water in the Gulf Stream. The simulations show that such a warm spot produces so-called Rossby waves.

Generally speaking, Rossby waves are large atmospheric waves-with wavelengths that stretch for more than 1,000 miles. They form when the path of moving air is deflected due to Earth’s rotation, a phenomenon known as the Coriolis effect. In a way similar to how gravity is the force that produces water waves on the surface of a pond, the Coriolis force is responsible for Rossby waves.

In the simulations, the warm water produces stationary Rossby waves, in which the peaks and valleys of the waves don’t move, but the waves still transfer energy. In the northern hemisphere, the stationary Rossby waves cause air to circulate in a clockwise direction just to the west of the warm region. To the east of the warm region, the air swirls in the counterclockwise direction. These motions draw in cold air from the north, balancing the heating over the warm ocean waters.

To gain insight into the mechanisms that control the atmospheric dynamics, the researchers speed up Earth’s rotation in the simulations. In those cases, the plume of cold air gets bigger-which is consistent with it being a stationary Rossby-wave plume. Most other atmospheric features would get smaller if the planet were to spin faster.

Although it’s long been known that a heat source could produce Rossby waves, which can then form plumes, this is the first time anyone has shown how the mechanism causes cooling that extends west of the heat source. According to the researchers, the cooling effect could account for 30 to 50 percent of the temperature difference across oceans.

This process also explains why the cold region is just as big for both North America and Asia, despite the continents being so different in topography and size. The Rossby-wave induced cooling depends on heating air over warm ocean water. Since the warm currents along western ocean boundaries in both the Pacific and Atlantic are similar, the resulting cold region to their west would be similar as well.

The next step, Schneider says, is to build simulations that more realistically reflect what happens on Earth. Future simulations would incorporate more complex features like continents and cloud feedbacks.

Wind can keep mountains from growing

Researchers sit atop a wind-formed ridge called a yardang located in the Qaidam Basin of Central Asia. The yardangs in that area can be as much as 40 meters (about 130 feet) tall and about a football field (100 meters) apart. -  Paul Kapp, University of Arizona.
Researchers sit atop a wind-formed ridge called a yardang located in the Qaidam Basin of Central Asia. The yardangs in that area can be as much as 40 meters (about 130 feet) tall and about a football field (100 meters) apart. – Paul Kapp, University of Arizona.

Wind is a much more powerful force in the evolution of mountains than previously thought, according to a new report from a University of Arizona-led research team.

Bedrock in Central Asia that would have formed mountains instead was sand-blasted into dust, said lead author Paul Kapp.

“No one had ever thought that wind could be this effective,” said Kapp, a UA associate professor of geosciences. “You won’t read in a textbook that wind is a major process in terms of breaking down rock material.”

Rivers and glaciers are the textbook examples of forces that wear down mountains and influence their evolution.

Wind can be just as powerful, Kapp said. He and his colleagues estimate wind can be 10 to 100 times more effective in eroding mountains than previously believed.

The team’s paper, “Wind erosion in the Qaidam basin, central Asia: implications for tectonics, paleoclimate, and the source of the Loess Plateau,” is in the April/May issue of GSA Today.

Kapp’s co-authors are Jon D. Pelletier and Joellen Russell of the UA; Alexander Rohrmann, formerly of the UA and now at the University of Potsdam in Germany; Richard Heermance of California State University, Northridge; and Lin Ding of the Chinese Academy of Sciences, Beijing. The American Chemical Society Petroleum Research Fund and a UA Faculty Small Grant funded the research.

The geoscientists figured out wind’s rock-sculpting abilities by studying gigantic wind-formed ridges of rock called yardangs.

Kapp first learned about yardangs when reviewing a scientific paper about Central Asia’s Qaidam Basin. To see the geology for himself, he booted up Google Earth — and was wowed by what he saw.

“I’d never seen anything like that before,” he said. “I didn’t even know what a yardang was.”

Huge fields of yardangs that can be seen from space look like corduroy. Wind had scoured long gouges out of the bedrock, leaving the keel-shaped ridges behind. Kapp wondered where the missing material was.

The team’s initial research was conducted using geological maps of the region and satellite images from Google Earth. Then Kapp and his team went to the Qaidam Basin to collect more information about the yardangs, the history of wind erosion and the dust.

“What we’re proposing is that during the glacials, when it’s colder and drier, there’s severe wind erosion in the Qaidam basin and the dust gets blown out and deposited downwind in the Loess Plateau,” Kapp said.

The term “loess” refers to deposits of wind-blown silt. Parts of the U.S. Midwest have large deposits of loess.

“Up until 3 million years ago, the basin was filling up with sediment,” he said. “Then like a switch, the wind turned on and basin sediments get sandblasted away.”

Known as the “bread basket of China,” the Loess Plateau is the largest accumulation of dust on Earth. Scientists thought most of the dust came from the Gobi Desert.

In contrast, Kapp and his colleagues suggest more than half of the dust came from the Qaidam Basin. Co-author Pelletier, a UA geomorphologist, created a computer model indicating that dust from the basin could have formed the plateau.

The wind is not having such effects now because the climate is different, Kapp said. Co-author Russell plus other research groups suggest the westerly winds shift north during interglacial periods like that of the current climate and shift toward the equator during glacial periods.

Therefore since the last Ice Age ended about 11,000 years ago, the winds have blown from the Gobi Desert toward the Loess Plateau. During glacial periods, the winds blew from the Qaidam basin toward the Loess Plateau instead.

“During the interglacials, the basin fills up with lakes. ? When it goes back to a glacial period, lake sediments blow away,” he said. “Our hypothesis is that you have lake development, then wind erosion, lake development, wind erosion, lake development – and so on.”

The team suggests wind erosion also influenced how fast the basin’s bedrock is folded. In Central Asia, bedrock folds and crumples because it’s being squeezed as the Indian plate collides with the Asian plate.

“The folding accelerated 3 million years ago,” Kapp said. “That’s when the wind erosion turned on. I don’t think it’s a coincidence.”

During the glacial periods, the winds whisked sediment out of the basin. As a result, the bedrock deformed faster because it was no longer weighed down by all the sediment.

Kapp calls the process “wind-enhanced tectonics.” The term “tectonics” refers to forces that cause movements and deformation of the Earth’s plates.

The whole process is driven by global climate change, he said. “The unifying theme is wind.”

Kapp and his team are quantifying the processes further as they analyze more samples they brought back from the Qaidam basin and Loess Plateau.

Volcanic eruptions affect rainfall over Asian monsoon region

This photo shows an 18-kilometer-high plume from one of a series of eruptions in 1991 at Mount Pinatubo. -  USGS
This photo shows an 18-kilometer-high plume from one of a series of eruptions in 1991 at Mount Pinatubo. – USGS

Scientists have long known that large volcanic explosions can affect the weather by spewing particles that block solar energy and cool the air.

Some suspect that extended “volcanic winters” from gigantic eruptions helped kill off dinosaurs and Neanderthals.

In the summer following Indonesia’s 1815 Tambora eruption, frost wrecked crops as far away as New England, and the 1991 blowout of the Philippines’ Mount Pinatubo lowered average global temperatures by 0.7 degrees F–enough to mask the effects of greenhouse gases for a year or so.

Now, in research funded by the National Science Foundation (NSF)’s Division of Atmospheric and Geospace Sciences, scientists have discovered that eruptions also affect rainfall over the Asian monsoon region, where seasonal storms water crops for nearly half of Earth’s population.

Tree-ring researchers at Columbia University’s Lamont-Doherty Earth Observatory (LDEO) showed that big eruptions tend to dry up much of Central Asia, but bring more rain to southeast Asian countries including Vietnam, Laos, Cambodia, Thailand and Myanmar–the opposite of what many climate models predict.

A paper reporting their results appears in an advance online version of the American Geophysical Union (AGU) journal Geophysical Research Letters.

The growth rings of some tree species can be correlated with rainfall. LDEO’s Tree Ring Lab used tree rings from some 300 sites across Asia to measure the effects of 54 volcanic eruptions going back about 800 years.

The data came from LDEO’s new 1,000-year tree-ring atlas of Asian weather, which has produced evidence of long, devastating droughts.

“We might think of the solid Earth and the atmosphere as two different things, but everything in the system is interconnected,” said Kevin Anchukaitis, the paper’s lead author. “Volcanoes can be important players in climate over time.”

Large explosive eruptions send up sulfur compounds that turn into tiny sulfate particles high in the atmosphere, where they deflect solar radiation.

The resulting cooling on Earth’s surface can last for months or years.

Not all eruptions have that effect, however. For instance, the continuing eruption of Indonesia’s Merapi this fall has killed dozens, but this latest episode is probably not big enough by itself to effect large-scale weather changes, scientists believe.

As for rainfall, in the simplest models, lowered temperatures decrease evaporation of water from the surface to the air. Less water vapor translates to less rain.

But matters are greatly complicated by atmospheric circulation patterns, cyclic changes in temperatures over the oceans, and the shapes of land masses.

Until now, most climate models incorporating changes in the sun and the atmosphere have predicted that volcanic explosions would disrupt the monsoon by bringing less rain to southeast Asia–but the researchers found the opposite.

They studied several eruptions, including one in 1258 from an unknown tropical site, thought to be the largest of the last millennium; the 1600-1601 eruption of Peru’s Huaynaputina; Tambora in 1815; the 1883 explosion of Indonesia’s Krakatau; Mexico’s El Chichón in 1982; and Pinatubo.

Tree rings showed that huge swaths of southern China, Mongolia and surrounding areas consistently dried up in the year or two following big events, while mainland southeast Asia received increased rain.

The researchers believe there are many possible factors involved.

“The data only recently became available to test the models,” said Rosanne D’Arrigo, one of the paper’s co-authors. “There’s a lot of work to be done to understand how all these different forces interact.”

In some episodes pinpointed by the study, it appears that strong cycles of the El Niño-Southern Oscillation, which drives temperatures over the Pacific and Indian Oceans and is thought to strongly affect the Asian monsoon, might have counteracted eruptions, lessening their drying or moistening effects. p>

But it could work the other way, too, said Anchukaitis; if atmospheric dynamics and volcanic eruptions come together with the right timing, they could reinforce one another–with drastic results.

“Then you get flooding or drought, and neither flooding nor drought is good for the people living in those regions,” he said.

Ultimately, said Anchukaitis, such studies should help scientists refine models of how natural and man-made forces might act together to shift weather patterns–a vital question for all areas of the world.

Earthquake Season in the Himalayan Front


Scientists have long searched for what triggers earthquakes, even suggesting that tides or weather play a role. Recent research spearheaded by Jean-Philippe Avouac, professor of geology and director of the Tectonics Observatory at the California Institute of Technology, shows that in the Himalayan mountains, at least, there is indeed an earthquake season. It’s winter.



For decades, geologists studying earthquakes in the Himalayan range of Nepal had noted that there were far more quakes in the winter than in the summer, but it was difficult to assign a cause. “The seasonal variation in seismicity had been noticed years ago,” says Avouac. Now, over a decade of data from GPS receivers and satellite measurements of land-water storage make it possible to connect the monsoon season with the frequency of earthquakes along the Himalaya front. The analysis also provides key insight into the timescale of earthquake nucleation in the region.



Avouac will present the results of the study on December 12 at the annual meeting of the American Geophysical Union (AGU) in San Francisco. They are also available online through the journal Earth and Planetary Science Letters, and will appear in print early next year.



The world’s tallest mountain range, the Himalaya continues to rise as plate tectonic activity drives India into Eurasia. The compression from this collision results in intense seismic activity along the front of the range. Stress builds continually along faults in the region, until it is released through earthquakes.



Avouac and two collaborators from France and Nepal–Laurent Bollinger and Sudhir Rajaure–began their earthquake seasonality investigation by analyzing a catalog of around 10,000 earthquakes in the Himalaya. They saw that, at all magnitudes above this detection limit, there were twice as many earthquakes during the winter months–December through February–as during the summer. That is, in winter there are up to 150 earthquakes of magnitude three per month, and in summer, around 75. For magnitude four, the winter average is 16 per month, while in summer the rate falls to eight per month. They ran the numbers through a statistical calculation and ruled out the possibility that the seasonal signal was due merely to chance.



“The signal in the seismicity is real; there is no discussion,” Avouac says. “We see this seasonal cycle,” he adds. “We didn’t know where it came from but it is really strong. We’re looking at something that is changing on a yearly basis-the timescale over which stress changes in this region is one year.”



Earlier studies suggested that seasonal variations in atmospheric pressure set off earthquakes, and this had been proposed for seasonal seismicity following the 1992 Landers, California, quake.



The scientists turned to satellite measurements of water levels in the region. Using altimetry data from TOPEX/Poseidon, a satellite launched in 1992 by NASA and the French space agency CNES (Centre National d’Etudes Spatiales), they evaluated the water level in major rivers of the Ganges basin to within a few tens of centimeters. They found that the water level over the whole basin begins its four-meter rise at the onset of the monsoon season in mid-May, reaching a maximum in September, followed by a slow decrease until the next monsoon season.


They combined river level measurements with data from NASA’s GRACE–Gravity Recovery and Climate Experiment–mission, which studies, among other things, groundwater storage on landmasses. The data revealed a strong signal of seasonal variation of water in the basin. Paired with the altimetry data, these measurements paint a complete picture of the hydrologic cycle in the region.



In the Himalaya, monsoon rains swell the rivers of the Ganges basin, increasing the pressure bearing down on the region. As the rains stop, the river water soaks through the ground and the built-up load eases outward, toward the front of the range. This outward redistribution of stress after the rains end leads to horizontal compression in the mountain range later in the year, triggering the wintertime earthquakes.



The final piece connecting winter earthquake frequency to season, and lending insight into the process by which earthquakes nucleate, lay in GPS data. Installation of GPS instruments across the Himalayan front began in 1994, and now they provide a decade’s worth of measurements showing land movement across the region. Instead of looking at vertical motions, which are widely believed to be sensitive to weather and the same forces that cause tides on Earth, the scientists concentrated on horizontal displacements. The lengthy records, analyzed by Pierre Bettinelli during his graduate work at Caltech, show that horizontal motion is continuous in the range front. Stress constantly builds in the region. But just as water levels near their lowest in the adjacent Ganges basin and earthquakes begin their doubletime, horizontal motion reaches its maximum speed.



“We had been staring at [the seasonal signal] for years, and then the satellite data came in and we deployed the GPS network and suddenly it became crystal clear,” says Avouac. “It’s like something you dream of.”



While many scientists have suggested that changing water levels can influence the earthquake cycle, a definitive mechanism had yet to be pinpointed. “There are two main avenues by which people have tried to understand the physics of earthquakes: Earth tides and aftershocks,” says Avouac. With the water level data, he could show that the rate at which stress builds along the rangefront, rather than the absolute level of stress, triggers earthquakes.



Although Earth tides induce stress levels similar to what builds up during seasonal water storage, they only vary over a 12-hour period. The Himalayan signal shows that it is more likely that earthquakes are triggered after stress builds for weeks to months, which matches the timescale of seasonal stress variation in that region.



About other earthquake-prone regions Avouac says, “seasonal variation has been reported in other places, but I don’t know any other place where it is so strong or where the cause of the signal is so obvious.”



Other authors on the paper are Pierre Bettinelli, Mireille Flouzat, and Laurent Bollinger of the Commissariat a l’Énergie Atomique, France; Guillaume Ramillien of the Laboratoire d’Etudes en Géophysique et Océanographie Spatiales, France; and Sudhir Rajaure and Som Sapkota of the National Seismological Centre in Nepal.



Avouac will present details of the group’s findings at AGU on Wednesday, December 12, at 2 p.m., Moscone West room 3018, in session T33F: Earthquake geology, active tectonics, and mountain building in south and east Asia.

Geologists Compile Longest Ever Record of Atlantic Hurricane Strikes





Graduate student Jonathan Woodruff of the WHOI Geology and Geophysics Department works to sink a coring tube into the sediments beneath Laguna Playa Grande in Vieques, Puerto Rico. - Photo Credit: Jeff Donnelly, Woods Hole Oceanographic Institution
Graduate student Jonathan Woodruff of the WHOI Geology and Geophysics Department works to sink a coring tube into the sediments beneath Laguna Playa Grande in Vieques, Puerto Rico. – Photo Credit: Jeff Donnelly, Woods Hole Oceanographic Institution

The frequency of intense hurricanes in the Atlantic Ocean appears to be closely connected to long-term trends in the El Niño/Southern Oscillation (ENSO) and the West African monsoon, according to new research from the Woods Hole Oceanographic Institution (WHOI). Geologists Jeff Donnelly and Jonathan Woodruff made that discovery while assembling the longest-ever record of hurricane strikes in the Atlantic basin.



Donnelly and Woodruff began reconstructing the history of land-falling hurricanes in the Caribbean in 2003 by gathering sediment-core samples from Laguna Playa Grande on Vieques (Puerto Rico), an island extremely vulnerable to hurricane strikes. They examined the cores for evidence of storm surges—distinctive layers of coarse-grained sands and bits of shell interspersed between the organic-rich silt usually found in lagoon sediments—and pieced together a 5,000-year chronology of land-falling hurricanes in the region.



In examining the record, they found large and dramatic fluctuations in hurricane activity, with long stretches of frequent strikes punctuated by lulls that lasted many centuries. The team then compared their new hurricane record with existing paleoclimate data on El Niño, the West African monsoon, and other global and regional climate influences. They found the number of intense hurricanes (category 3, 4, and 5 on the Saffir-Simpson scale) typically increased when El Niño was relatively weak and the West African monsoon was strong.



“The processes that govern the formation, intensity, and track of Atlantic hurricanes are still poorly understood,” said Donnelly, an associate scientist in the WHOI Department of Geology and Geophysics. “Based on this work, we now think that there may be some sort of basin-wide ‘on-off switch’ for intense hurricanes.”



Donnelly and Woodruff published their latest results in the May 24 issue of the journal Nature.



Donnelly and his colleagues have pioneered efforts to extend the chronology of hurricane strikes beyond what can be found in historical texts and modern meteorological records and previously applied their methods to the New England and the Mid-Atlantic coasts of the United States.



Their research area, Laguna Playa Grande, is protected and separated from the ocean during all but the most severe tropical storms. However, when an intense hurricane strikes the region, storm surges carry sand from the ocean beach over the dunes and into Laguna Playa Grande. Such “over-topping” events leave markers in the geological record that can be examined by researchers in sediment core samples.



The geological record from Vieques showed that there were periods of more frequent intense hurricanes from 5,000 to 3,600 years ago, from 2,500 to 1,000 years ago, and from 1700 AD to the present. By contrast, the island was hit less often from 3,600 to 2,500 years ago and from 1,000 to 300 years ago.


To ensure that what they were seeing was not just a change in the direction of hurricanes away from Vieques—that is, different storm tracks across the Atlantic and Caribbean—the scientists compared their new records with previous studies from New York and the Gulf Coast. They saw that the Vieques record matched the frequency of land-falling hurricanes in New York and Louisiana, indicating that some Atlantic-wide changes took place.



Donnelly and Woodruff, a doctoral student in the MIT/WHOI Joint Graduate Program, then decided to test some other hypotheses about what controls the strength and frequency of hurricanes. They found that periods of frequent El Niño in the past corresponded with times of less hurricane intensity. Other researchers have established that, within individual years, El Niño can stunt hurricane activity by causing strong winds at high altitudes that shear the tops off hurricanes or tip them over as they form. When El Niño was less active in the past, Donnelly and Woodruff found, hurricane cycles picked up.



The researchers also examined precipitation records from Lake Ossa, Cameroon, and discovered that when there were increased monsoon rains, there were more frequent intense hurricanes on the other side of the Atlantic. Researchers have theorized that frequent and stronger storms over western Africa lead to easterly atmospheric waves moving into the Atlantic to provide the “seedlings” for hurricane development.



Much media attention has been focused recently on the importance of warmer ocean waters as the dominant factor controlling the frequency and intensity of hurricanes. And indeed, warmer sea surface temperatures provide more fuel for the formation of tropical cyclones. But the work by Donnelly and Woodruff suggests that El Niño and the West African monsoon appear to be critical factors for determining long-term cycles of hurricane intensity in the Atlantic.



The research by Donnelly and Woodruff was funded by the National Science Foundation, the Risk Prediction Initiative, the National Geographic Society, the WHOI Coastal Ocean Institute, and the Andrew W. Mellon Foundation.



The Woods Hole Oceanographic Institution is a private, independent organization in Falmouth, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the ocean’s role in the changing global environment.