The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow in the

tropics The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

in the lower atmosphere (

troposphere The troposphere is the lowest layer of the atmosphere of Earth. It contains 80% of the total mass of the Atmosphere, planetary atmosphere and 99% of the total mass of water vapor and aerosols, and is where most weather phenomena occur. From the ...

). According to this model, parcels of air follow a closed circulation in the zonal and vertical directions. This circulation, which is roughly consistent with observations, is caused by differences in heat distribution between ocean and land. In addition to motions in the zonal and vertical direction the tropical atmosphere also has considerable motion in the meridional direction as part of, for example, the Hadley Circulation. The Walker circulation is associated with the pressure gradient force that results from a high pressure system over the eastern Pacific Ocean, and a low pressure system over

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...

. The Walker circulations of the tropical Indian, Pacific, and Atlantic basins result in westerly surface winds in northern summer in the first basin and easterly winds in the second and third basins. As a result, the temperature structure of the three oceans display dramatic asymmetries. The equatorial Pacific and Atlantic both have cool surface temperatures in northern summer in the east, while cooler surface temperatures prevail only in the western Indian Ocean. These changes in surface temperature reflect changes in the depth of the thermocline. Changes in the Walker circulation with time occur in conjunction with changes in surface temperature. Some of these changes are forced externally, such as the seasonal shift of the sun into the Northern Hemisphere in summer. Other changes appear to be the result of coupled ocean-atmosphere feedback in which, for example, easterly winds cause the sea surface temperature to fall in the east, enhancing the zonal heat contrast and hence intensifying easterly winds across the basin. These anomalous easterlies induce more equatorial

upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted sur ...

and raise the thermocline in the east, amplifying the initial cooling by the southerlies. From an oceanographic point of view, the equatorial cold tongue is caused by easterly winds. Were the Earth climate symmetric about the equator, cross-equatorial wind would vanish, and the cold tongue would be much weaker and have a very different zonal structure than is observed today. The Walker circulation was discovered by Gilbert Walker. The term "Walker circulation" was coined in 1969 by the Norwegian-American meteorologist Jacob Bjerknes.

Walker's methodology

Gilbert Walker was an established applied mathematician at the

University of Cambridge The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...

when he became director-general of observatories in India in 1904. While there, he studied the characteristics of the

Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...

, the failure of whose rains had brought severe

famine A famine is a widespread scarcity of food caused by several possible factors, including, but not limited to war, natural disasters, crop failure, widespread poverty, an Financial crisis, economic catastrophe or government policies. This phenom ...

to the country in 1899. Analyzing vast amounts of weather data from India and the rest of the world, over the next fifteen years he published the first descriptions of the great seesaw oscillation of atmospheric pressure between the Indian and

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five Borders of the oceans, oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is ...

, and its correlation to

temperature Temperature is a physical quantity that quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

and rainfall patterns across much of the Earth's tropical regions, including India. He also worked with the Indian Meteorological Department especially in linking the monsoon with Southern Oscillation phenomenon. He was made a Companion of the Order of the Star of India in 1911. Walker determined that the time scale of a year (used by many studying the atmosphere) was unsuitable because geospatial relationships could be entirely different depending on the season. Thus, Walker broke his temporal analysis into December–February, March–May, June–August, and September–November. Walker then selected a number of "centers of action", which included areas such as the Indian Peninsula. The centers were in the hearts of regions with either permanent or seasonal high and low pressures. He also added points for regions where rainfall, wind or temperature was an important control. He examined the relationships of the summer and winter values of pressure and rainfall, first focusing on summer and winter values, and later extending his work to the spring and autumn. He concludes that variations in temperature are generally governed by variations in pressure and rainfall. It had previously been suggested that sunspots could be the cause of the temperature variations, but Walker argued against this conclusion by showing monthly correlations of sunspots with temperature, winds, cloud cover, and rain that were inconsistent. Walker made it a point to publish all of his correlation findings, both of relationships found to be important as well as relationships that were found to be unimportant. He did this for the purpose of dissuading researchers from focusing on correlations that did not exist.

Oceanic effects

The Walker Circulations of the tropical Indian, Pacific, and Atlantic basins result in westerly surface winds in Northern Summer in the first basin and easterly winds in the second and third basins. As a result, the temperature structure of the three oceans display dramatic asymmetries. The equatorial Pacific and Atlantic both have cool surface temperatures in Northern Summer in the east, while cooler surface temperatures prevail only in the western Indian Ocean. These changes in surface temperature reflect changes in the depth of the thermocline. Changes in the Walker Circulation with time occur in conjunction with changes in surface temperature. Some of these changes are forced externally, such as the seasonal shift of the Sun into the Northern Hemisphere in summer. Other changes appear to be the result of coupled ocean-atmosphere feedback in which, for example, easterly winds cause the sea surface temperature to fall in the east, enhancing the zonal heat contrast and hence intensifying easterly winds across the basin. These enhanced easterlies induce more equatorial upwelling and raise the thermocline in the east, amplifying the initial cooling by the southerlies. This coupled ocean-atmosphere feedback was originally proposed by Bjerknes. From an oceanographic point of view, the equatorial cold tongue is caused by easterly winds. Were the earth climate symmetric about the equator, cross-equatorial wind would vanish, and the cold tongue would be much weaker and have a very different zonal structure than is observed today. The Walker cell is indirectly related to

off the coasts of

Peru Peru, officially the Republic of Peru, is a country in western South America. It is bordered in the north by Ecuador and Colombia, in the east by Brazil, in the southeast by Bolivia, in the south by Chile, and in the south and west by the Pac ...

and

Ecuador Ecuador, officially the Republic of Ecuador, is a country in northwestern South America, bordered by Colombia on the north, Peru on the east and south, and the Pacific Ocean on the west. It also includes the Galápagos Province which contain ...

. This brings

nutrient A nutrient is a substance used by an organism to survive, grow and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

-rich cold water to the surface, increasing fishing stocks.Jennings, S., Kaiser, M.J., Reynolds, J.D. (2001) "Marine Fisheries Ecology." Oxford: Blackwell Science Ltd.

El Niño–Southern Oscillation

The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern Pacific Ocean and a low pressure system over

. The Walker circulation causes an upwelling of cold deep sea water, thus cooling the sea surface. El Niño results when this circulation decreases or stops, as the impaired or inhibited circulation causes the ocean surface to warm to above average temperatures. A markedly increased Walker circulation causes a La Niña by intensifying the upwelling of cold deep sea water; which cools the sea surface to below average temperatures. During non-El Niño conditions, the Walker circulation is seen at the surface as easterly trade winds that move water and air warmed by the sun toward the west. This also creates ocean upwelling off the coasts of

and

and brings nutrient-rich cold water to the surface, increasing fishing stocks.Jennings, S., Kaiser, M.J., Reynolds, J.D. (2001) "Marine Fisheries Ecology." Oxford: Blackwell Science Ltd. The western side of the equatorial Pacific is characterized by warm, wet, low-pressure weather as the collected moisture is dumped in the form of

typhoons A typhoon is a tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere and which produces sustained hurricane-force winds of at least . This region is referred to as the Northwestern Pacific Basin, accounting for a ...

and thunderstorms. The ocean is some higher in the western Pacific as the result of this motion. A scientific study published in May 2006 in the journal ''

Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...

'' indicates that the Walker circulation has been slowing since the mid-19th century. The authors argue that

global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...

is a likely causative factor in the weakening of the wind pattern. However, a 2011 study from The Twentieth Century Reanalysis Project shows that, aside from El Niño–Southern Oscillation cycles, the overall speed and direction of the Walker circulation remained steady between 1871 and 2008.The Twentieth Century Reanalysis Project. Quarterly Journal of the Royal Meteorological Society, 137: 1–28. , http://onlinelibrary.wiley.com/doi/10.1002/qj.776/abstract

References

General references

* Walker Institute, University of Reading, UK. http://www.walker-institute.ac.uk/about/sir_gilbert.htm * Walker, JM. Pen Portrait of Sir Gilbert Walker, CSI, MA, ScD, FRS. Weather 1997 (Volume 52, No.7, pp. 217–220) * Walker, G.T. and Bliss, E.W., 1930. World Weather IV, Memoirs of the Royal Meteorological Society, 3, (24), 81–95. * Walker, G.T. and Bliss, E.W., 1937. World Weather VI, Memoirs of the Royal Meteorological Society, 4, (39), 119–139. * Walker, G.T., 1923. Correlation in seasonal variations of weather, VIII. A preliminary study of world weather. Memoirs of the India Meteorological Department, 24, (4), 75–131. * Walker, G.T., 1924. Correlation in seasonal variations of weather, IX. A further study of world weather. Memoirs of the India Meteorological Department, 24, (9),275–333. http://www.rmets.org/about/history/classics.php * Katz, R.W. Sir Gilbert Walker and a Connection between El Nino and Statistics. Statistical Science, 17 (2002), 97–117. http://amath.colorado.edu/courses/4540/2004Spr/walkerss.pdf * Climate research summary �
Walker Circulation: a tropical atmospheric circulation slow-down
Text and graphics from

NOAA The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with forecasting weather, monitoring oceanic and atmospheric conditions, charting the seas, conducting deep-sea exploratio ...

/ Geophysical Fluid Dynamics Laboratory
Slowdown in tropical Pacific flow pinned on climate change
– press release from University Corporation for Atmospheric Research.
Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing
4 May 2006 in

. * Associated Press news story, 3 May 2006: "Global Warming Cited in Wind Shift"
Tropical convective transport and the Walker circulation
29 October 2012 in Atmospheric Chemistry and Physics {{DEFAULTSORT:Walker Circulation Tropical meteorology fr:Circulation atmosphérique#Circulation de Walker

Walker's methodology

Oceanic effects

El Niño–Southern Oscillation

See also

References

General references