Tropical Meteorology: Intra-Seasonal Oscillation (Madden-Julian
Oscillation)
Roland A. Madden
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
The global trade winds of the Northern and Southern Hemispheres meet near the
equator in the Inter-Tropical Convergence Zone (ITCZ). Here, the surface air
rises in large convective storms over preferred regions of South America,
Africa, and the Malay Archipelago, and then moves north and south and east and
west at upper levels. The circulations in the north and south directions form
the Hadley Cells and those in the east and west directions which are centered on
the Malay Archipelago form the Walker Cells. The first are named after George
Hadley who described the circulation from tropics to mid-latitudes in the 1700s.
The Walker Cells are named after Sir Gilbert Walker, a British climatologist.
Walker wrote in the 1920s and 1930s about an east to west oscillation in
pressure between the Indian/West Pacific Oceans and the Southeast Pacific Ocean.
It was later learned that this pressure oscillation was accompanied by a
longitudinal shift in the main region of upward motion in the east-west
circulation cells. They were named the Walker Cells. The pressure oscillation
that Walker studied has a dominant time-scale on the order of two to seven years
and is now called the 'Southern Oscillation', a part of the El
Niño/Southern Oscillation (ENSO). However, circulation in the Walker Cells
changes on all time scales, and here we describe a variation on an intraseasonal
time scale.
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Hongjun Zhang:
zhangho@ucar.edu