Editing Atmospheric circulation (section)

== Longitudinal circulation features==
[[Image:Diurnal wind change in coastal area.png|thumb|400px|Diurnal wind change in local coastal area, also applies on the continental scale.]]

While the Hadley, Ferrel, and polar cells (whose axes are oriented along parallels or latitudes) are the major features of global heat transport, they do not act alone. Temperature differences also drive a set of circulation cells, whose axes of circulation are longitudinally oriented. This atmospheric motion is known as '''zonal overturning circulation'''.

Latitudinal circulation is a result of the highest solar radiation per unit area (solar intensity) falling on the tropics.  The solar intensity decreases as the latitude increases, reaching essentially zero at the poles. Longitudinal circulation, however, is a result of the heat capacity of water, its absorptivity, and its mixing. Water absorbs more heat than does the land, but its temperature does not rise as greatly as does the land. As a result, temperature variations on land are greater than on water.

The Hadley, Ferrel, and polar cells operate at the largest scale of thousands of kilometers ([[Synoptic scale meteorology|synoptic scale]]). The latitudinal circulation can also act on this scale of oceans and continents, and this effect is seasonal or even [[decade|decadal]].  Warm air rises over the equatorial, continental, and western Pacific Ocean regions. When it reaches the tropopause, it cools and subsides in a region of relatively cooler water mass.

The Pacific Ocean cell plays a particularly important role in Earth's weather.  This entirely ocean-based cell comes about as the result of a marked difference in the surface temperatures of the western and eastern Pacific.  Under ordinary circumstances, the western Pacific waters are warm, and the eastern waters are cool. The process begins when strong convective activity over equatorial [[East Asia]] and subsiding cool air off [[South America]]'s west coast create a wind pattern which pushes Pacific water westward and piles it up in the western Pacific. (Water levels in the western Pacific are about 60&nbsp;cm higher than in the eastern Pacific.).<ref>{{cite web
 |title=Envisat watches for La Nina 
 |publisher=BNSC 
 |date=2006-03-03 
 |url=http://www.bnsc.gov.uk/content.aspx?nid=5989 
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}}</ref><ref>{{cite web
  | title =The Tropical Atmosphere Ocean Array: Gathering Data to Predict El Niño
  | work =Celebrating 200 Years
  | publisher =NOAA
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  | url =http://celebrating200years.noaa.gov/datasets/tropical/welcome.html
  | access-date =2007-07-26  }}</ref><ref>{{cite web|title=Ocean Surface Topography |work=Oceanography 101 |publisher=JPL, NASA |date=2006-07-05 |url=http://sealevel.jpl.nasa.gov/gallery/presentations/oceanography-101/ocean101-slide14.html |access-date=2007-07-26 |url-status=dead |archive-url=https://web.archive.org/web/20090414022153/http://sealevel.jpl.nasa.gov/gallery/presentations/oceanography-101/ocean101-slide14.html |archive-date=April 14, 2009 }}</ref><ref>{{cite web
  | title =ANNUAL SEA LEVEL DATA SUMMARY REPORT JULY 2005 – JUNE 2006
  | work =THE AUSTRALIAN BASELINE SEA LEVEL MONITORING PROJECT
  | publisher =Bureau of Meteorology
  | url =http://www.bom.gov.au/fwo/IDO60202/IDO60202.2006.pdf
  | archive-url =https://web.archive.org/web/20070807235141/http://www.bom.gov.au/fwo/IDO60202/IDO60202.2006.pdf
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The daily (diurnal) longitudinal effects are at the [[Mesoscale meteorology|mesoscale]] (a horizontal range of 5 to several hundred kilometres).  During the day, air warmed by the relatively hotter land rises, and as it does so it draws a cool breeze from the sea that replaces the risen air. At night, the relatively warmer water and cooler land reverses the process, and a breeze from the land, of air cooled by the land, is carried offshore by night.

=== Walker circulation ===
{{Main|Walker circulation}}
The Pacific cell is of such importance that it has been named the '''Walker circulation''' after [[Sir Gilbert Walker]], an early-20th-century director of British observatories in [[India]], who sought a means of predicting when the [[monsoon]] winds of India would fail. While he was never successful in doing so, his work led him to the discovery of a link between the periodic pressure variations in the [[Indian Ocean]], and those between the eastern and western Pacific, which he termed the "[[Southern Oscillation]]".

The movement of air in the Walker circulation affects the loops on either side. Under normal circumstances, the weather behaves as expected. But every few years, the winters become unusually warm or unusually cold, or the frequency of [[hurricane]]s increases or decreases, and the pattern sets in for an indeterminate period.

The Walker Cell plays a key role in this and in the [[El Niño]] phenomenon. If convective activity slows in the Western Pacific for some reason (this reason is not currently known), the climates of areas adjacent to the Western Pacific are affected. First, the upper-level westerly winds fail. This cuts off the source of returning, cool air that would normally subside at about 30° south latitude, and therefore the air returning as surface easterlies ceases. There are two consequences. Warm water ceases to surge into the eastern Pacific from the west (it was "piled" by past easterly winds) since there is no longer a surface wind to push it into the area of the east Pacific. This and the corresponding effects of the Southern Oscillation result in long-term unseasonable temperatures and precipitation patterns in North and South America, Australia, and Southeast Africa, and the disruption of ocean currents.

Meanwhile, in the Atlantic, fast-blowing upper level Westerlies of the Hadley cell form, which would ordinarily be blocked by the Walker circulation and unable to reach such intensities. These winds disrupt the tops of nascent hurricanes and greatly diminish the number which are able to reach full strength.<ref>{{Cite web|title=The Walker Circulation: ENSO's atmospheric buddy {{!}} NOAA Climate.gov|url=https://www.climate.gov/news-features/blogs/enso/walker-circulation-ensos-atmospheric-buddy|access-date=2020-10-03|website=www.climate.gov}}</ref>

==== El Niño – Southern Oscillation ====
{{Main|El Niño-Southern Oscillation}}

''El Niño'' and ''La Niña'' are opposite surface temperature anomalies of the Southern Pacific, which heavily influence the weather on a large scale. In the case of El Niño, warm surface water approaches the coasts of South America which results in blocking the upwelling of nutrient-rich deep water. This has serious impacts on the fish populations.

In the La Niña case, the convective cell over the western Pacific strengthens inordinately, resulting in colder than normal winters in North America and a more robust cyclone season in [[South-East Asia]] and [[Eastern Australia]]. There is also an increased upwelling of deep cold ocean waters and more intense uprising of surface air near South America, resulting in increasing numbers of drought occurrences, although fishermen reap benefits from the more nutrient-filled eastern Pacific waters.