Al Stahler: Next winter’s weather | TheUnion.com
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Al Stahler: Next winter’s weather

 

The warmest ocean water on Earth sloshes around the far western Pacific, over by Australia and Indonesia. Super-warm water, on the other side of the Pacific, is one of many factors that drive California’s climate of dry summers, wet winters.

Look out over a sun-baked parking lot, and the air shimmers as plumes of warm air rise up off the hot pavement: Warm air rises.

Like a ginormous parking lot, Earth’s tropics bake in the hot sun, and warm air rises big-time. Warm air rises, especially, over the super-warm water on the far side of the Pacific.



What goes up, must come down. The warm air that rose in the tropics comes down in all sorts of places, more in some laces than others. Where it especially descends, the air piles up. Those piles of air act like rocks in a stream. Piles of air force winds, force storms to go around them. An especially large pile of air, out in the Pacific, blocks storms from reaching California in summer.

When the sun heads south, in winter, air rises and descends – pies up – differently; the new arrangement allows storms to reach California … at least, in a good winter.



Over the tropical Pacific, the trade winds blow east-to-west, pushing warm water towards Asia, piling up that warm water, on the far side of the Pacific. The super-warm water, in turn, warms the air … and warm air rises … and then comes down, forming mountains of air that, among other things, block California-bound storms in summer.

The surface water that blows across the Pacific leaves behind space for colder water to rise from below, making the surface of the central Pacific … cold … usually.

Every few years, though, the trade winds slow … sometimes they stop entirely … or even shift into reverse.

Water does not remain piled-up by itself. If the trade winds stop blowing, the pile of warm water on the far side of the Pacific collapses, and the warm water sloshes eastward, back toward the central Pacific, toward South America. The central Pacific warms … which changes where warm air rises … which, in turn, changes where air sinks and piles up.

Which, finally, changes the paths of storms … changes how easily storms can get into California.

This relaxation of the trade winds, and sloshing-back of warm water to the central Pacific – thus changing weather patterns – is El Niño.

El Niño – the little boy – has a sister: La Niña.

Have you ever been in a situation – say, applying for a job – where you really want people to think you’re normal. You therefore work – hard – to act super-normal.

It’s hard to act super-normal without coming off as very, very strange.

Unlike El Niño, which re-organizes the tropical Pacific – warming cool regions, cooling warm ones – La Niña is the tropical Pacific trying to act super-normal. The trade winds blow strong. Water on the far side of the Pacific – normally warm – grows super-warm. Water in the central Pacific – normally cool – grows cooler yet.

Acting super-normal, La Niña over-does it. And the resulting pattern of rising and sinking air – all-too-often – steers wintertime storms away from California.

The Pacific was in La Niña mode last winter, and last winter was, indeed, a droughty one.

Then – last spring – La Niña faded away.

The Climate Prediction Center (CPC) – part of the National Oceanographic and Atmospheric Administration (NOAA!) is charged with forecasting what the climate will look like, a season ahead. For some months now, the central Pacific has been growing cooler – first beneath the surface, now right on top.

Last Thursday, the CPC issued its latest monthly climate forecast: La Niña is back.

La Niña and El Niño are not the only patterns that show up in the ocean and atmosphere. Many patterns come and go, steering storms this way and that.

Back in the winter of 2017 … the Oroville Dam over-topped … hundreds of thousands of people in the flood plain of the Feather River were evacuated. And 2017 was a La Niña year. I asked oceanographer Bill Patzert, recently retired from JPL (NASA’s Jet Propulsion Lab) in Pasadena, about La Niña’s reputation for droughty weather.

“La Niña often means below average rainfall for Southern California, but La Niña’s not necessarily bad news for northern California. It was last winter … but not every winter. Northern California is right on the border between the Pacific Northwest and southern California. Often, during a La Niña, northern California can get a pretty good drenching.”

And now … one small thing. Recall that it’s the behavior of the trade winds – revving up, slowing down, shifting into reverse – that pushes water around, determining where the tropical Pacific gets warm or cold … where air will rise or fall.

Even as the trade winds determine the pattern of warm and cold, in and over the Pacific … that pattern of warm and cold in the ocean determines how hard, and which direction, the trade winds blow. Chicken … egg … ?

Al Stahler enjoys sharing science and nature with friends and neighbors in The Union and on KVMR-FM. He teaches classes for both kids and grown-ups, and can be reached at a.a.stahler11@gmail.com

El Niño and La Niña are patterns of warm and cool water in the tropical Pacific. Both influence California's weather.
Photo courtesy NASA

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