SIO 210 Talley Topic 8: Monsoons, El Nino

Lynne Talley, 2000
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Outline

This material is not being covered in the 2000 SIO 210 course except for a brief introduction to the monsoons.

A comprehensive report on the climate problems being considered by CLIVAR (monsoons - Asian, American, African; El Nino; decadal change) can be accessed at:
http://www.clivar.org/publications/other_pubs/iplan/brochure/contents.htmand in much more detail at:
http://www.clivar.org/publications/other_pubs/iplan/climp.htm
The entry point for a description of all CLIVAR programs is http://www.clivar.org/.

1. Monsoons

Thermally direct circulation in the tropical atmosphere (like Walker cell and sea/land breeze). Rising air over warm earth and sinking over cool with surface air flow from sinking to the rising region.

In summer, land is warmer than ocean so surface wind is from ocean to land. In winter, the reverse.

Indian (Asian-Australian) monsoon: late summer conditions are strong air flow from the Arabian Sea northeastward into India ("Southwest monsoon"), accompanied by large precipitation over land. Wind along Arabia is especially intense ("Findlater jet"), like an atmospheric western boundary current. The Findlater jet forces major upwelling along the Arabian coast (offshore Ekman flow). Circulation in the Arabian Sea is anticyclonic and the northward Somali Current (western boundary current) is fully developed.
In autumn, the sea-air temperature contrast decreases. The Findlater jet swings to the south and blows eastward ("Transition"). During the Transition, a strong eastward surface jet develops in the ocean along the equator.
In winter, the wind blows from land to sea ("Northeast monsoon"). Upwelling in the Arabian Sea ceases. Circulation in the Arabian Sea weakens and the Somali Current can reverse.

Other monsoon regions: Asia up through China and Japan is part of the same monsoon system ("Asian-Australian monsoon"). The western part of Mexico up through Arizona experiences the "Pan-American monsoon". The southern hemisphere portion of this monsoon affects western South America in austral summer. The African monsoon system affects tropical Africa with major rainfall in the northern hemisphere in boreal summer and in the southern hemisphere in austral summer.

Draft versions of CLIVAR's detailed descriptions of the
Asian-Australian Monsoon (click to Volume 2 and then look through table of contents)
and the American Monsoon (also click to Vol. 2 and look at table of contents). are available online. The full text of the CLIVAR plans for all timescales is available at http://www.clivar.org/publications/other_pubs/iplan/climp.htm

2. El Nino

Many excellent websites now exist concerning El Nino. A good entry point produced by NOAA is:
http://www.pmel.noaa.gov/toga-tao/el-nino/
Another excellent entry point, based on satellite altimetry data, and produced by NASA is:
http://topex-www.jpl.nasa.gov/elnino/elnino.html

(The following text is from my article written 12/98 for junior high level encyclopedia, slightly edited)

The tropical Pacific is the seat of the global climate cycle variously known as "El Nino/La Nino" or the El Nino Southern Oscillation, which occurs every two to seven years. The name El Nino originated long ago from the recognition of increases in rainfall and decreases in fisheries along Peru around Christmas time. It has since been learned that these changes are part of a much larger climate pattern. The atmosphere and ocean are closely coupled in the equatorial region. Normal conditions include trade winds blowing from east to west; they result from air rising over a pool of warm water in the west ("warm pool") and air sinking over a tongue of cold water ("cold tongue") along the equator in the east (Walker cell). Clouds form and precipitate where air rises, and so normally there is rainfall in the western equatorial Pacific and dryness in the eastern Pacific. Associated with the trade winds, the atmospheric pressure is high in the east and low in the west.

The trade winds force two things in the equatorial ocean: a westward surface current which carries warm water westward right along the equator (South Equatorial Current), and secondly upwelling along the equator due to poleward flow of the upper 30 to 50 meters of water just off the equator (Ekman transport). The poleward flow results from the earth's rotation which causes flow to be to the right of a force like the wind in the northern hemisphere and to the left in the southern hemisphere. Because the surface water carried to the west creates a thick layer of warm water there, the upwelled water is colder in the east than in the west. (Also, sea level is higher in the west due to the pileup of water there.) Thus the winds maintain the ocean temperature difference which then drives the winds.

Under normal conditions, winds blowing equatorward along the west coast of the Americas cause the surface waters to move offshore (Ekman layer). This results in upwelling along these coasts, which draws up water from about 100 meters depth which is rich in nutrients (Peru and California Currents). This sustains large fisheries in these coastal regions.

During an El Nino, the normal easterly trade winds weaken. In the western equatorial region they actually shift to being westerlies. The difference in atmospheric pressure between the central and western Pacific thus also decreases. (The pressure difference between Tahiti and Darwin, Australia, called the Southern Oscillation Index, is often taken as a measure of El Nino, hence the commonly-used name El Nino/Southern Oscillation.) The westward flow of ocean water at the equator then slows, which leads to a draining of the western warm pool towards the east. Equatorial ocean upwelling is reduced, which results in warmer sea surface temperatures in the eastern Pacific. As the western warm pool cools slightly and the central and eastern equatorial Pacific warm, this further decreases the strength of the tradewinds. This is therefore a positive feedback and the El Nino keeps growing.

During an El Nino, the large atmospheric convection cell over Indonesia moves eastward. This results in drought in the western Pacific, including over Indonesia and Australia, and increased rainfall in the central and eastern Pacific, for instance at Christmas Island, the Galapagos and Ecuador.

During an El Nino, the warm water in the eastern Pacific spreads to the west coast of the Americas and splits to flow north and south there. The normal upwelling off northern Peru weakens and also draws up only warm, nutrient-poor equatorial water. The result is a decline in production in this important fisheries area. If the El Nino is particularly strong, its effect in the ocean can reach as far north as the California coast.

The opposite phase of the El Nino is called La Nina. (The name was coined to mean the opposite of El Nino.) La Nina is particularly strong occurrence of "normal" conditions. La Nina and El Nino are the two opposite phases of the same cycle. During La Nina, the tradewinds are especially strong, and the warm pool in the western Pacific and cold water at the equator in the central and eastern Pacific are particularly well-developed. Sea level is especially high in the western Pacific. Rainfall is strong in the western Pacific, with little rainfall in the eastern Pacific.

El Nino/La Nina affects from the tropical Pacific can reach far to the northeast and southeast through the atmosphere. The cycle thus affects South and Central America and when especially strong, can impact the United States. During an especially strong El Nino, such as occurred in 1997, record rainfall is found in California, tornados and storms in the southeast U.S.

El Nino occurs irregularly, but generally every three to seven years, which is the timescale for east-west propagation of large, slow ocean waves across the equatorial region. Major progress has been made in predicting an El Nino about one year in advance because the sequence of events in an El Nino is often the same. Thus detection of early signs of El Nino, such as the appearance of warm water in the eastern tropical Pacific or a change in the strength of the trade winds, often allows prediction of changes in rainfall and air temperature later in the year throughout the Pacific region. An ocean-atmosphere observing network and computer models now assist in observing and forecasting El Nino occurrences.

The strength of El Nino varies greatly over an even more irregular time scale of about ten to thirty years. For instance El Nino's in the 1940's were strong, followed by several decades of weak events, and then followed by very strong El Nino's again in the 1980's and 1990's. Long records of El Nino's have been extracted from atmospheric pressure observations at Tahiti and Darwin, and from growth and properties of the annual accretion in coral heads in the tropical Pacific. This so-called decadal modulation of El Nino is much less well-understood at this time than El Nino itself.

Switch from El Nino to La Nino: mechanism not clear.

Involvement of ITCZ not detailed here. Kelvin wave mechanism for sloshing of warm pool eastward during El Nino.

3. Mid-latitude variability - decadal change

No text prepared for class notes yet - no opportunity in the 2000 course to cover this topic.

CLIVAR is the international program organized by the World Climate Research Programme for studying climate change. Under CLIVAR, climate time scales from El Nino to decadal are being studied. The scientific plan is still being shaped. For information about CLIVAR, check the website http://www.clivar.ucar.edu/hp.html.