This is the best explanation of El Nino that I have read, and with so many reports of an El Nino event occurring this year, I thought I would share.
This is an excerpt from the book California Marine Life by Marty Snyderman
“Every so often our planet experiences a severe dislocation of its largest and most dominant weather patterns, with dramatic worldwide ramifications. This climatic phenomenon is know as El Nino… The name is derived from a Spanish reference to Christ (“The Child”), and refers to the fact that a dramatic increase in water temperature along the coast of South America, usually the first indication of an El Nino event, often occurs near Christmas. Interspersed with both periods of colder-than-normal temperatures (known as “La Nina”) and periods of “normal” weather, El Nino events usually take place once every three to seven years and are highly variable in intensity.
During El Nino, the trade winds that normally push equatorial surface waters westward slow, stop, or even change directions. This causes warm water that is usually piled up along the tropical coasts of the western Pacific Ocean to slosh back eastward toward the coast of South America. Some of this warm water then tends to move northward in a current called the El Nino Current. Water temperatures and ocean currents affect wind and rain patterns as well, so weather patterns are often drastically affected worldwide.
Because El Nino exerts so many complicated effects on the weather, the net effect in California (as well as other areas of the world) can vary from severe droughts, such as those experienced during the 1976-1977 El Nino, to the torrential winter storms that inundated the state during the intense El Nino of 1982-1983. Local effects are not always negative: the El Nino that began in 1991 may have played a large part in ending a long California drought. What swimmers and divers are most likely to notice is that water temperatures are often elevated 4 degrees F to 10 degrees F, and can be increased as much as 14 degrees F. That may not sound like a lot compared to fluctuations in air temperatures, but if you consider that water requires 3,600 times as much heat per volume as does air to bring about a comparable temperature change, you will begin to realize how much energy the current contains.
These dramatic changes can be devastating to California’s marine life, as well as that of other parts of the world. The increased temperature directly causes two problems for marine life: (1) many plants and animals cannot withstand the stress of temperatures that rise too high or too fast, and (2) the warm layer of water places a density cap on colder deep water, cutting off the normal influx of nutrients from below. During the notorious El Nino of 1982-1983, miles of healthy kelp forests and associated kelp communities were decimated by the rise in water temperature and lack of nutrients. Two years later, many kelp beds were just beginning to reestablish themselves. These biological crises create a domino effect up the food chain; El Nino events have been linked to enormous losses in commercial fisheries, unprecedented mortality rates among local seabirds, and even an increase in sea lion and northern elephant seal beach strandings and deaths.
An additional change in marine communities has been the invasion of species that usually are found only much farther south. As temperatures rise, these animals migrate into California waters that are normally too cold for them. Wahoo, triggerfish, butterfly fish, and angle fish were common sights at some of the Channel Islands during the 1982-1983 El Nino. Howard Hall, a well-known underwater photographer, and Jon Hardy, a well-known authority in the sport diving business, enjoyed the good fortune to film manta rays at Anacapa Island and Catalina Island, respectively. Mantas, like many of the other above-mentioned species, are seldom sighted farther north than the southern tip of Baja, nearly 1,000 miles south of the Channel Islands.
El Nino events normally last for one to two years. However, some scientists believe that effects of the 1982-1983 El Nino may have displaced part of the Kuroshio Current 200 miles northward in 1992- nine years later. Although El Nino years are believed to be part of a normal, cyclical pattern of nature, recently the incidence seems to be increasing. In fact, El Ninos of 1991-1992, 1993, and 1994-1995 have been separated by a matter of only months. Scientists debate whether this unusual pattern is caused by global warming or is merely a normal extreme of global weather cycles.”