May 26, 2017
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Newly Identified Climate Pattern May Have Caused California’s Drought

Research by scientists in Colorado describes five pairs of pressure cells in the upper atmosphere that can influence both droughts and floods. And soil moisture in California could provide a way to monitor the phenomenon.

By Matt Weiser

WHAT CAUSED THE worst drought in California history? This question will haunt the state’s water managers, even as they begin to put the five-year drought behind them.

Now a pair of federal researchers may have the beginnings of an answer to the question. In two new papers, they describe a new wave pattern in the upper atmosphere that may be responsible both for the long drought and the freight train of storms that ended the drought this winter.

The scientists at the National Center for Atmospheric Research (NCAR) have given this new pattern the somewhat sleepy name “wavenumber-5,” or Wave-5 in abbreviated form. It consists of five pairs of alternating high- and low-pressure features that encircle the globe about 6 miles (10km) above the ground. They are very large-scale planetary waves that have strong impacts on local weather systems by moving heat and moisture between the tropics and higher latitudes, as well as between oceanic and inland areas.

The waves at times become almost stationary, thereby influencing where storms occur. The result can be persistent weather patterns that produce droughts, floods and heat waves.

An example was the so-called Ridiculously Resilient Ridge, a large and persistent high-pressure ridge that blocked storms from reaching California during much of the drought.

Water Deeply recently interviewed Haiyan Teng, a scientist at NCAR and co-author of the recent studies. She emphasized more research is necessary, but in a few years Wave-5 could be useful to predict future weather events.

Water Deeply: How did you start to learn about this new climate phenomenon?

During the recent California drought, people have paid a lot of attention to the ridge and the deep trough in the Central and Eastern U.S. With more data, we found this ridge and trough are part of this global Wave-5 pattern. It consists of five pairs of high-pressure and low-pressure systems along the West Coast.

Water Deeply: How did we not know about this pattern before?

Teng: It’s not like people haven’t noticed this before. Scientists have documented this pattern before. This Wave-5 pattern occurs often. It’s like an intrinsic atmospheric pattern. It’s just that, in the day-to-day weather map, you are observing local weather. You are not observing low-frequency circulation anomalies.

In other words, when you look at a California weather map, you see around the jet stream there are lots of wiggle patterns moving from the west toward the east. But by looking at this weather map you cannot process how different this January average is from last January’s average.

Water Deeply: What drives this Wave-5 pattern? What causes it to form and change?

Teng: This pattern does not need special forcing, like from climate change or tropical forcing. It forms just from the mid-latitude atmosphere internal dynamics. But with certain kicks in the external forcing, it can last longer and become stronger.

In the paper, we only focused on the tropical precipitation forcing. The tropics can act like a kick to the mid-latitude internal dynamics. When there is anomalously strong precipitation in the Western Pacific, it can cause this Wave-5 pattern.

We are also looking at other factors in California, like soil moisture. This is ongoing work, and we haven’t written up this part of the results yet. But our preliminary results show that when California is very dry and the soil moisture is low, this can also strengthen this Wave-5 pattern. Soil moisture provides some kind of feedback to the pattern. Certain factors can make this pattern more persistent.

Water Deeply: Really? Dry soil in California affects this weather pattern?

Teng: Yes. This is an ongoing research project. Part of our research goal is to find out all these sources that may provide skills for seasonal or even subseasonal climate prediction. So we are turning knobs to see whether it can be a predictor. It’s a hot topic. Grant [Branstator], my coauthor, and I are involved in the NOAA drought task force, and this is a hot topic we’re discussing right now: How soil moisture can affect the seasonal to subseasonal weather forecast.

Water Deeply: Does that mean soil moisture in California could predict this Wave-5 pattern, the way ocean temperatures predict El Niño?

Teng: That’s right. In the paper we only focus on how tropical precipitation may enhance the strength of this Wave-5 pattern. But we don’t know to what extent we can predict these tropical Pacific anomalies. So right now we’re working on the soil moisture impact of these wave patterns. For example, if we know the California soil moisture is extremely dry, this may add additional information to the seasonal forecast.

Water Deeply: How much advance warning could it offer?

Teng: You need to think about it in terms of probability forecasts. It’s like if we know there is a Wave-5 pattern, it will shift the odds that a high-pressure ridge will occur.

We know this Wave-5 pattern has a stronger connection with the California drought. But this Wave-5 pattern is less predictive than El Niño. It is still too early to think about an operational forecast for this Wave-5 pattern. We’re still trying to understand the mechanism. Forecasts will be our ultimate goal, but I think there’s still a long way to go to understand the mechanism.

What we are doing is just trying to find out factors like how tropical precipitation or soil moisture can affect the probability of the Wave-5. Say, if we know California is very dry, and a dry California can increase the probability of the Wave-5, then that means once the Wave-5 occurs it will add to the chance of drought, and this can be like a feedback loop.

Water Deeply: Will you be able to use this pattern to warn about the next drought?

Teng: Possibly, if the soil moisture can provide us some skills on the weather patterns. If we can push forward the soil moisture research and if we know dry soil can cause Wave-5 patterns, we should be able to see it as a predictor for drought. But I don’t think it’s proper to make any promises at this stage, because this is still ongoing work.

This image shows the location of the five pressure cells that characterize the Wave-5 climate pattern (each highlighted by black arrows). The top image from January 2014 shows a large high-pressure cell (red) parked off the coast of California and the Pacific Northwest, which greatly aggravated the drought. The bottom image from January 2017 shows a shift: Now a low-pressure cell (blue) is parked in the same location, which allowed a series of heavy storms to reach California. (Image courtesy National Center for Atmospheric Research)

Water Deeply: Is Wave-5 connected in any way to other weather patterns, such as El Niño?

Teng: El Niño can influence mid-latitude atmospheric circulation. But this Wave-5 pattern is a mid-latitude intrinsic pattern, meaning it’s governed by a set of atmospheric movement equations. This Wave-5 pattern does not owe its existence to El Niño. It’s independent from it. And actually that’s one reason why we need to pay attention to this Wave-5 pattern.

In operational forecasts, the largest factor for variability in the mid-latitudes is El Niño. And people rely on El Niño information to provide seasonal forecasts. But because this Wave-5 pattern has a strong connection with this California drought ridge, you also need to take this Wave-5 pattern into consideration.

But the problem is, we don’t know how predictive this pattern is. The ocean is a slow process. We see warm water emerging from the Pacific and we know El Niño is coming, and it will impact California rainfall. But this Wave-5 pattern is an atmospheric-intrinsic pattern. It is definitely more difficult to predict than El Niño. That’s why we still don’t now how much this Wave-5 pattern can improve our forecasts. But because it has close connections to the original weather, we need to understand it.

Water Deeply: Once it forms, how long does this Wave-5 pattern last?

Teng: It lasts two weeks to 20 days. But it can also last longer. If you have persistent forcing – for example, surface temperature or soil moisture – it can persist for a whole season. That’s why, during the California drought, it had this longer pattern.

Water Deeply: Does Wave-5 cause other weather problems elsewhere on the planet?

Teng: We only focused on the California drought. But if you look at the model simulations, indeed these Wave-5 patterns are connected not only to the California drought, but it can also cause drought or flood situations along the jet stream in the mid-latitudes global wide. So it could be involved in the weather all over the place.

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