Lynne Talley, 1997
Back to SIO 210 index.
Reading and study questions
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.ucar.edu/vol1/contents.html
.
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
and the
American Monsoon
are available online. The full text of the CLIVAR plans for
all timescales is also available at
http://www.dkrz.de/clivar/climp.html
Flow on equator driven by trade winds, pushing surface water westward. Water piles up in west in the "warm pool", which creates a pressure gradient which forces water eastward, in the undercurrent.
El Nino condition:
The trades weaken, as marked by a change in the Southern Oscillation Index (SOI) towards higher pressure in Australia and lower pressure in the central Pacific.
The westward equatorial surface flow weakens, warm pool surges eastward. Sea level drops in western Pacific and rises in central Pacific. The thermocline flattens. Warm sea surface temperature (SST anomalies appear along equator. Upwelling weakens in the east as well, causing warm SST anomaly there.
This SST change weakens the trades further and shifts the convection cell eastward. Western Pacific is drier and central Pacific is wetter.
Warm anomalies in the eastern Pacific spread poleward along the eastern boundary (Kelvin wave). Thermocline deepens along the boundary so even if coastal winds are upwelling-favorable, the nutrient-rich water does not reach the surface.
El Nino condition affects the mid-latitudes through a teleconnection through the atmosphere.
La Nina condition: Extreme case of the "normal" situation.
Switch from El Nino to La Nino: mechanism not clear.
Many excellent websites now exist concerning El Nino. A good entry
point is:
http://www.pmel.noaa.gov/toga-tao/el-nino/
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 .