Re: What causes the effects of El Nino? How did it get this name?

"El Nino" means "The (Christ) Child". Peruvian fishermen have known for a long time that once every few years, the sea surface temperature off their coast gets much warmer, and at the same time all the fish die, causing economic hardship. The event gets its name because it happens around Christmastime.

Why does it happen? Normally, the equatorial Pacific has warm waters near Indonesia, and colder surface water near Peru. This heats and cools the atmosphere above it, causing warmed air to rise near Indonesia and cool air to sink off Peru. This sets up an east-west "Walker Circulation" in which air rises in the west, travels eastward, sinks in the east, and travels westward again. This wind pattern helps to carry more warm water to the west, maintaining the sea-surface temperature (SST) pattern. This is the non-El Nino state, often called "La Nina" (The Girl).

However, once every few years, the SST pattern collapses: the warm water at Indonesia travels across the Pacific as a "Kelvin wave", leaving the surface uniformly warm. This keeps cool, nutrient-rich water from rising to the surface off Peru, thereby creating the phenomenon the fishermen noticed. Also, without the "cold east, warm west" SST pattern, the Walker Circulation ceases or even reverses. It takes several years for the system to return to normal.

Here's a diagram of the two phases of the El Nino/Southern Oscillation (ENSO):

This diagram, along with a much more thorough discussion of the El Nino phenomenon, can be found at the Lamont-Doherty Earth Observatory's El Nino page.

How does El Nino affect weather throughout the world? Many (but not all!) scientists believe that the changes in the Walker Circulation can generate a pattern of "stationary waves" (like the ripples that form behind a rock in a stream, but thousands of kilometers across) which can propagate around the globe, changing wind and precipitation patterns. Some studies suggest that El Nino can have detectable influences on weather outside the equatorial Pacific. However, in my opinion these influences are nowhere near as strong as the TV weathermen would like you to believe. It seems like every strong weather event this winter is being attributed to El Nino; in reality, El Nino appears to have a small effect (a few percent) on rainfall and temperatures in the southern part of the U.S., the effect is almost impossible to detect over most of North America. In non El Nino years, the same unusual weather events are attributed to Global Warming or somesuch. As another example, the North Atlantic Oscillation (NAO) pattern has a much stronger contribution than El Nino to wintertime temperatures and precipitation in the east coast of North America and especially Western Europe. We really don't understand enough about the climate system to be able to predict the likelihood of floods and droughts and cold and warm winters reliably yet, and El Nino and the NAO is only the first in what will probably be a very long list of factors influencing short-term climate change.