Winds at the jetstream level (small arrows), five to fifteen miles above sea level, change their course between normal (above) and El Niño (below) winters. A ridge of high pressure over North America's west coast during El Niño winters keeps temperatures above normal in the orange region and steers storms that would otherwise pass through Washington and Oregon northward toward the coast of Alaska as indicated by the heavy arrow. El Niño also creates a favorable environment for storms to develop in the Gulf of Mexico, bringing heavy rains to much of the southern United States. An analogous strengthening of the westerlies in the Southern Hemisphere during its winter season brings heavy precipitation to parts of southern Brazil and northern Chile and Argentina (not pictured).

To download a postscript file of this figure for printing, click here.