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. The atmosphere is heated which creates favorable conditions for convection and precipitation. 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). Weak Walker circulation = El Niño 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. The Southern Oscillation—an oscillatory exchange of atmospheric mass between the eastern south Pacific and Indonesia—and the Walker Circulation—its counterpart in wind circulation—have a time-scale of years and are manifestations of a near-global variation in circulation, clouds and precipitation, centered in the equatorial eastern Pacific.

Walker Circulation Reversal Kelvin waves cause the shallow thermocline in the eastern Pacific to deepen, and with a deeper thermocline, upwelling will transport warmer than normal water to the surface.  As shown below, this will essentially disrupt and reverse the normal pattern known as the Walker Circulation, causing El Niño conditions to form.  balances the Earth's Coriolis force against a topographic boundary such as a coastline, or a waveguide such as the equator. A feature of a Kelvin wave is that it is non-dispersive, i.e., the phase speed of the wave crests is equal to the group speed of the wave energy for all frequencies. This means that it retains its shape in the alongshore direction over time. . Note that for motion toward the west, the Coriolis force would not restore a northward or southward deviation back toward the equator; thus, equatorial Kelvin waves are only possible for eastward motion (as noted above). Both atmospheric and oceanic equatorial Kelvin waves play an important role in the dynamics of El Nino-Southern Oscillation, by transmitting changes in conditions in the Western Pacific to the Eastern Pacific. 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 ? The Southern Oscillation Index is computed as the ratio between the atmospheric pressure at Tahiti (circled in the map) and Darwin.

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