The media occasionally blames extreme weather events on El Niño or its near opposite, La Niña. Although these processes clearly influence weather events, they do not cause of most of our weather. Uneven heating from the sun causes most weather.
Nearly all of the heat in Earth’s atmosphere comes from the sun, but the shape of the Earth allows sunlight to strike some parts of the earth more directly than others. Further, the surface of the earth and clouds reflect some sunlight back into space and some parts of the earth reflect or absorb sunlight more readily than others. Uneven reception and absorption of sunlight causes some regions become warmer than others. More direct sunlight in the tropics than higher latitudes makes them warmer on average.
Currents in the atmosphere and ocean interact to mix the warm and cold air masses, reducing the temperature difference between warm regions and cold ones. Some of this mixing in the global atmosphere is driven by large clusters of thunderstorms located near the warmest regions of the tropics. Meteorologists call the mixing associated with these thunderstorms “convection”. Air rises in the region of rainfall in the convection, then moves away through the upper atmosphere and descends gradually back toward the surface of the earth. This descending air makes rain less likely in desert regions of the subtropics (such as the Sahara of Africa).
Not all regions near the equator have organized thunderstorm activity all of the time. Although the eastern Pacific Ocean along the equator is well within the tropics, the tongue of cold water that is normally located there makes that region a desert most of the time. The western equatorial Pacific region is usually more warm and rainy. Thus air frequently rises over the western Pacific. This rising air serves as an anchor to weather patterns across the North Pacific and North America.
Air rising in convection over the western Pacific region has to descend somewhere. If the earth did not spin on its axis, then that rising air would spread and gradually sink toward the poles. Since the earth rotates on its axis, however, the air moving toward the poles begins to turn toward the east and rotate. As the air continues to move farther away, it begins to rotate in the opposite direction. This pattern of swapping rotation direction becomes a train of waves in the atmosphere that originates over the western Pacific and extends over thousands of miles toward North America (See the rough drawing in Figure 1).
This wave train helps to organize weather events over North America. The wave train moves when the rain across the western tropical Pacific moves. El Niño is associated with unusually warm conditions along the equator in the eastern Pacific Ocean. El Niño influences our weather because it is associated with reduced rain across the west Pacific and more than normal rain across the eastern Pacific. This pattern of warm ocean water and enhanced rainfall over the eastern Pacific results in a wave train that extends toward the north and east from the middle of the Pacific Ocean instead of the western part:
The resulting trough of low pressure across eastern North America is associated with more frequent cool conditions and storms coming up the east coast.
When an El Niño is developing, forecasters frequently use average patterns during past El Niño events to predict the average weather for the coming winter season. These forecasts are occasionally helpful to farmers and investors in energy markets. However, no two El Niño events evolve in exactly the same way, and sometimes nature throws the forecasters a curved ball. Much of the winter of 2010-2011, for example, La Niña conditions were in place. Those conditions are often associated with above average warmth in the eastern United States. However, much of that season a trough developed over the region, yielding conditions colder than anticipated.
All El Niño or La Niña events are different, but also, these events are only part of the big picture. Other processes also influence the weather. Thus although El Niño might set the stage for an east coast blizzard, El Niño does not determine exactly when, where, or whether a blizzard will occur. So, blame weather events on the weather, not El Niño. At the same time, recognize that El Niño or La Niña can make such events more or less likely.