TIP
Questions about El Niño and La Niña are very common on the APES exam. Be sure you know
these two processes!El Niño (Warm Phase)
The steps of forming an El Niño are:
■ Air pressure patterns reverse direction, causing trade winds to decrease in
strength.
■ This causes the normal flow of water away from western South America
to decrease and “pile up.”
■ As a result, the thermocline off western South America becomes deeper
and results in a decrease in the upwelling of nutrients, which causes
extensive fish kills.
■ A band of warmer-than-average ocean water temperatures develops off
the Pacific coast of South America.
■ Effects are strongest during the Northern Hemisphere winter because
ocean temperatures worldwide are at their warmest.
■ Increased ocean warmth enhances convection, which then alters the jet
stream resulting in: – enhanced precipitation across the western U.S.
(including California) and the southern U.S.
– winter temperatures that are often cooler than normal in the southeast
U.S.