El Niño winter

This week I had a business trip to southern California. The winter “down south” has been very active this winter, particularly in January. Heavy rain, strong winds, thunderstorms, hail, and even waterspouts and tornadoes have pelted the Southland. If I didn’t know better, I’d think this was an El Niño winter!

In fact, it is. This year the normally cool waters in the tropical Pacific off South America became a lot warmer. On the west side of the Pacific, near Indonesia, the bathtub-like water became a little less warm. Warmer in the east, cooler in the west. That’s El Niño!

The El Niño is sometimes called a “warm event” because of the way the eastern Pacific warms up. But warm events are just part of a giant ocean-atmosphere thing called the “El Niño-Southern Oscillation,” or ENSO. The other phase of ENSO is a “cool event,” or “La Niña.” A cool event is literally the opposite of a warm event: the eastern Pacific (normally rather cool) gets even cooler; the warm western Pacific gets even warmer.

Now imagine a map of the Pacific (or look at one) and pay attention to the ocean between 10 degrees north and south latitude, from South America to Indonesia. Pretty big, right? And very important. This area, often called the “Pacific Warm Pool,” is the largest source of heat to the atmosphere of any place on earth. Heat escapes from water much faster than it escapes from land, and it escapes from warm water much faster than it escapes from cool water. So what do you get when you have a giant area of warm water? An area with a dominant effect on weather and climate worldwide!

El Niño and La Niña represent shifts in the locations of the warmest and coolest water, and this can cause significant shifts in wind patterns: the Trade Winds, which affect the tropics and subtropics; and the jet stream, which affects mid-latitude weather by moving and guiding our big winter storms.

But something else happens: the overall temperature of the Pacific changes as ENSO phases change. During an El Niño, the average temperature of the Pacific Warm Pool goes up; so do average global temperatures. La Niña events tend to bring cooler than average temperatures to the Warm Pool and to the globe.

Locally, El Niño and La Niña bring weather extremes to places throughout the world. Take Southern California: the wettest winters occur during El Niño events. Wild, extreme conditions are more likely. This year is no exception. Some people actually predicted the wild weather down there.

In the Northwest, our driest, mildest winters occur during an El Niño. But early-season weather is generally stormier than usual. So we tend to have wet autumns with rain and mountain snow, dry and mild mid-winters, and warm (but wet) springs. This year the progression has followed the script pretty well:

• Autumn was active, with lots of early-season mountain snow. The ski areas opened very early.

• The December “Arctic blast” was similar to other extreme cold events at the end of an El Niño, such as December, 1972 (the last time our temperatures went below zero) and December, 1998 (Arctic event following the 1997-98 El Niño).

• Mid-winter. We just finished one of the warmest Januaries ever. Temperatures never got below 32 degrees. It was a wet month, but very mild — just about as much above normal as December was below normal.

So if this year’s weather has closely followed the script, what does the script say for the remainder of the year? Well, don’t call this a “prediction,” but here’s what has happened “typically”:

A mild spring, wetter than average
A little more rain than usual in summer
A cooler than average summer

In six months, we’ll know if the script was followed!

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