Pacific wind pattern driving el Niño slows due to global warming
May 3, 2006
Global warming has caused a key wind circulation pattern over the Pacific Ocean to weakened by 3.5% since the mid-1800s and scientists warn that it be further diminished by another 10% by 2100.
The Walker circulation, an atmospheric circulation of air at the equatorial Pacific Ocean which spans almost half the circumference of Earth, pushes the Pacific Ocean's trade winds from east to west, generating rainfall in Indonesia while creating ocean upwelling off the coast of South America that nourishes marine life. Scientists with the National Center for Atmospheric Research (UCAR) say that "changes in the circulation, which varies in tandem with El Niño and La Niña events, can have far—reaching effects."
"The Walker circulation is fundamental to climate across the globe," says lead author Gabriel Vecchi, a scientist with the University Corporation for Atmospheric Research (UCAR).
Vecchi, along with Brian Soden (University of Miami) and the Geophysical Fluid Dynamics Laboratory team of Andrew Wittenberg, Isaac Held, Ants Leetmaa, and Matthew Harrison, used computer climate model simulations to verify the slowdown and determine whether the cause is human-induced climate change.
A news release describing their research appears below.
- - -
This diagram shows the Walker Circulation, a vast loop of air above the equatorial Pacific Ocean. See below for an alternate depiction. (Illustration by Gabriel Vecchi, UCAR.)
45% chance Gulf Stream current will collapse by 2100 finds research
New research indicates there is a 45 percent chance that the thermohaline circulation in the North Atlantic Ocean could shut down by the end of the century if nothing is done to slow greenhouse gas emissions. Even with immediate climate policy action, say scientists, there would still be a 25 percent probability of a collapse of the system of currents that keep western Europe warmer than regions at similar latitudes in other parts of the world.
Change in Atlantic circulation could plunge Europe into cold winters
The Atlantic Ocean circulation that carries warm waters north and returns cold waters south is slowing, putting Europe at risk of colder temperatures, according to research published in Nature. The Atlantic Heat Conveyor, the system of currents in the Atlantic Ocean that result in a net transport of warm water into the northern hemisphere, keeps western Europe warmer than regions at similar latitudes in other parts of the world. A weakening of the system, which includes the Gulf Stream, could cause a cooling in northwest Europe.
Tropical Atlantic cooling and deforestation correlate to drought in Africa
Against the backdrop of the Montreal Summit on global climate being held this week, an article on African droughts and monsoons, by a University of California, Santa Barbara scientist and others, which appears in the December issue of the journal Geology, underlines concern about the effects of global climate change. Tropical ocean temperatures and land vegetation have an important effect on African monsoon systems, explains first author Syee Weldeab, a post-doctoral fellow in the Department of Earth Science at the University of California, Santa Barbara. The monsoons are critical to sustaining agriculture in equatorial Africa.
May 3, 2006
BOULDER, Colorado—The vast loop of winds that drives climate and ocean behavior across the tropical Pacific has weakened by 3.5% since the mid-1800s, and it may weaken another 10% by 2100, according to a study led by University Corporation for Atmospheric Research (UCAR) scientist Gabriel Vecchi. The study indicates that the only plausible explanation for the slowdown is human-induced climate change. The findings appear in the May 4 issue of Nature.
The Walker circulation, which spans almost half the circumference of Earth, pushes the Pacific Ocean's trade winds from east to west, generates massive rains near Indonesia, and nourishes marine life across the equatorial Pacific and off the South American coast. Changes in the circulation, which varies in tandem with El Niño and La Niña events, can have far—reaching effects.
"The Walker circulation is fundamental to climate across the globe," says Vecchi.
In their paper, "Weakening of Tropical Pacific Atmospheric Circulation Due to Anthropogenic Forcing," the authors used observations as well as state-of-the-art computer climate model simulations to verify the slowdown and determine whether the cause is human-induced climate change. The work was performed at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL), where Vecchi is stationed through the UCAR Visiting Scientist Programs. His coauthors include Brian Soden (University of Miami) and the GFDL team of Andrew Wittenberg, Isaac Held, Ants Leetmaa, and Matthew Harrison.
"This could have important effects on ocean ecosystems," Vecchi says. "The ocean currents driven by the trade winds supply vital nutrients to the near-surface ocean ecosystems across the equatorial Pacific, which is a major fishing region."
Matching theory and observations
Several theoretical studies have shown that an increase in greenhouse gases should produce a weakening of the Walker circulation. As temperatures rise and more water evaporates from the ocean, water vapor in the lower atmosphere increases rapidly. But physical processes prevent precipitation from increasing as quickly as water vapor. Since the amount of water vapor brought to the upper atmosphere must remain in balance with precipitation, the rate at which moist air is brought from the lower to the upper atmosphere slows down to compensate. This leads to a slowing of the atmospheric circulation.
Based on observations since the mid-1800s, the paper reports a 3.5% slowdown in the Walker circulation, which corresponds closely to the number predicted by theory. To establish whether human-induced climate change is at work, Vecchi and colleagues analyzed 11 simulations using the latest version of the GFDL climate model spanning the period 1861 to 2000. Some of the simulations included the observed increase in greenhouse gases; others included just the natural climate-altering factors of volcanic eruptions and solar variations. Only the simulations that included an increase in greenhouse gases showed the Walker circulation slowing, and they did so at a rate consistent with the observations.
This diagram and the one at top show two different views of the Walker Circulation, a vast loop of air above the equatorial Pacific Ocean. (Illustration by Gabriel Vecchi, UCAR.)
Simulation results depend on the assumptions and conditions within different models. However, the agreement of theory, observations, and models for the past 150 years lends support to this outlook, say the authors.
What about El Niño?
The study sends mixed signals on the future of the El Niño—Southern Oscillation—the system of ocean-atmosphere linkages that produces the worldwide weather of El Niño and its counterpart, La Niña.
"The circulation has been tending to a more El Niño-like state since the 1860s," says Vecchi. "However, the dynamics involved here are distinct from those of El Niño."
This article contains a modified news release from UCAR.