Thursday, 3 March 2016

El-Nino in California

El Niño’s Long-Awaited Grand Performance Is On Its Way to California



By: Bob Henson , 6:07 PM GMT on March 02, 2016
     
After a crushingly dry February, it looks as if early to mid-March is likely to bring California some of the serious moisture it needs from the 2015-16 El Niño event--and perhaps some unwanted flooding and mudslides. Long-range models are increasingly confident that the low-latitude jet stream that’s been dodging the California coast for weeks will finally plow into the state over the next 10 to 15 days, hauling copious amounts of Pacific moisture inland with it. The last few runs of the GFS and ECMWF models have become especially bullish on the development of one or two atmospheric rivers (ARs) heading into California over the next week or two. Roughly30% to 50% of annual precipitation in the West Coast states occurs from just a few AR events per year.

The first significant storm should plow into northern and central California this coming weekend, followed by a stronger series of storms affecting most of the state during the following week. The 0Z Wednesday operational run of the GFS model doused parts of the central and northern California and Sierra Nevada with 10” to 20” of precipitation over the ten-day period ending at 7:00 pm EST Friday, March 11. The GEFS and ECMWF ensembles, though less dramatic than individual runs, still paint a very wet picture for the state. It remains unclear how far into southern California the biggest rains and mountain snows will extend. The outlook for very heavy precipitation is a bit more confident from central California all the way north to Washington. Already, some California reservoirs are 
releasing water: though this may seem odd while the region is still in drought, it’s a long-employed strategy to help reduce the odds of flooding when torrential rains are predicted.

The stormy weather is coming just in time for the close of the 
El Niño Rapid Response Field Campaign being conducted by NOAA’s Earth System Research Laboratory. The project includes flights from a NOAA Gulfstream IV hurricane-hunting aircraft, as well as hundreds of radiosonde and dropsonde launches profiling the remote Pacific atmosphere. An extra week has been added to the project’s flight schedule, which will now run through March 10. See our blog post from January 12 for more on this project.

Figure 1. The 7-day forecast for precipitation from 12Z (7:00 am EST) Wednesday, March 2, 2016, through Wednesday, March 9. Another round of intense precipitation may also affect California late next week, just beyond the range of this forecast. Image credit: NOAA/WPC.

With a little help from the MJO
The Madden-Julian Oscillation can be credited, at least in part, for the precipitation prospects over California. This recurrent cycle of tropical showers and thunderstorms has an active phase that has recently pushed eastward to the eastern Pacific, with a suppressed phase on the opposite side of the global tropics, over the Maritime Continent. Acting in tandem with the state of El Niño itself, the active MJO phase will enhance the moisture available to be drawn from the tropics into the subtropics, where the juicy air can be entrained into midlatitude storms and any atmospheric rivers approaching California. By later in March, when the active phase has moved to the Maritime Continent, a suppressed phase should follow on its heels over the eastern Pacific. “This could temper the favorable intraseasonal/seasonal state that drives California precipitation,” said Michael Ventrice (The Weather Company). As a result, West Coast rains and mountain snows may well decrease again in the latter half of March, at least for a week or two.

Figure 2. Integrated water vapor transport (IVT) projected to be heading toward California by the 00Z Wednesday GFS model run at 00Z Sunday, March 6 (7:00 pm EST Saturday). IVT incorporates the amount of moisture in the atmosphere as well as how quickly it’s moving. The channel of moisture heading toward central California includes IVT of greater than 750 kg/m/s. IVT is used by many researchers and forecasters to identify and track the evolution of atmospheric rivers. Image credit: NOAA/ESRL.

The flop that was February in California
Only a month ago, there was good reason for drought-stricken Californians to believe that the state was on its way to at least partial recovery after four years of punishing drought. Several major storms had pushed snowpack just above the seasonal average in the crucial Sierra Nevada range, which supplies about a third of the state’s water supply through snowmelt. The hope was that February and March would consolidate these early gains with at least a few more wet-but-not-damaging storms.

Then came February--a disappointingly dry month for any winter, much less an El Niño one. The month was dominated by an upper-level ridge that blocked Pacific storms and allowed temperatures to hit record levels beneath clear, sunny skies. It wasn’t quite the winter-long Ridiculously Resilient Ridge that marked the last couple of years, but it was enough to tamp down the modest precipitation surpluses across many areas. Most parts of the state got only a paltry 10-20% of average precipitation in February, as noted by Jan Null (Golden Gate Weather Services). Four major cities--San Diego, Los Angeles (Downtown), Sacramento (Executive Airport), and San Jose--saw their warmest February on record. By month’s end, the Sierra snowpack had dwindled to 85% of its typical water equivalent for the date.

What’s especially striking is how the November-to-February period turned the West Coast precipitation pattern typical of strong El Niño events on its ear. Seattle gets about twice the moisture of Los Angeles between November and February in an average year--roughly 21” vs. 10” (see Figure 3). Strong El Niño events tend to boost LA’s rainfall substantially, with little effect on Seattle’s. This time around, Los Angeles netted just 4.54” from November through February, while Seattle racked up an amazing 32.91”. In fact, by some measures, Seattle is having 
the wettest winter in its history, and even more heavy rain is on the way (see Figure 1 above). WU weather historian Christopher Burt takes a closer look at the precipitation to date across California in his latest post.

Figure 3. November-February precipitation totals for Seattle, San Francisco, and Los Angeles for each of the El Niño seasons classified as “strong” or “very strong” by NOAA since 1950. Image credit: Christopher Burt.

What kept the rain away?
It’s not uncommon to get a two- or three-week break between winter storms over California, especially toward the south, but the duration of the February mini-drought in the midst of a powerful El Niño was a puzzler for experts and everyday Californians alike. Data gathered throughout the month by NOAA’s field project may help shed light on what happened. One obvious factor: a change in the equatorial showers and thunderstorms (convection) that typically lead to El Niño impacts on a broader scale. Instead of the convection shifting to the eastern tropical Pacific, as is typical during a strong El Niño, most of the storminess in February was centered over the central Pacific. The downstream effects of this dislocation likely played a role in the West Coast ridging and the dearth of major California storms.

Figure 4. Sea surface temperatures (relative to seasonal average) for February 15, 1998 (top) and February 15, 2016 (bottom). Image credit: NASA.

Along with the displaced convection, the west-to-east gradient in sea-surface temperature across the tropical Pacific has been substantially weaker than it was during other strong El Niño events of modern times. The eastern tropical Pacific has been plenty warm: sea surface temperatures in the crucial Niño3.4 measuring region were 3.1°C (5.6°F) above average in the week of November 18. That’s the warmest weekly departure in NOAA records 
going back to 1990. What’s been absent, for the most part, is the horseshoe-shaped region of cooler-than-average water that typically cradles the west end of the El Niño equatorial warming, extending from Indonesia toward the northeast and southeast (see Figure 4, top, from February 1998). “These anomalies just never materialized, perhaps due to the background warming trend, or some other currently unknown reason,” said Klaus Wolter (NOAA/ESRL).

Likewise, the east-to-west trade winds across the tropical Pacific have not weakened or reversed as much as in previous strong El Niño events. “In January 1983 and 1998, it was very cool west of the Date Line, especially in the Northern Hemisphere,” pointed out Kevin Trenberth (National Center for Atmospheric Research). “This year it’s at least 1°C warmer. The gradients along the equator are much less. So the reversal in the trade winds is nowhere near as extensive or as strong as it was in those two events.”

All of these factors have thrown sand in the cogs of the El Niño machine, cutting back on its ability to synchronize ocean and atmosphere across the tropical Pacific. “Every El Niño has its own character and gets modulated by other effects,” Trenberth noted. Despite its idiosyncrasies, the El Niño machine of 2015-16 is far from broken right now. Although the West Coast response to this El Niño hasn’t followed the playbook, many other parts of the world have seen prototypical El Niño conditions over the last few months. These include:

Drought, fires and severe air pollution over Indonesia late last year
—A 
reduced summer monsoon over India, plus catastophic autumn rains in far southeast India (both consistent with El Niño)
Increasing heat and drought across northeast Brazil, along with significant drought relief over the hard-hit Sao Paolo region
—The exacerbation of a 
severe multiyear drought in southern Africa
—Warmer-than-average winter temperatures across 
Canada and the northern U.S. (in fact, virtually all of the contiguous U.S. saw a warmer-than-average winter)
—Wet conditions and 
enhanced severe weather across the Gulf Coast and Florida

Figure 5. The mean value of model-generated departures from average precipitation for each month from November through April during strong El Niño events. The maps are based on an ensemble of 130 simulations of weather from 1979 to 2014. Image credit: Andrew Hoell, NOAA/ESRL.

It’s the whole wet season that counts
Ever since last autumn, when it became clear this El Niño was likely to be among the biggest on record, experts warned not to expect an entire winter chock-full of heavy rain over California. 
Seasonal forecasts from the NOAA Climate Prediction Center trended toward a wet winter, to be sure, but they were amply caveated with the small but real chance that precipitation could wind up below average. The importance of El Niño’s strength in determining the California outcome was highlighted in a paper published this winter in Geophysical Research Letters. To get around the problem of small sample size, lead author Andrew Hoell (NOAA/ESRL) and colleagues classified each winter from 1979 to 2014 based on its Niño3.4 temperatures through that period. Then they simulated the weather over that 35-year period a total of 130 times, using three different models primed with the actual evolution of SSTs, sea ice, greenhouse gases, and ozone. The resulting ensemble thus offered 130 different takes on how a given series of El Niño events might influence California rain and snow.

Not too surprisingly, strong El Niño events are most likely to make California wet. During a strong event, the study found that Northern California is roughly five times more likely to get a winter with 150% of average precipitation, with a near-zero chance of getting less than 50% of average. The less intuitive outcome is that moderate and weak El Niños showed only a small (and less reliable) ramp-up in average precipitation. In southern California, the effect of a strong El Niño was even greater: for such events, the models showed a near-zero chance of getting less than 75% of normal November-to-April precipitation.

If it were to stay most dry in southern California this March and April, that near-zero chance would become a reality. However, it appears that March is just as climatologically likely as any other month to produce a bumper crop of rainfall relative to average across the region during a strong El Niño (see Figure 5 above). There is still plenty of room for a “Miracle March” to boost the entire water-year outlook, as this month just might manage to do.

We’ll be back with our next post on Friday.

Bob Henson

Figure 6. Panorama of the scanning X-Band radar installed in San Francisco in support of the NOAA 2016 El Niño Rapid Response Field Campaign Image credit: Francesc Junyent, CSU/CIRA.

Figure 7. Participants confer at NOAA’s Earth System Research Laboratory in Boulder at a daily forecast briefing for the NOAA 2016 El Niño Rapid Response Field Campaign. Image credit: Barb DeLuisi.



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