Walker Circulation

Mean Tropical Overturning Circulations

(Figure obtained from Introduction to Tropical Meteorology, 2nd Edition, © 2011 COMET.)

Base State Pacific Zonal Circulation

(Figure obtained from Introduction to Tropical Meteorology, 2nd Edition, © 2011 COMET.)

Base State Global Walker Circulation

(Figure obtained from Introduction to Tropical Meteorology, 2nd Edition, © 2011 COMET.)

Oceanic Transport

W

Consider a 2-D fluid bounded by land masses to the west and east with temperatures decreasing with increased depth. Easterly winds at the surface of that fluid will attempt to carry the fluid westward. At the eastern edge of the fluid, it is replaced by cooler fluid from below. At the western edge, the fluid is forced downward.

z

warming cooling

In reality, oceanic transport is far more complicated than in the simplistic diagram above. In particular, Ekman transport results in a rightward deflection (NH) of the oceanic transport vector, resulting in modest equatorial upwelling under easterly wind conditions. This becomes important when considering El Nino conditions.

Mean Daily Precipitation: Jan-Mar

Figures obtained using the freely-available NCEP-DOE Reanalysis II dataset.

Enhanced precipitation found to the west, especially with Pacific circulation and over South America.

Mean Daily Precipitation: Jul-Sep

Figures obtained using the freely-available NCEP-DOE Reanalysis II dataset.

Northward shift of enhanced precipitation occurs but still favors the western branches of the three major zonal circulations.

Other heating-induced features also contribute to precipitation patterns, as we will discuss later.

Linear Approximation: Walker Circulation

(Figure obtained from Introduction to Tropical Meteorology, 2nd Edition, © 2011 COMET.)

Linear Approximation: Walker Circulation

(Figure obtained from Gill (1980), © 1980 Royal Meteorological Society/Wiley Interscience.)

(u,v) - vectorsw - contours

(u,v) - vectorsp - contours

Meridionally-integrated flow

Linear Approximation: Hadley Circulation

(Figure obtained from Gill (1980), © 1980 Royal Meteorological Society/Wiley Interscience.)

zonal velocity

streamfunction

pressure perturbation