What are the consequences of El Niño? There are both local effects, arising because of the warm eastern Pacific, weakened trade winds, and shifted regions of convection, and distant effects, mostly arising because the atmosphere carries the signal of El Niño far afield. We'll talk about the local effects first.
Local effects of El Niño
El Niño brings significantly warmer water to the eastern tropical Pacific, and this water spreads both north and south along the coast, giving a detectable signal in the SST as far north as California and Oregon. There is a direct effect of this change on Pacific marine life: Warm-water species have an extended range of habitat, whereas cold-water species try to move poleward or into deeper water, and the reduced upwelling off the South American coast produces fewer nutrients, fewer fish, and fewer sea-birds that feed off the fish. The shift of fish populations is a problem for the fishing industry, if only because the species tend to move away from established fisheries, and overall there is a loss of commercially important species. Pinnipeds (e.g., fur seals and sea lions) are also affected as far north as California (for example, in the Channel Islands off Los Angeles) because they may have difficulty finding adequate food supplies. Coral bleaching (the whitening of corals because of the loss of or changes in the protozoa living within them) in the Galapagos also occurs in El Niño years.
The warm ocean also affects the atmosphere above it; the warming is part of the ENSO cycle itself. During a warm event, the main convective areas move eastward, and the western parts of South America, especially close to the equator in northern Peru and Ecuador, experience significantly more rainfall, especially from December to February, when the events typically reach their peak. Significant flooding may occur when the event is a strong one. In contrast, northern Australia and the Indonesian archipelago experience significantly less rainfall than normal throughout the entire region and for an extended period of time. During a La Niña, the situation
EL NIÑo IN A NuTSHELL
1. El Niño refers to the warming of the surface waters in the eastern equatorial Pacific. The interval between warm events is typically from two to seven years but is quite irregular and can be longer.
2. El Niño events are associated with a weakening of the trade winds and a shifting eastward of the region of convection. The atmospheric side of the events is known as the Southern Oscillation, and the entire phenomenon is known as the El Niño-Southern Oscillation, or ENSO.
3. In contrast to an El Niño event, from time to time the far western equatorial Pacific becomes warmer than usual and the eastern Pacific, colder, which is known as a La Niña event.
What causes el niño events?
1. El Niño is caused by the mutual interaction between the atmosphere and ocean in the equatorial Pacific, involving a positive feedback between the sea-surface temperatures and the strength of the trade winds.
2. The trade winds normally blow westward, but during an El Niño event the trade winds weaken, allowing the temperatures in the eastern equatorial Pacific to rise, further weakening the trade winds and so on.
What are some consequences of el niño?
1. The global surface temperature in an El Niño year is up to 0.5°C higher than normal.
2. Convective rainfall in North Australia and Indonesia is suppressed in El Niño years, whereas rainfall is enhanced in western tropical South America.
3. During strong El Niño events, the atmospheric storm track in the eastern Pacific is stronger and further south than normal, bringing heavy rain to central and southern California and penetrating inland across the southern United States.
4. The Atlantic storm track may also be affected, moving south and bringing conditions resembling a negative NAO index.
5. El Niño years are associated with the suppression of Atlantic hurricanes.
is almost reversed, with warmer and wetter weather in the Indonesian archipelago and northern Australia and generally cooler and dryer weather in coastal Peru and Ecuador.
By distant effects we mean the effects of El Niño on other regions of the globe that are not in themselves part of the ENSO cycle. Some of these effects are noticeable, especially if the El Niño event is strong, but others are weak and ambiguous and only emerge after averaging over a number of El Niño events to remove the natural variability—or noise—that causes one year to have different weather from another in any case. We first note the overall effect: In an El Niño year, the globally averaged surface temperature can be as much as 0.5°C higher than the years before and after, and this increase is accounted for by the fact that the surface temperatures in the eastern tropical Pacific can be several degrees higher than normal.
One effect that certainly arises during strong El Niño events is that the storm track over the eastern North Pacific becomes stronger, wetter, and further south in winter, bringing heavy rain to central and southern California and additional snow to the southern Sierras. In both the 198283 and 1997-98 events, extensive flooding and landslides occurred. In some contrast, further north in Oregon the snow pack was less than usual because of a warm winter but no enhanced precipitation, and Canada tends to experience warmer and drier winters in El Niño years. These effects are not nearly as pronounced in years with weaker El Niño events, and it is often hard to definitively attribute anomalous winter rainfall to an El Niño event.
The storm track does not end when it reaches California, and New Mexico, Arizona, and indeed much of the southern United States can receive enhanced precipitation in El Niño years. (Indeed, New Mexico has some of the best tree-ring records of El Niño.) The storm track may pick up again over the Atlantic and bring enhanced precipitation to Europe. Certainly, there is a correlation between El Niño events and the phase of the North Atlantic Oscillation, with warm events associated with a negative phase of the NAO. Elsewhere in the world, the effects are weaker, but there is some evidence that parts of East Africa (particularly Kenya and Tanzania) experience wetter weather during warm events.
Finally, we mention another tropical effect, a nonlocal one but one that seems to be quite robust. It is that
Atlantic hurricanes tend to be suppressed during El Niño years. Why should this be? Hurricanes form over warm tropical waters, and when the SST in the Atlantic is anomalously high, then the Atlantic hurricane season is more active than usual. However, hurricanes do not respond to the sea-surface temperature alone. Rather, they are a kind of heat engine, and a significant factor in driving hurricanes is the temperature difference between the sea surface and the upper atmosphere. (The effect is analogous to the fact that midlatitude weather systems respond to the temperature gradient between the equator and the poles.) During El Niño events, the Pacific region becomes particularly warm, and this warmth spreads throughout the tropics in the upper atmosphere, and in particular the upper tropical atmosphere over the Atlantic becomes anomalously warm. Hence, the temperature difference between the surface and the upper atmosphere that drives Atlantic hurricanes is reduced, and the Atlantic hurricane season in an El Niño year tends to be less active than usual.
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