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El Niño Events results from weakening easterly trade winds. The easterly trade winds are driven by a surface pressure pattern of higher pressure in the eastern Pacific and lower pressure in the west. When this pressure gradient weakens, so do the trade winds. The weakened trade winds allow warmer water from the western Pacific to surge eastward, so the sea level flattens out.

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Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years. The average period length is five years. When this warming occurs for seven to nine months, it is classified as El Niño "conditions"; when its duration is longer, it is classified as an El Niño "episode".

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La Niña keeps hot, humid air concentrated over the south-central U.S., increasing the likelihood of tornadoes and damaging hailstorms.

When La Niña occurs, less wind shear is found in the developmental regions of the Atlantic, increasing the potential for a higher-than-normal amount of tropical systems.

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La Niña puts more emphasis on the northern jet stream, weakening the southern jet stream. The change yields more frequent storms in the northern U.S.

Western Canada and the Northwest endure the stormiest conditions during a La Niña winter, but the systems generally weaken as they travel over the northern Rockies. If a storm is potent enough, it can reach eastward and deliver snow, but on average the systems lack enough moisture to produce a major storm.

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To forecast an El Nino, scientists monitor temperatures in the upper 656 feet of the ocean. They are watching for the temperature to shift from the western Pacific to the eastern Pacific.

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An El Niño creates stronger wind-shear and more-stable air over the Atlantic, which makes it harder for hurricanes to form. However, the warmer-than-average ocean temperatures boost eastern Pacific hurricanes, contributing to more-active tropical storm seasons. Strong El Niños are also associated with above-average precipitation in the southern tier of the United States from California to the Atlantic coast. The cloudier weather typically causes below-average winter temperatures for those states, while temperatures tilt warmer-than-average in the northern tier of the United States. Record rainfall often strikes Peru, Chile and Ecuador during an El Niño year.
El Niño also affects precipitation in other areas, including Indonesia and northeastern South America, which tend toward drier-than-normal conditions. Temperatures in Australia and Southeast Asia run hotter than average. El Niño-caused drought can be widespread, affecting southern Africa, India, Southeast Asia, Australia, the Pacific Islands and the Canadian prairies.

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an irregularly occurring and complex series of climatic changes affecting the equatorial Pacific region and beyond every few years, characterized by the appearance of unusually warm, nutrient-poor water off northern Peru and Ecuador, typically in late December.

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a cooling of the water in the equatorial Pacific, which occurs at irregular intervals, and is associated with widespread changes in weather patterns complementary to those of El Niño, but less extensive and damaging in their effects.

In the United States, a La Niña winter means more rain in the Pacific Northwest, brief periods of below-average temperatures in the Northeast and generally dry and mild conditions for the southern tier.