Walker
Circulation and
the Monsoon
Season
The impact of the El Niño-Southern Oscillation (ENSO)
Ben Pearson
Physical Oceanography, Fall 2012
Walker Circulation
• The Walker circulation is the result of a surface pressure and
temperature difference over the western and eastern tropical
Pacific ocean.
• Under normal Walker circulation, the tropical western Pacific
is warm and wet due to prevailing low pressure system.
• The eastern Pacific lies under a high pressure system for cool
and dry conditions
• An easterly trade wind blows warm water towards the west.
• Warm water is piled in the upper ocean of the western
tropical Pacific near Indonesia and the Australian continent,
up to 60 millimeters higher on the western coast.
Figure courtesy of United Nations Environmental Program, GRID-Arendal Maps and Graphics Library;
http://www.unep.org/
Walker Circulation Strength
• When the Walker circulation reverses every few years, it weakens
which causes the wind to also weaken. Warm water of the western
Pacific flows to the east.
• Under strong Walker conditions, winds are stronger across the
Pacific which causes cooler ocean temperature through upwelling
that occurs in the eastern Pacific.
• The Walker circulation reverses as part of the El Niño-Southern
Oscillation (ENSO).
o Weak Walker circulation = El Niño
o Strong Walker circulation = La Niña
• 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.
Walker Circulation Reversal
El Niño occurs with the combination of relaxed trade winds over equatorial Pacific
with an eastward movement of waves, specifically as Kelvin waves.
Image Courtesy of Pennsylvania State University
http://www.personal.psu.edu/czn115/blogs/meteo241/2010/10/e-portfolio-2.html
Global impact
• The Walker circulation usually brings areas of low pressure to
the western Indian Ocean but, in years when El Niño occurs, this
pattern can get shifted eastward, bringing high pressure over
India and suppressing the monsoon, especially in spring when
the monsoon begins to develop.
• India and many other
developing countries
bordering the Pacific
ocean rely on the rainy
season and normal
strength Walker conditions
for a productive
agricultural economy and
fishing.
Image Courtesy of the Australian Government, Bureau of Meteorology
http://www.bom.gov.au/climate/enso/history/ln-2010-12/three-phases-of-ENSO.shtml
Walker Circulation and
ENSO
• Prediction of ENSO events could help countries prepare and
help avoid negative consequences based on the prevailing
weather patterns.
• Currently only able to predict weather patterns after on-set of
altered circulation.
• Positive Southern Oscillation Index tends to yield La Niña
conditions while a negative index yields El Niño conditions.
• Does Eurasian warming and a southwest shift of the Walker
Circulation offset the effect of El Niño on the monsoon
season.
• Will climate change intensify El Niño or La Niña ?
References
• Collins, Mat, et al. "The impact of global warming on the tropical
Pacific Ocean and El Niño." Nature Geoscience 3.6 (2010): 391-397.
• Kumar, K. Krishna, Balaji Rajagopalan, and Mark A. Cane. "On the
weakening relationship between the Indian monsoon and
ENSO." Science 284.5423 (1999): 2156-2159.
• Rasmusson, Eugene M., and John M. Wallace. "Meteorological
aspects of the El Nino/southern oscillation." Science 222.4629
(1983): 1195-1202.
• Saravanan, R., and Ping Chang. "Interaction between tropical
Atlantic variability and El Nino-Southern Oscillation." Journal of
Climate 13.13 (2000): 2177-2194.
• Webster, Peter J., and Song Yang. "Monsoon and ENSO: Selectively
interactive systems." Quarterly Journal of the Royal Meteorological
Society118.507 (2006): 877-926.