This winter has been a record winter with snow piled up to enormous heights across the state. In the southern Sierra Nevada, the amount of snow is literally off the charts.
“The graph had to extend vertically, along the y-axis,” Benjamin Hatchett, an earth system scientist at the Western Regional Climatic Center and Desert Research Institute, told Monday’s drought webinar.
The bounty wasn’t due to a single storm this winter — it was due to a continuous barrage of storms over the past few months that resulted in astonishing results for scientists.
A rare triple La Niña forecast began in winter, with experts predicting months of dry weather in California. But other climate patterns also affect the weather. Scientists say the location of the high and low pressure systems in the Pacific this winter has left the storm door wide open for months.
“Storm patterns continue to reappear,” said Dan Cayan, a climate scientist at the Scripps Institution of Oceanography at the University of California, San Diego.
catch the wave
Cayan and other scientists have pointed to abnormal convection (the vertical transport of heat and moisture) as one potential factor in this winter’s heavy rain and snow in the western tropical Pacific Ocean. Despite the turmoil thousands of miles from California, it still affects what’s happening on the West Coast.
“Heat causes these giant atmospheric waves to propagate toward the poles and affect our weather,” said Amy Butler, an atmospheric scientist at the National Oceanic and Atmospheric Administration’s Laboratory for Chemical Sciences.
Marybeth Arcodia, an atmospheric scientist at Colorado State University, explains that the effect is similar to dropping a stone into a pond and creating ripples on the surface.
“Instead of dropping stones,” Arkodia said, “we have these huge storm systems that disrupt the upper atmosphere and send these waves.”
Waves influence the path of the jet stream — winds orbit the planet and rise for miles through the atmosphere — how California’s weather patterns set. This winter, these effects and more could have overwhelmed any signs of dryness that La Niña conditions usually mean for the state.
Scientists hypothesize that what happened in the tropics may have formed an anticyclonic ridge, which formed and reformed throughout the winter in the Gulf of Alaska and the eastern North Pacific off the west coast. may have strengthened the troughs of the cyclones that were formed.
“That valley is deep enough that many storms are tracking further south,” Kayan said.
These storms brought in frigid air from the Gulf of Alaska, bringing chills and above-normal snow to California.
“It just leaves the storm door open, and the cold air often mixes with the storm and keeps the snow falling,” Hatchett said.
Meet MJO
Rena Suzuki/The Chronicle
Gabriel Lurie/The Chronicle
Above: A toddler sits under the rain cover of a double stroller while being pushed by an adult with an umbrella in the rain on Wednesday, March 29, 2023 in San Francisco, California. Valencia Avenue in San Francisco, California during heavy rain on Tuesday, March 28, 2023. / Lee Suzuki, Chronicle.Gabriel Lurie, Chronicle
The tropical disturbances behind these atmospheric waves are part of a typically 20- to 100-day-long cycle known as the Madden-Julian Oscillation (MJO). The pattern begins as the storm system rotates near the Indian Ocean, moves eastward, and then fades away around the International Date Line.
“These can range from 5 to 10,000 kilometers,” says Arcodia. “That’s one to two times the width of the United States.”
Scientists find that MJO activity in the western edge of the Pacific is associated with atmospheric rivers reaching the coast of California days later, along with high west-to-east winds in the equatorial stratosphere. Did.
“This fits well with what we’ve observed this winter,” said Chris Castellano, a meteorologist at the Scripps Institute of Oceanography and the Western Center for Meteorology and Water Extremes at the University of California, San Diego.
MJO activity increased in the Western Pacific in December and January. In a similar time frame, an atmospheric river parade marched through California. These narrow plumes of tropical moisture fueled a storm that dropped more than 30 trillion gallons of water in three weeks. State Average Total Precipitation.
“We had a very strong MJO in the western Pacific in March, and about … five days later we saw the first major (atmospheric riverine) event,” Castellano said. “And less than a week later, another attack followed, and finally a third attack occurred around March 20.”
Castellano co-authored a Scripps-led study published earlier this month that described tools for predicting atmospheric rivers a few weeks ago, based on MJO activity and stratospheric wind direction. But Castellano said the predictions work best when the winds are blowing in the opposite direction to what they were this winter.
Predicting wet weather is a difficult science, as there are many interrelated weather patterns and even random weather phenomena. More research is needed to understand exactly what combination of wet conditions this year.
“There is no recipe for an atmospheric river that causes a major flood in California,” said Arcodia.
future winter
Scientists are still investigating how the MJO, and its relationship to other climate patterns, might change in a warming world, Arcodia said.
But there are indications that climate change has played a role in some of this winter’s most influential events.
“The nature of storms is changing,” said Michael Wehner, a climate scientist at Lawrence Berkeley National Laboratory. Tweeted.
California’s Mediterranean climate naturally fluctuates between wet and dry seasons, but as the earth continues to warm, the extremes become wet and dry. So even after such a wet winter, improving the state’s drought and climate resilience remains critical, Hatchett said.
“We’re in really good shape, but we should always think about the future,” Hatchett said.
Contact Jack Lee: jack.lee@sfchronicle.com
