La Niño

	El Niño & La Niña
	What is El Niño? El Niño is a warming of the Pacific Ocean between South America and the Date Line, centered directly on the Equator, and typically extending several degrees of latitude to either side of the equator. Coastal waters near Peru also warm. The warming is expressed as a departure from long-term average ocean temperatures, which are generally cool in the region, due to upwelling. El Niño is thus associated with a slackening, or even cessation, of the cold upwelling conditions which typically prevail in that area. During a typical El Niño, the ocean warms a degree or two (C) above its climatological average. A strong El Niño can warm by 3-4 degrees C over large areas, and even 5 degrees C in smaller regions.
Typically, El Niño is first noticed along the South American coast around Christmas (hence the origin from Peruvian fishermen of its Spanish name ("the child"). Farther west, in the open ocean, El Niño typically begins to appear about a month later (near the Galapagos) to about 4 months later (near the Date Line) than near the coast.
What is La Niña? La Niña is essentially the opposite of El Niño. La Niña exists when cooler than usual ocean temperatures occur on the equator between South America and the Date Line. The name La Niña ("the girl child") was coined to deliberately represent the opposite of El Niño ("the boy child"). The terms El Viejo and anti-El Niño are also sometimes used. La Niña occurs almost as often as El Niño, but has been lesser known. La Niña and El Niño are but two faces of the same larger phenomenon. Stronger than usual trade winds accompany La Niña. These winds, from the east, push the ocean water away from the equator in each hemisphere. (This is caused by the rotation of the earth.) Cold water from below rises to replace the warm surface water which has moved away from the equator. The cool water acts as an impediment to the formation of clouds and tropical thunderstorms in the overlying air. This suppression of rain-producing clouds leads to dry conditions on the equator in the Pacific Ocean from the Date Line east to South America.
Normal (Long-term Average) Conditions The colored areas on the top of the block diagram portion denotes sea surface temperatures (SST) during normal conditions. The red colored area in the western Pacific basin (120°E) denotes the highest SST. These highest SST occur under [(considerable cloudiness) (clear skies)] in the tropical Pacific. This SST pattern is caused by relatively strong trade winds pushing sun-warmed surface water westward, as indicated by the direction of surface currents. Warm surface water transported by the wind away from South America coast (80°W) is replace by cold water rising from below in a process called upwelling. Upwelling of cold deep water results in relatively [(high) (low)] SST in the eastern Pacific basin (80°W) compared to the western Pacific basin (120°E).
		Normal Pacific pattern. Equatorial winds gather warm water pool toward west. Cold water upwells along South American coast. (NOAA / PMEL / TAO)

Tropical Pacific During El Niño Compared to normal (long-term average conditions), the area of stormy weather during El Niño has moved [(eastward) (westward)]. While no two El Niño episodes are exactly alike, all of them exhibit most of the characteristics shown in the El Niño schematic. In response to changes in surfaces currents, sea surface heights in the eastern tropical Pacific basin (80°W) are higher than during normal conditions. At this time, the arrival of the warmer water in the east causes the surface warm-water layer to thicken. Evidence of this is the [(shallower) (deeper)] depth of the thermocline to the east compared with normal conditions.
		El Niño Conditions. Warm water pool approaches South American coast. Absence of cold upwelling increases warming.
Tropical Pacific During La Niña At times the tropical Pacific experiences trade winds that are stronger than normal conditions with SST lower than usual in the eastern tropical Pacific basin (80°W) and higher than usual in the western tropical Pacific basin (120°E). Because stronger trade winds produce stronger surface currents during La Niña, the warm water is pushed westward and colder water wells up to cause below-average sea surface temperatures in the eastern tropical Pacific basin (80°W). It also follows that sea surface temperature in the western tropical Pacific basin (120°E) must be [(above) (below)] its normal condition average.
		La Niña Conditions. Warm water is farther west than usual.
How El Niño and La Niña affect U.S. storms

		While El Niño usually means warm/dry winters in the Pacific Northwest, it frequently means a very stormy winter in Los Angeles, especially once you get into January and February. The rest of the South also gets a wetter winter as those storms continue their general trek along the Pacific Jet Stream. Meanwhile the northern U.S. gets a break with a milder winter.
El Niño years in the Pacific Northwest are typically marked by warmer temperatures and below normal rainfall with little -- if any -- lowland snowfall. It's these winters where mountain snowpack typically suffers as we're drier to begin with, but what precipitation does fall usually comes with a higher snow level.


		In contrast, the Pacific Northwest is more likely to be wetter than normal in the late fall and early winter with the presence of a well-established La Niña. Additionally, on average La Niña winters are warmer than normal in the Southeast and colder than normal in the Northwest.

La Niña often features drier than normal conditions in the Southwest in late summer through the subsequent winter. Drier than normal conditions also typically occur in the Central Plains in the fall and in the Southeast in the winter.
Atmospheric Circulation and Surface Currents