El NiĆ±o was first observed by Western scientists as an unusual ocean warming along the coasts of Peru and Ecuador in South America. The timing of this warming often coincided with the Christmas season, so it was called El NiĆ±o for the coming of the Christ Child. We now know that this pool of unusually warm water can shift atmospheric circulation patterns around the globe, leading to changes in temperature and precipitation in areas far away from the warm pool of water. Each El NiĆ±o event is a little different from the next, with variations in the timing of the warming and the position of the warmest water. ENSO, El NiĆ±o-Southern Oscillation, refers to the fluctuations in the temperature and pressure of the surface waters between the eastern and western tropical Pacific as part of the Walker circulation.
|Figure A: Normal vs. El NiĆ±o Conditions
El NiĆ±o events typically occur every three to seven years and start when the trade winds blowing from the northeast, near the equator in the Northern Hemisphere, weaken. In a strong event, the winds can actually reverse and flow from the west. The trade winds weaken because the air pressure gradient between the eastern and western tropical Pacific decreases. As the winds die down, warm water from the western equatorial Pacific sloshes back towards South America, thickening the layer of warm surface water and cutting off the upwelling of nutrients from the deep ocean near Peru and Ecuador. This leads to the death of fish and the birds that feed on the nutrients, leading to economic impacts on commercial fishing and guano gathering for fertilizer from the nesting areas of the birds.
An area of thunderstorms and intense rainfall often accompanies the pool of warmest water, leading to changes in precipitation amounts along the South American coast. This causes flooding in western South America where it is usually dry and makes it excessively dry in the western Pacific where precipitation normally falls.
|El Nino Climate Changes Felt Around the World, Image from www.ncdc.noaa.gov|
All of these changes in the Pacific Ocean change the global atmospheric circulation, which in turn affects the weather and climate patterns all over the globe. Since the air flows unimpeded around the earth a shift in the atmospheric patterns at one location (similar to placing a large rock in a river) can lead to shifts in the flow of air and atmospheric patterns downstream. Figure B shows some of the known variability in climate associated with the occurrence of an El NiĆ±o episode.
In addition, the pool of warm water in the eastern Pacific ocean aids in the transfer of heat from this location. This also affects weather patterns all over the world. Areas usually receiving great amounts of rainfall could potentially dry out during an El NiĆ±o event and vice versa for other areas that are usually dry. Over the central Pacific Ocean, typhoons usually increase during strong El NiĆ±o events due to the ample supply of warm water at the surface. Since El NiĆ±o usually reaches its maximum strength in winter, El NiĆ±o has the strongest impacts on the weather in December, January, and February in the Southeast, although it can continue to impact weather regionally for several months.
|Figure C: Moderate El NiĆ±o Winter Conditions|
|Image from NOAA, Moderate El Nino Winter Conditions|
The effects of El NiĆ±o also impact the location of the jet streams. The subtropical jet stream moves up into the southern United States during the winter months of an El NiĆ±o year. This jet brings warm, moist air into southern California. During the wintertime, southern California would then be mostly wet. South-central Texas and the southeast US will be mostly cool and wet because the cold, moist air from the Pacific Ocean entering the southern states enhances clouds and rainfall and cools temperatures due to lack of direct sunlight. In turn, much of the northern United States and Canada will be warm during an El NiĆ±o event because the polar jet stream swings farther east over the northeastern United States. For much of the southeastern United States during the summer of an El NiĆ±o event, the climate is usually warmer and drier. The strong subtropical jet stream also tends to disrupt the development of hurricanes in the Atlantic Ocean and Gulf of Mexico. Because of that, fewer hurricanes are likely during an El NiĆ±o year in the tropical Atlantic.