7 Things You Need To Know About El Niño and the Drought | KCET
7 Things You Need To Know About El Niño and the Drought
We've had several years of increasing drought here in California. Lawns are going brown and reservoirs running dry. So it's no surprise that Californians are eagerly awaiting the possible arrival of a wet winter brought by El Niño.
Those hopes have been stoked in recent days, as forecasters increase the confidence in their long-range predictions of a strong El Niño developing, with rainstorms bringing as much as two years' worth of rain to parts of the West Coast. One meteorologist went so far in August as to call the possible shift in winter weather a "Godzilla El Niño," and media outlets wasted no time in spreading the meme. And not without justification: it's starting to look like the winter of 2015-16 might be the strongest El Niño ever recorded.
But those predictions' greater confidence shouldn't prompt you to haul your lawn sprinklers out of storage. Even in the best possible circumstances, El Niño won't cure California's long-term water woes... and it may cause some huge problems of its own. Here are seven things you need to know about this year's El Niño.
El Niño isn't a storm.
The phenomenon we know as El Niño is part of one of the planet's most influential long-term weather cycles, the El Niño/Southern Oscillation, or ENSO. While Californians tend to think of El Niño as consisting primarily of a wet Californian winter, the truth is more complex.
The "Oscillation" in ENSO refers to a cycle in surface temperatures in the Pacific Ocean, as well as in the winds that blow across the ocean's surface. During a normal year, equatorial trade winds blow west across the Pacific, driving surface water before them. That surface water, warmed by the sun, accumulates in a "warm pool" northwest of Australia and New Guinea. The warm pool heats the air above it, which rises. That rising air reinforces the trade winds, as surrounding air is drawn into the area above the warm pool by convection.
Meanwhile, a similar process takes place beneath the ocean surface: as surface water is pushed westward by the winds, colder water from the ocean deeps flows upward along the Pacific Ocean's east edge, also known as the west coasts of North and South America. That cold water upwelling is ecologically important: it cools the surface waters, and brings deep-sea nutrients to fish and other marine life closer to the surface.
The trade winds keep the warm pool pushed to the west, and the warm pool boosts the trade winds: a neat example of positive feedback. But every few years, for reasons that are not well understood, that feedback falters. The trade winds slacken and the warm pool extends eastward across the Pacific, and which one of those two changes sparks the other is a matter of debate among meteorologists.
As the warm pool moves eastward, that positive feedback loop breaks down. Upwelling slows along the American coast, and nearby waters hold fewer of the sardines and anchovies that usually gorge on the bounty of plankton the upwelling provides. (Peruvian fishermen noticed that the sudden warmer waters and absence of fish happened in late December, and named the phenomenon El Niño, "the little boy," a reference to the Christ child.
This may turn out to be the strongest El Niño on record.
El Niños vary in strength, and meteorologists gauge the strength of an El Niño cycle by measuring ocean temperatures in an area along the Equator southeast of Hawaii, and well east of the usual location of the warm pool. If surface water there is warmer than usual, that means the warm pool is "sloshing east," an indication of an El Niño in progress.
Forecasters start expecting El Niño events when temperatures in that part of the Pacific reach half a degree Celsius above average. (That's just under one degree Fahrenheit.) The higher the temperature above average, the stronger the El Niño. In the last week of August 2015, ocean surface temperatures in the area had reached 2.2°C (3.6°F) above average. That's the strongest El Niño signal ever recorded in August, and one of the strongest ever recorded in any month.
Forecasters are now saying this may be one of the most powerful El Niños ever recorded. But they're still steering clear of predicting a drought-busting winter for California. For one thing, the trade winds haven't yet stopped: they've weakened, but they'll have to weaken more for a full-fledged strong El Niño to make California wetter.
Even a strong El Niño doesn't guarantee a wet winter for the whole state of California.
At this point you may be asking how El Niño's warmer water off the South American coast causes wet winters in California? The answer: it doesn't. At least not directly. The most El Niño does is increase the chances that existing storms will bring their rain here instead of the Pacific Northwest or Central America.
What actually brings the rain is a shift in the position and strength of the jet stream. Or more specifically, what meteorologists call the "subtropical branch" of the jet stream. The West Coast of North America often has two jet streams blowing eastward over it; one, lopping down from Alaska, is called the polar branch, while the subtropical branch tends to come in from farther south in the Pacific Ocean. (The atmosphere is a complicated place, so both branches aren't always present at the same time, and sometimes neither branch shows up.)
During a normal year, California gets most of its winter precipitation from storms brought into the state by the polar branch of the jet stream, which wobbles southward from the usually rainy Pacific Northwest. It's that polar jet stream that's responsible for California's usual distribution of winter rain and snow: more in the north, less in the south.
The warmer Eastern Pacific conditions brought by El Niño change that up by strengthening the subtropical branch of the jet stream, and that usually takes energy from the polar branch. During a weak El Niño event, that can result in California getting less rain and snow; a weakened polar branch of the jet stream generally won't wobble into California as much.
However, a strong El Niño will strengthen the subtropical branch of the jet stream and deflect it so that it pushes available storms toward California more often. They'll be heading up from the subtropics, which means the southern part of the state will get them first.
And that means that there's a chance that even a strong El Niño will deliver extra rainstorms only to the southern part of the state, leaving the north drier than average -- and the north is where most of the state's reservoirs are.
As of this week, many meteorologists are pretty confident the whole state will have a wetter than average winter. That would be great for our rivers, streams, and dwindling reservoirs. But here's the thing: even with our growing ability to predict it, the weather can still outsmart us. There are still a few months for things to go wrong.
That "Ridiculously Resilient Ridge" that's kept the state dry for years? It might be toast.
The last few years have seen extreme weather across North America, with both a record-breaking drought and heat in California and the Southwest and record lows, including the "Polar Vortex" winter of 2013, in the midwest. Both have at least part of their origins in a persistent area of high atmospheric pressure off the coast of the Pacific Northwest, a so-called "blocking high," dubbed the "Ridiculously Resilient Ridge" by meteorologist Daniel Swain.
The Ridge, which has been in place since late 2012, stretches from coastal Alaska to extreme northern California. It's essentially an obstruction in the jet stream, like a big rock dropped in the middle of a creek, forcing the water to flow around it. Without the Ridge, the jet stream would likely have brought winter storms to California over the last three or four years. With the Ridge blocking its way the jet stream has looped up around the Ridge's north end, dumping all of California's water in the Pacific Northwest, even causing flooding in Washington state. And then it loops back down across the midwest from the north, bringing Arctic air to the plains and Great Lakes in mid-winter -- the so-called Polar Vortex.
There's also the related phenomenon called "The Blob," a patch of unusually warm water in the eastern Pacific that extends from Alaska to Mexico. Though it's not 100 percent certain, it is likely The Blob is there because of the persistence of the Ridge, which has deflected storms out of the area and prevented the kind of wind-driven mixing of surface and deep water that would otherwise have cooled The Blob some time ago.Though some news outlets are characterizing the upcoming fall as a contest between El Niño and The Ridge, that contest may already be over. According to Daniel Swain, the Ridge has likely been kept in place the last several years by the very same warm surface temperatures in the Western Pacific -- the warm pool near Indonesia -- that El Niño is ending.
As for The Blob? Its weeks are numbered. During the first week of September, for the first time in recorded history, the eastern Pacific held three simultaneous hurricane-strength storms. (At this writing, that's down to two, with two strong tropical storms keeping them company.) Wave action along the California coast is about as vigorous as you might expect from having four big storms offshore creating swells. Waves stir up cold water from beneath the surface, and that will quite likely erode The Blob's high temperatures.
One El Niño winter won't end the drought.
Okay, we admit that may seem counterintuitive. If heavy rains don't fix the drought, what can? It's all in the numbers. An exceptionally strong El Niño might bring one and a half times the state's average rainfall, or -- like the strong El Niño in 1965-66 -- it could bring a period of merely average rainfall. But we've had four years of drought, more if you count 2011 as a wet year in the middle of a drought rather than the beginning of the current drought and the end of the previous one. Some parts of California have been deprived of the equivalent of two years' average precipitation.
That means that while getting some water on our native landscapes and in our reservoirs will be quite welcome, an optimal strong El Niño won't make up the state's water deficit.
There's good news to add here, and then some bad news on top of that. Many El Niño events last for more than one year, and forecasters now say there's an 85 percent chance this El Niño will last well into spring 2016. That would give us a chance to catch up even farther on the rainfall we've done without , but it probably won't get us all the way there.
The bad news is that drought may well have limited the landscape's ability to hang on to that water.
Flooding and landslides are a serious threat.
California is dry. And just as a kitchen sponge that gets too dry can repel water, dry landscapes often have trouble letting rain soak in. Especially if it falls all at once.
Some dry soils, such as the montmorillonite clay common throughout coastal California, actually shed water if they get too dry. It takes slow, gentle rain to moisten such soils enough that they'll be able to absorb even more water. Vegetation plays a role here as well: a thriving landscape of forest or chaparral will serve to retain rainwater. The vegetation, and its associated network of roots and organic matter act as sponges.
But where that vegetative cover has been damaged, either through drought-spurred wildfires or because the plants simply died of thirst, that water retention doesn't work as well. That means more rain will run off instead of soaking into the ground.
During our last strong El Niño, in February 1998, much of the state got a foot and a half of rain or more in a single month. Catastrophic flooding resulted, and that wasn't even on the heels of a multi-year drought. (The previous year had also brought heavy rains that caused flooding.)
If we get storms to match those we got in February 1998, we can expect even worse flooding, along with increased debris flows and mudslides as the torrents of rain hit soil that's unprotected by vegetation.
Even if we get enough El Niño storms to re-fill our reservoirs this time, drought may well be the new normal.
The Earth's climate is changing, and California's climate is changing along with it. Though it's far from certain, most climate models anticipate that as the Earth's average temperature climbs, California will see more dry spells like the one we're in now. There will likely still be heavy rains: a warmer climate means higher ocean temperatures, and it's possible that those warmer ocean waters will propel more and stronger El Niños to buffet our state with winter rains every few years.
That may seem reassuring, but trading our old California where we can count on some rain during the winter for a new California where we parch four years out of five and then flood the fifth year won't be comfortable. There will be things we can do to adapt to the New Normal, but you're probably going to have to get used to the idea that lawns belong in Ohio. At least you can tell yourself that the drought's good for the wine.
For ongoing environmental coverage in March 2017 and afterward, please visit our show Earth Focus, or browse Redefine for historic material.
KCET's award-winning environment news project Redefine ran from July 2012 through February 2017.
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