Why does El Niño often lead to a warmer winter in Madison?
Forecasts of a relatively mild winter are being tied to a phenomenon called “El Niño” in the tropical Pacific Ocean. El Niño is so often invoked to support predictions of a warm winter that it begins to sound like Greek mythology. What is an El Niño, and why would it influence winter weather in the Midwest?
El Niño is a combined atmosphere/ocean circulation anomaly in the tropical Pacific Ocean in which unusually warm surface water extends westward from the coast of Peru into the mid-Pacific. The warm waters support persistent tropical thunderstorms in a region where they are ordinarily rare.
Because the Earth spins like a merry-go-round on its axis, air near the equator (like the outer edge of the merry-go-round) has higher momentum than air at, say, 30 degrees north latitude. Thus, when equatorial air is exported toward the pole, the wind must speed up because it must conserve momentum as it circulates on a shorter radius. At the top of these thunderstorms, air with high momentum is exhausted into the upper atmosphere and heads north, where it alters the position of the jet stream, which tends to warm our winter.
In other words, unusually warm water causes thunderstorms in the Pacific Ocean, which move the jet stream, and that brings warmer winter weather to the Midwest.