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Walker Circulation, El Nino and La Nina

The reason for this wiki is that the walker circualtion has a profound affect on our weather. We do not get the extremes of Australia and America, but we are affected.

Walker Circulation

The Walker circulation is an ocean-based system of air circulation that influences weather on the Earth. The Walker circulation is the result of a difference in surface pressure and temperature over the western and eastern tropical Pacific Ocean. Normally, the tropical western Pacific is warm and wet with a low pressure system, and the cool and dry eastern Pacific lie under a high pressure system. This creates a pressure gradient from east to west and causes surface air to move east to west, from high pressure in the eastern Pacific to low pressure in the western Pacific. Higher up in the atmosphere, west-to-east winds complete the circulation.

The warm waters of the western Pacific Ocean in East Asia heat the air above it and supply it with moisture. On average, the air rises, forms clouds, and then flows to the east across the Pacific, losing moisture to rainfall. The air then sinks off the west coast of South America and returns to the west along the surface of the ocean, back to the western Pacific Ocean.

The Walker circulation contributes to normal weather conditions in the tropical Pacific Ocean: warm, wet weather in the western Pacific and cool, dry weather in the eastern Pacific.

The Walker circulation reverses every few years, as part of a phenomenon called the El Niño-Southern Oscillation (ENSO). When the Walker circulation weakens, the winds also weaken and the warm water of the western Pacific spreads to the east. These conditions are called El Niño. During times when the Walker circulation is particularly strong, called La Niña, the winds are stronger across the Pacific. These strong winds cause cooler ocean temperatures because of upwelling in the eastern Pacific. El Niño and La Niña impact the weather in North and South America, Australia, and Southeast Africa, and can cause flooding, droughts, and increases or decreases in hurricane activity.

Gilbert Walker

Sir Gilbert Walker

Sir Gilbert Walker was a classic late Victorian polymath with an extraordinary range of artistic and scientific interests, such as painting, ice skating, gliding and the study of bird flight. He was fascinated by the flute and made a small improvement to its design. He spent 10 years working on the properties of the boomerang, countless examples of which he had shipped over from Australia and from which he developed new theories about the nature of gyroscopic motion.

Walkers contribution to our understanding of the weather was to apply the art of statistics long before the days of number crunching computers, what he had available was a large number of assistants in the Indian Met. Service. He set them the task of checking through weather and ocean data from around the world, and of statistically analyzing it and identifying any significant correlation between meteorological and oceanographic events. From this study there emerged a link between the monsoon's severity and the time of onset and the relative air pressures over the Indian and Pacific oceans. He found that high pressure over the Pacific tended to to mean low in the Indian Ocean all the way from Africa to Australia and vice versa. He immediately saw this as proof of the monsoon being linked to a global system, and he named this oscillation of pressure between the oceans the “Souther Oscillation”. Walker spent years building up date to establish that his oscillation correlated with the changes in rainfall and wind in the Pacific and Indian oceans, and with changes in temperature in Africa, southern Canada and the USA. Sadly his attempts to use it to predict the monsoon failed and his evidence of a link was called into question by other meteorologists. The problem was he simply did not have enough data to make the pattern clear, although years later it is now obvious that he was very much on the right track, and today the circulatory pattern that he identified has become known as the Walker Circulation. Its name is a tribute to his remarkable achievements, for what he did was far more then provide the first clues to predicting the monsoon, rather he set out the very foundations for our understanding of the global climate as a whole

Sir Gilbert Walker continued his studies of yearly weather and climate change after his retirement from India (in 1924) and acceptance of a professorship in meteorology at Imperial College London. He served as president of the Royal Meteorological Society from 1926 to 1927.

Walker’s results fell into oblivion until Jacob Bjerknes, in 1960, expressed his interests in discovering the secrets behind El Niño.

Gilbert Walker died at Coulsdon, Surrey on 4 November 1958. He was 90 years old.

What's in a name?

Scientific types refer to what the public thinks of as El Niño as ENSO – El Niño combined with Southern Oscillation - reflecting Bjerknes' finding that the entire phenomenon depends on an interaction between the atmosphere and the ocean, just as Walker had predicted 50 years earlier.

Jacob Bjerknes

Jacob Bjerknes

It took another 50 years, but in the late 1960s a Norwegian meteorologist, Jacob Bjerknes, put the whole picture together. As a professor at the University of California, he was the first to see a connection between unusually warm sea-surface temperatures and the weak easterlies and heavy rainfall that accompany low-index conditions.

Relying upon the data of 1957, Bjerknes went further than Walker in stating his thesis on El Niño. Only a thin sheet of water separates the atmosphere from the ocean depths. Thus, the difference in temperature between that sea surface and the air above it is what causes heat to flow and water vapor to be exchanged. Bjerknes proposed in 1966 that the relaxed trade winds of 1957 not only caused the appearance of unusually warm surface waters over the tropical Pacific, but were in turn caused by the increase in sea surface temperatures (SST) during that year. This amazing circular argument implied that this phenomenon was neither strictly atmospheric nor oceanic but a product of interactions of the two.

Dr. Richard T. Barber beautifully summed it up when he said, “The ocean is clearly driving the atmosphere.”

Analysis of His Findings

The differences between the two distant pressure systems (Tahiti minus Darwin) have been converted into an index called the Southern Oscillation Index (SOI). Usually, there is a low pressure system in the region of Indonesia and northern Australia, centered near Darwin. This system brings storminess to the region, providing some parts of Australia, the driest inhabited continent for sustaining its settlements, wildlife, and ecosystems. At the same time, there is a high pressure system in the southeastern Pacific, centered near Tahiti. The sea level pressure at each of these two points is related to each other. When one is increasing, the other is usually decreasing. Therefore, the difference in seal level pressure (Tahiti minus Darwin) is used as an index that characterizes ENSO.

Thus, El Niño is related to the negative phase of SOI and the cold event to the positive phase. This index, the SOI, has been reliable associated with a number of climate related events: when the sea level pressure at Darwin increases, the likelihood of drought in the Australian-Indonesian region also increases; when the pressure at Tahiti decreases, the likelihood of more rainfall in the equatorial central Pacific region increases. These particular sea level pressure changes appear to set the stage for a possible onset of El Nino. Along with pressure changes come changes in wind speed and direction, shifts in the location of pools of warm and cold oceanic surface water, changes in the strength of coastal upwelling, and shifts in the location of biological productivity in the ocean which, in turn, alters the location of various fish populations. It is important to realize that although there is no one-to-one correlation between observed El Nino events and the SOI, this relationship is very strong.

How the SOI is calculated

There are a few different methods of how to calculate the SOI. The method used by the Australian Bureau of Meteorology is the Troup SOI which is the standardised anomaly of the Mean Sea Level Pressure difference between Tahiti and Darwin. It is calculated as follows:

       [ Pdiff - Pdiffav ]

SOI = 10 ——————-



  Pdiff   =   (average Tahiti MSLP for the month) - (average Darwin MSLP for the month),
  Pdiffav   =   long term average of Pdiff for the month in question, and
  SD(Pdiff)   =   long term standard deviation of Pdiff for the month in question. 

The multiplication by 10 is a convention. Using this convention, the SOI ranges from about –35 to about +35, and the value of the SOI can be quoted as a whole number. The SOI is usually computed on a monthly basis, with values over longer periods such a year being sometimes used. Daily or weekly values of the SOI do not convey much in the way of useful information about the current state of the climate, and accordingly the Bureau of Meteorology does not issue them. Daily values in particular can fluctuate markedly because of daily weather patterns, and should not be used for climate purposes

In December 2010, the SOI reached a record level with an average value for December 2010 of +27 (La Nina). This is the highest average monthly December SOI value ever recorded and the highest value for any single month since November 1973.

La Nina brought floods to Queensland Australia and the Philippines

What are El Niño and La Niña?

El Niño is a natural feature of the global climate system. Originally it was the name given to the periodic development of unusually warm ocean waters along the tropical South American coast and out along the Equator to the dateline, but now it is more generally used to describe the whole “El Niño - Southern Oscillation (ENSO) phenomenon”, the major systematic global climate fluctuation that occurs at the time of an “ocean warming” event. El Niño and La Niña refer to opposite extremes of the ENSO cycle, when major changes in the Pacific atmospheric and oceanic circulation occur.

When neither El Niño nor La Niña are present, (usually referred to as “neutral” or normal conditions or the Walker Circulation, see above), trade winds blow westward across the Pacific, piling up warm surface water so that Indonesian sea levels are about 50 cm higher than those in Ecuador. Cool, nutrient-rich sea water “wells up” off the South American coast, supporting marine ecosystems and fisheries. Relatively cold sea temperatures also extend along the equator from South America towards the central Pacific. High rainfall occurs in the rising air over the warmest water to the west, whereas the colder east Pacific is relatively dry.

Normal Tropical Pacific Conditions, Walker Circulation

El Niño

During El Niño events, the trade winds weaken, leading to a rise in sea surface temperature in the eastern equatorial Pacific and a reduction of “up-welling” off South America. Heavy rainfall and flooding occur over Peru, and drought over Indonesia and Australia. The supplies of nutrient rich water off the South American coast are cut off due to the reduced up-welling, adversely affecting fisheries in that region. In the tropical South Pacific the pattern of occurrence of tropical cyclones shifts eastward, so there are more cyclones than normal in areas such as the Cook Islands and French Polynesia.

Tropical Pacific Conditions during El Niño

La Niña

During La Niña events, the trade winds strengthen, and the pattern is a more intense version of the “normal conditions”, with an even colder tongue of sea surface temperatures in the eastern equatorial Pacific.

Courtesy of Niwa:

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living_in_new_zealand/walker_circulation.txt · Last modified: 2011/08/26 07:44 by art
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