The Walker circulation, also known as the Walker cell, is a conceptual model of the air flow in the tropics in the lower atmosphere (troposphere).  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.  It was discovered by Gilbert Walker.  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 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 when he became director-general of observatories in India in 1904. While there, he studied the characteristics of the Indian Ocean monsoon, the failure of whose rains had brought severe famine 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, and its correlation to temperature 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 upwelling off the coasts of Peru and Ecuador. This brings nutrient-rich cold water to the surface, increasing fishing stocks.

El Niño 

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 Indonesia. 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.

A scientific study published in May 2006 in the journal Nature indicates that the Walker circulation has been slowing since the mid-19th century. The authors argue that global warming 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.

See also
Atmospheric circulation
Earth's atmosphere

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 / 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 Nature.
 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

Tropical meteorology

fr:Circulation atmosphérique#Circulation de Walker
워커 순환() 또는 워커 세포()는 대류권의 공기 흐름에 대한 개념적 과학 모형으로, 공기 덩어리가 일정한 지역에서 닫힌 형태로 순환을 이룬다는 것이다. 워커 순환은 대륙과 해양 사이의 열 분배 차이에 의해 발생한다. 워커 순환은 관측 결과와 대략 부합하나, 실제는 자오선 방향으로의 공기 이동 또한 일어나, 해들리 순환의 형태를 띈다.

워커 순환은 길버트 워커가 처음 제안하였으며, 워커 순환이라는 용어는 1969년 기상학자 야콥 비에르크네스가 만들었다.

워커의 방법론 
길버트 워커는 케임브리지 대학교 소속의 저명한 응용수학자였다. 워커는 1904년 인도의 관측소 소장을 맡게 되었는데, 여기서 인도양 계절풍과 이로 인한 1899년~1900년 인도 대기근을 연구하였다. 워커는 15년 간 인도와 세계 각지의 기상 자료를 분석한 후, 인도와 태평양 지역 기압의 진동과, 열대 지방의 온도 및 강우 형태의 관련성을 발표하였다. 이후 워커는 인도 기상국에서 근무하며, 계절풍과 남방진동 현상을 연결하는 연구를 진행했다. 1911년에는 인도의 별 훈장을 수여받았다.

워커는 계절에 따라 국부적 관련성이 달라질 수 있다는 것을 기반으로, 당시 기상학 연구에서 사용하던 연 단위 분석법 대신, 1년을 네 계절(12월~2월, 3월~5월, 6월~8월, 9월~11월)로 나눠 분석하였다. 지역적으로는 '행동 중심'을 지정했는데, 이 중심은 항상 고기압이나 저기압이 형성되는 지점, 계절에 따라 고기압이나 저기압이 바뀌는 지점, 강우나 풍향이 중요한 지점이었다. 워커는 계절별로 기압과 강우량의 관계를 분석하였으며, 그 결과 온도의 변화는 기압과 강우의 변화에 의한 것이라는 결론을 내렸다. 일각에서 제기했던, 온도 변화가 흑점의 변화에 의한 것이라는 이론에 대해서는, 흑점과 온도, 바람, 구름, 강우 자료를 대조하며 일치하지 않음을 증명하였다.

워커는 중요한 관련성에 더해, 중요하지 않은 관련성도 모두 출판했는데, 이는 다른 연구자들이 존재하지 않는 관련성을 찾으려고 하지 않게끔 하려는 목적이었다.

해양의 영향 

워커 순환으로 인해 북반구의 여름에 인도양에서는 서풍, 태평양과 대서양에서는 동풍이 부는데, 이로 인해 세 대양의 온도는 불균형을 이룬다. 태평양과 대서양에서는 북반구의 여름에 동쪽 전반의 해수면 온도가 낮으나, 인도양에서는 서쪽에서만 온도가 낮다. 해수면 온도의 차이로 인해, 수온약층의 깊이에도 차이가 발생한다.

워커 순환의 변화는 지표면 온도의 변화와 맞추어 일어난다. 계절의 변화처럼 외부 요인도 있지만, 해양과 대기의 자체 피드백 효과도 있는데, 예를 들어 동풍이 불면 동쪽의 해수면 온도가 감소하므로 기온 차이가 증가해 바람의 세기가 강해진다. 이러한 변칙적 현상에 의해 적도에서의 용승이 촉진되어 수온약층이 상승한다. 만약 지구의 기후가 적도를 기준으로 대칭이었다면, 적도를 통과하는 바람은 없었을 것이며, 현재 관측되는 것과 비교해 냉각 현상이 약하고 구조 또한 달랐을 것이다. 워커 순환은 태양에 의해 가열된 해수와 공기를 무역풍을 통해 서쪽으로 이동시키는 형태로 나타나며, 페루와 에콰도르 해안에서 차갑고 영양 염류가 풍부한 해수를 표면으로 용승시켜, 어업량을 증가시킨다.

엘니뇨 

워커 순환은 동태평양의 고기압과 인도네시아의 저기압 사이의 기압 경도력에 의해 발생한다. 워커 순환으로 인해 차가운 해양심층수의 용승이 촉진되어, 해수면 온도는 하강하게 된다. 엘니뇨는 워커 순환이 감소하거나 멈추었을 때, 해수면 온도가 상승하는 현상이며, 반대로 워커 순환이 증가하면 용승이 증가해 해수면 온도가 하강하는 라니냐가 일어난다.

2006년 5월 네이처에 발표된 논문에서는 워커 순환이 19세기 중반부터 느려지고 있음을 밝혔으며, 논문 저자들은 지구 온난화가 워커 순환 감속의 원인이라고 주장하였다. 하지만 20세기 기록을 2011년 분석한 연구에서는, 엘니뇨 남방진동으로 인한 변동을 제외하고, 1871년부터 2008년까지 워커 순환의 세기는 일정하게 유지되었다고 주장하였다.

같이 보기 
 대기 순환
 지구 대기권

각주 

참고 자료
 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 / 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 Nature.
 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

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