Drought, Deluge and Wavenumber-5

It seems the drought and the record-breaking winter are connected. A recent study links long dry spell and heavy rains to same climatological phenomenon, Wavenumber-5.

By Daniel Merino

Recent study links long dry spell and heavy rains to same climatological phenomenon.

With recent spring storms dropping even more rain on soaked California, a record has been broken as the winter of 2016-17 has become the wettest in California history. Surpassing the El Nino winter of ‘82-’83, the intense rains have helped California emerge from a crippling drought.

It has been a dramatic turnaround in two short years. To experience such extremes back to back is unusual for California’s climate, and a new study, published in the Journal of Climate shows that both the drought and the record-setting rain may be caused by the same climatological phenomenon: a high-altitude system dubbed Wavenumber-5.

In the spring of 2015, Jay Famiglietti, a water scientist at the NASA Jet Propulsion Lab, warned, that the drought could spell disaster.

“Right now the state has only about one year of water supply left in its reservoirs, and our strategic backup supply, groundwater, is rapidly disappearing,” Famiglietti said in an interview with the LA Times in 2015. “California has no contingency plan for a persistent drought like this one -- let alone a 20-plus-year mega-drought -- except, apparently, staying in emergency mode and praying for rain.”

After three years, it seemed normal for California to be living in a waterless world, and while many prayed for rain, it did not fall in enough abundance for another 20 months. Last year’s El Nino was a normal year for precipitation, but after the bone dry 3 years prior, was not nearly enough to deal with the water deficit.

This past winter has been nothing if not eventful. Mudslides, flooding, and road closures shut down whole towns all over the state. Residents experienced long power outages, emergency evacuations, and, if caught in the mountains, the real meaning of being snowed in.

And the rain has kept coming through spring.

While not as intense as the winter storms earlier in the year, the April rains pushed California past the previous record for most rainfall in a water year (October 1st - September 30th) as measured by the Sierra 8-Station index, a group of measuring sites spread around North-Eastern California.

In coastal areas, rainfall numbers are sitting between 150-190 percent of average precipitation to date. The sierras have been hit much harder, with totals around 200 percent of average and some places nearing 300 percent.

To repeat, in some places three times the amount of water fell from the sky this year compared to the historical average.

And compared to the drought years?

On April 7th, 2015, the snow water equivalent, a measure of total water held in the Sierra snowpack, was just 6 percent of average. Two years later it is at 161 percent.

What happened? How did California go from death-by-drought to please-no-more-rain in two short years?

A new study published in the Journal of Climate offers an answer, or at least part of one.

In the paper, titled “Causes of Extreme Ridges That Induce California Droughts,” Haiyan Teng and Grant Branstator posit that a recently discovered climatological phenomenon called Wavenumber-5 may be an important driver behind the weather extremes.

"This wave pattern is a global dynamic system that sometimes makes droughts or floods in California more likely to occur," said Teng, lead author of the paper.

“Wavenumber-5 consists of five pairs of alternating high- and low-pressure features that encircle the globe about six miles (10 kilometers) above the ground,” says David Hosansky of the National Center for Atmospheric Research.

“It is a type of atmospheric phenomenon known as a Rossby wave, a very large-scale planetary wave that can 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 and by influencing where storms occur.”

During the drought years as well as this recent winter, wavenumber-5 was present and at least partially responsible for the weather extremes. In both situations the wave stalled, leaving either a high- or low-pressure system in place for long periods of time. When stuck in a high-pressure zone, as was the case in the ‘13-’14 and ‘14-’15 winters, California sees little rain as any storm that tries to impact the coast is blocked by the persistent high-pressure ridge.

This past winter was the exact opposite. With a near permanent low-pressure zone sitting over California, the storm door was wide open, allowing both north pacific storms and pineapple express atmospheric river events to slam into California.

The resulting downpours lasted for weeks, and while destructive, were just the medicine needed for drought-stricken California.

With reservoirs overflowing and the snowpack in the Sierras historic, it is easy to put the drought out of mind. Yet caution is needed when planning for the future.

Governor Jerry Brown officially declared the drought over on April 7th but warned that the future is uncertain with regards to water in California.

“This drought emergency is over, but the next drought could be around the corner,” Brown said. “Conservation must remain a way of life.”


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