California desperately needs a wet winter, but a variety of atmospheric patterns could keep it from happening
SACRAMENTO, Calif. – A New Year of Water began October 1, marking the start of a pivotal year in water security.
The summer trend will continue for the long term forecast. Aside from a rare soaking storm in September, it has been a very dry year. In fact, the years 2020-2022 have been the driest on record, according to John Abatzoglou, professor of climatology at UC Merced.
While drought conditions have improved since last October, the entire state is still experiencing some level of drought. Another mild, dry winter is a big concern as the state tries to escape a fourth straight year of drought.
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The coming winter
The El Niño Southern Oscillation (ENSO) is the primary driver of atmospheric patterns controlling jet stream ripple and is generally considered a respectable indicator of what a region can expect during the winter months.
The rare La Niña “triple-dip” is making headlines, but there are other atmospheric patterns and movements, or oscillations, that could keep California dry this coming winter.
Four other lesser-known oscillations can also impact California weather: the Arctic Oscillation, the Madden-Julian Oscillation, the Pacific/North American Pattern, and the Pacific Decadal Oscillation have all been linked to the incidence of California winter weather.
Bob Henson, meteorologist, author and journalist at Yale Climate Connections, described ENSO as the “great enchilada” of oscillations, but also acknowledged the predictive value of the other four.
Here’s a look at each, including the expected impacts for California this winter:
Arctic Oscillation (AO)
The main characteristic of the Arctic Oscillation is the undulation of the jet stream associated with the positioning of low or high pressure in the Arctic.
During positive phases, low pressure dominates arctic regions and the jet stream is faster and less wavy, preventing storm systems from constantly deepening in California.
The data shows that the swing is currently strongly positive, which is generally bad news for California.
Madden–Julian Oscillation (MJO)
Henson described the MJO as “a pulse of showers, thunderstorms, convection that circumnavigates the globe over a period of, say, four to six weeks on average for six to eight weeks.”
The MJO is expected to increase activity after a dormant period in the coming weeks.
“I would say MJO just modulates when you’re going to have those rainy spells and a wet winter,” Henson said. The effects of the MJO are amplified when it coincides with an area of existing low pressure, which means more rain.
However, it is difficult to predict the influence of the MJO this winter in California due to its innate variability.
Research by Professor Da Yang of UC Davis predicts a 54% increase in MJO-induced rainfall variability by the year 2100 under high emission climate scenarios.
Pacific North American Scheme (PNA)
Henson described the PNA as a “descriptor” more than anything. The PNA is a more variable process than the ENSO, which means that it changes more regularly than the other oscillations.
The PNA is currently in its positive phase, which is marked by resilient high pressure over California sending thunderstorm systems northward.
A negative phase generally results in “a much wetter pattern for California,” according to Henson. This opens the door to storms creeping into the area, in a sense.
Pacific Decadal Oscillation (PDO)
The AOP is a 20 to 25 year oscillation consisting of a warm or positive phase and a cold or negative phase.
The “cool” phase is characterized by a cold wedge with lower than normal sea surface heights/ocean temperatures in the eastern equatorial Pacific and a warm horseshoe pattern with sea surface heights of above-normal seas linking the north, west and south Pacific, according to NASA’s Jet Propulsion Lab.
The wobble is a measure of ocean temperature and can work in conjunction with La Niña to amplify drier-than-normal conditions, according to Henson.
The current state of AOP is the cold, negative phase that has been linked to the California drought. “It was not only a three-year stretch with strong La Nina, but also very strong negative AOP. So both hammered, you know, California and other places,” Henson said.
What does all this mean?
Oscillation studies are a generally new frontier of atmospheric research, particularly with respect to the impacts of climate change. The ability of forecasters to make sense of these oscillations is improving, as are the capabilities of models.
Research continues into these atmospheric oscillations and the potential power they hold in future seasonal forecasting skills.
While no promises can be made based on these oscillations and their current state, there is a good chance that California will get the rain and snow it needs to escape the drought.
“You definitely have some kind of train wreckage of drought-producing influences,” Henson said, referring to the presence of La Niña, the negative phases of PDO and PNA, and the positive phase of AO.
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