How Much Will El Niño Help to Quench California’s Grinding Drought?

By: Bob Henson and Jeff Masters , 10:46PM,GMT on January 29,2016



 

With hillsides greening up fast, and chunks of coastline falling into the sea, it’s obvious that generous rains (and mountain snows) have returned to large parts of California this winter. But it remains an open question exactly how much this winter will help the state recover from a brutal four years of drought.

We can thank El Niño for the moisture return. One of the three strongest El Niño events since reliable records began in 1950 is still keeping sea surface temperatures (SSTs) much warmer than average over the central and eastern tropical Pacific. Those warm waters will continue to generate showers and thunderstorms thousands of miles east of their usual equatorial home. In turn, that displacement will keep forcing the atmosphere to adjust in ways that reverberate for thousand of miles, including northward into North America.


Figure 1. Boulders help shore up the base of an eroding cliff below an apartment complex (top) that residents were forced to evacuate in Pacifica, Calif., on January 27, 2016. Crashing waves and powerful rainstorms during this El Niño winter have put homes perched atop coastal bluffs near San Francisco in danger. Image credit: AP Photo/Jeff Chiu.


In the United States, El Niño tends to produce a stronger-than-usual subtropical jet stream. On average, this leads to wintertime conditions that are wetter than average from California across the Sun Belt to the Gulf States, with drier-than-average conditions across the northern tier of states. Temperatures during a strong El Niño typically segue from a warm central and eastern U.S. in November and December to a mild north-cool south tendency in January and February. These temperature patterns have largely materialized, albeit with a lot of variability. There’s been no lack of Gulf Coast moisture recently--especially in southern and central Florida, where this month is among the soggiest Januarys on record in many locations. And after a fire hose of Pacific moisture drenched the Pacific Northwest in the autumn, northern and central California have done very well in January, as has the immediate San Diego area.

Snowpack in the Sierras best in four years, but California drought still dire
Snowpack in the Sierras is 115 percent of normal, said the California State Department of Water Resources on Tuesday. This is the highest since 2011, when the snowpack was at 129 percent. The Sierra snowpack is a critical source of water to the state, contributing about 1/3 of the state’s water supply. Key reservoirs in the northern half of the state are still quite low, but beginning to rise. For example, Lake Oroville, the State Water Project's largest reservoir, is at 60 percent of its historical average for this time of year. This winter’s rains are still a long way from putting a significant dent in California’s four-year drought, though; the latest weekly Drought Monitor for California (January 26, 2016) showed 40 percent of the state in the highest level of drought, “Exceptional”, down from 46 percent at the beginning of the rainy season four months ago.


Figure 2. Change in drought conditions over the Western U.S. for the three-month period ending on January 26, 2016. Washington has seen a dramatic improvement in drought conditions, but California has seen very little improvement in its four-year drought. Image credit: National Drought Mitigation Center.


Will February come through for Southern California?
One part of California has largely missed out on the El Niño bounty thus far: the hugely populated coastal strip from Santa Barbara to Orange County, including the Los Angeles area. After a relatively dry autumn, this area has seen only near-normal precipitation for the month through January 29. Experts on El Niño and regional climate have been urging Southern Californians to be patient for some time, and they’re largely adhering to that advice.

Using the two maps below, John Monteverdi (San Francisco State University) pointed out that the strongest El Niños tend to bring Southern California their heaviest rains in Feburary and March, rather than January. “What I conclude from this is that, so far, what has happened in January is basically what climatology said would happen,” Monteverdi said. “Now if February and March precipitation fails, then this strongest or nearly-strongest El Niño in the record since 1950 will have failed to produce the precipitation we expected.”


Figure 3. Left: Departures from average precipitation (in inches) for the 30-day period from December 28, 2015, to January 26, 2016. Right: Departures from average precipitation in January for the five strongest El Niño events between 1950 and 2014. Image credit: NOAA Regional Climate Centers (left); NOAA/ESRL/PSD and CIRES-CU (right); images courtesy John Monteverdi.


Figure 4. Departures from average in land and ocean temperature for December 2015. Darkest red indicates areas of record-warm temperatures. Image credit: NOAA/NCEI.

Looking at the global scale, what jumps out like a sore (red) thumb is the vast swaths of warmer-than-average temperatures across the world’s oceans, especially across the tropics and subtropics. “Things are very, very warm globally, and subtropical ridging has been unusually pronounced this year,” said Daniel Swain (Stanford University), author of the California Weather Blog. “This ridging includes the semi-permanent Pacific High, which has been largely responsible for keeping the storm track aimed at Northern rather than Southern California.”

Swain added: “On a basic level, warm El Niño forcing puts a lot of heat into the tropical atmosphere and ultimately causes strengthening/straightening of the Pacific jet. That has happened this year, though it has occurred 500 or so miles north of what many people were expecting--a small shift in subtropical ridging by global climate standards, but a critically important one if you happen to live in Los Angeles.”

What’s different about this El Niño
Every El Niño has its peculiarities, and the blockbuster event of 2015-16 is no different. Figure 5 (below) shows the SST departures for the six strongest El Niño events since 1950. One notable difference between the current event and past ones is the dramatically warmer Niño4 region--the westernmost area, far out in the equatorial Central Pacific. This toasty Niño4 helps explain the turbocharged Central Pacific hurricane season of 2015, which sloshed into January 2016 with the unseasonable development of Hurricane Pali--the first hurricane known to prowl within one of the Niño regions shown in the map. It’s not yet clear how the distinctive SST pattern of the Niño regions this winter is intersecting with other factors, including the widespread oceanic warmth elsewhere.

One thing is clear: this isn’t an El Niño Modoki. That subcategory of El Niño, whose downstream effects can differ from those of a “classic” event, is defined by SSTs that are above average in the central Pacific (Niño4) but below average in the far eastern Pacific (Niño1+2). Right now, the Niño1+2 region is still running well above average.



Figure 5. A look at temperatures in the four Niño regions of the equatorial tropical Pacific (see map at top) during the last six El Niño events classified as “strong” or “very strong”, including the current El Niño through December 2015. Years shown in each graphic are the beginning year of the event. Image credit: NOAA Climate Prediction Center (top), courtesy Jan Null, @ggweather (bottom).

Some hopeful signals for the weekend and beyond
The latest Pacific storm to approach California is trending southward in model projections, a good sign that the Los Angeles area has a shot at its healthiest rains in almost a month. Another good sign: this storm has a robust channel of moisture extending southwestward to the tropical Central Pacific. Local forecasters expect as much as 1” in coastal areas and up to 2” in the mountains and foothills, and those numbers may prove low. If this storm comes through as expected, it will bump the area up to well-above-average totals just in time to close out January. Things may quiet down again for the first few days of February, but that’s not so unusual for SoCal, where even a torrentially wet winter by local standards may see rainfall on only a few days.

In its most recent experimental forecast discussion for weeks 3-4, issued on Friday afternoon and valid for the period Feb. 13-26, NOAA points to model indications that the subtropical jet will undercut Pacific ridging and enhance the odds of above-average rainfall and snowfall over far Southern California and much of the Southwest. As shown in Figure 6 (below), the week 3-4 probabilities have that certain El Niño look to them.

We’ll have a new post on Monday. Have a great weekend, everyone!

Bob Henson and Jeff Masters



Figure 6. NOAA’s experimental extended precipitation outlook (departures from average) for the two-week period of Feb. 13-26, 2016. Image credit: NOAA Weather Prediction Center.