Weird Wild Western Weather - Why?
California had its MoJO working - and no one saw it coming
There have been lots of western weather stories this winter, and the focus has shifted very quickly from too little water to too much. The winter of 2022/23 was exceptional for rainfall along the California coast and for snowfall in the Sierra Nevada Mountains.
And one of the strangest things about this is that no one saw it coming. Predictions last fall were for a fourth consecutive dry winter.
Both print and broadcast media (citations under Sources below) were highlighting a seasonal prediction from the NOAA Weather Service that looked like this:
All the sources cited the well-established relationship between the state of the El Niño-Southern Oscillation (ENSO) system and rainfall in California. The Central Pacific Ocean was firmly in a La Niña phase (and had been for more than 2 years). The cooler than normal sea surface temperatures in the equatorial Pacific that define a La Niña event have traditionally (and statistically) meant drier than normal winters for California.
So what happened? I'll get back to that (and to the MoJO reference) in a moment. First, just how much wetter than normal was this winter?
The water year in California runs from October through September to capture the peak mid-winter rainy season. From the beginning, the year looked wetter than predicted, and by early January I posted a good news story here about how reservoirs in the California water system were beginning to refill, and especially that the critical accumulation of snow in the Sierra Nevada mountains was ahead of normal for that time of year.
Then things got wilder and more dangerous as storm after storm hit the coast and mountains.
The extreme weather was linked to a series of atmospheric rivers, a phenomenon discovered or defined only a little more than 20 years ago. These rivers in the sky appear to be launched as far away as the Indian Ocean, gathering moisture as they cross the Pacific, and, when conditions are right, delivering that moisture to California as rain and snow. The discovery of these rivers, combined with the rediscovery of the worst series of floods ever to strike the state, in 1861/62, led to an earlier essay about predicting when the next big one - meaning statewide flooding - might strike.
This year has not yet equaled 1861/62, when the entire central valley was deep in runoff water, but the mountain snows have yet to melt, and the atmospheric rivers seem to keep coming. And just this week, media stories were beginning to report tens of thousands of acres of Central Valley farmland under water. From severe drought to severe flooding in a single season.
So what have all those atmospheric rivers delivered? Kudos to the California Data Exchange Center for making the detailed information on the state's water system presented here publicly available on their website.
For precipitation, totals by region through the end of February, and before a rainy March, were well above long-term averages.
The accumulation of snow in the Sierra Nevada Mountains to the east of the Central Valley is critical both for reducing the immediate flood threat from big storms and providing a relatively slow and continuous source of water as the snows melt across the spring and early summer.
Accumulations this year have been even more extreme than statewide precipitation. Current water content of snow across three regions of the Sierra Nevadas ranges from 182% to 286% of the long-term average for this date.
For comparison, totals on this date in 2015 were 5% or less of those averages!
Numbers are also available for water storage in the extensive set of reservoirs that have been constructed, again, to prevent flooding and provide water for the dry summer months. These figures compare current storage with historical averages (the green line) for each site and date. Values were generally below historical averages at the beginning of the 2022/23 water year (1 October 2022), but even before the onset of the upcoming spring snowmelt season, both early melt and excessive winter rainfall have pushed many of those reservoirs to near or above seasonal averages.
Allocation of water from the Colorado River has been a hot topic throughout the southwest as the prolonged western drought has reduced allowable river withdrawals. Currently, California draws 27% of the river's flow for both domestic use and irrigation.
And there may be better news here as well. The NOAA site that tracks snow accumulations and predicted river runoff shows that the largess of snow in California has also extended into the Rocky Mountains. Water supply predictions are anywhere from 85% to as much as 350% of the long-term average.
And now we come back to the Why.
Those early predictions of another dry winter were based on some pretty solid historical evidence. On average, rainfall in California has been lower in a La Niña year and higher in an El Niño year (when sea surface temperatures are higher, as opposed to the cooler temperatures in the La Niña image above).
There had been one major exception. The winter of 2015/16 recorded the strongest El Niño signal of all time, and yet the ongoing drought that had begun in 2013 persisted. This unusual break in the relationship was attributed to "The Blob" a large region of unusually warm ocean water in the far northern Pacific.
El Niño versus The Blob made for some interesting media cartoons!
What this really says is that the relationship between El Niño and California rainfall was really just statistical, rather than based on some unalterable physical relationship. The pattern could be overturned if the forces behind the relationship changed. That happened in 2015/16 (The Blob), and again, in spades, in 2022/23. Record storms, rains, and snowfall happened during an ongoing La Niña episode.
One of the great things about science is that it's OK, and sometimes even a good thing, to be wrong. One of my favorite science quotes is from Enrico Fermi: "If your experiment confirms your hypothesis, then you have made a measurement. If your experiment contradicts your hypothesis, then you have made a discovery!”
So what was discovered as a result of the failure of the El Niño-rainfall hypothesis?
The proximate cause of the deluge was confirmed - a continuous series of atmospheric rivers charging onshore for several months. But what caused those rivers?
Here is where we get to the MoJO - or actually - the MJO.
The El Niño/ENSO oscillation is just one of many cyclical events in the climate system. The Madden-Julian Oscillation (MJO) is another, but of a different flavor. Discovered only in the 1970s, the MJO is not a cycle occurring in one location, like El Niño, but rather a wave of energy and moisture that develops first in the Indian Ocean and then moves eastward across the Pacific and can reach as far as the shores of the Americas. Both oscillations involve a cycle of upward and downward air movement. In the ENSO system, the cycle sits in the equatorial Pacific and can spin more rapidly (La Niña) or more slowly (El Niño). An MJO wave can vary in strength and speed as it moves across a vast stretch of ocean.
This site offers a great analogy that captures the difference between these two. It describes El Niño as a rider on a stationary bike, cycling in place, and the MJO as a road rider, moving past the stationary bike; entering stage left (the Western Pacific) and exiting stage right (The American coast).
As the wave moves eastward, enhanced convection and heavy rainfall follow. MJO "waves" tend to take 30-60 days to complete a west-to-east passage. Some waves are stronger than others, and there are periods when the system goes dormant.
One interesting pattern that is emerging is that the MJO may be more active, or more likely to hit the California coast, when the ENSO system is in the cooler, La Niña phase. Perhaps the speed of the stationary bike (La Niña) can deflect the path of the road bike (MJO)?
So it seems we now have two very different, interacting oscillations to consider when trying to forecast California's next winter - and we can't yet predict either one accurately. We may be surprised again!
And what of climate change?
The fact that the MJO draws its energy from the warm waters of the equatorial Indian and Pacific Oceans, and that these waters are warming, suggests increasing strength for future MJOs. The Wikipedia site notes that residence time for MJO waves over the Indian Ocean has decreased while time spent traversing the Pacific has increased, altering rainfall patterns globally. Perhaps this winter's California experience will become the new normal, or will at least become more common.
We have much to learn about the MJO - and oscillations in general. We can predict weather with some accuracy for maybe 5-7 days into the future, and we know very well where the planet is headed in terms of decadal shifts in global air temperature, ocean temperature, sea level rise and loss of ice from glaciers and ice caps. But Oscillations occupy an odd time frame (months to years) that cannot be predicted using super-high-resolution computer models, and may be powerful enough to deflect our path into our known climate future. We already know that the state of the El Niño/ENSO system can alter global average temperature.
I think of it this way - We know where we are headed, but the pathway from here to there might be noisy.
To my mind, the interactions of a warming planet with these oscillations is the biggest unknown in the science story of our climate future. They seem unlikely to alter the 2100 end point significantly, but how each one cycles, and, in particular, how the different oscillations interact (and we know little of this) can reshape that squiggly line (to use the technical term) into our future.
It is intriguing to me that the current state of the science around oscillations seems to be more like traditional climatology than rigorous first-principles modeling. Researchers are discovering patterns and fitting those patterns into known physical relationships, but predictive models have yet to emerge.
So while recent stories about climate change in the western U.S., influenced by the recent extended drought, have suggested a drier, more flammable future, this winter's MJO story offers another option - a cycling between drought and flood. Neither seems appealing, but then California has always been a region of extremes.
In terms of oscillations like ENSO and MJO, stay tuned, as they say on the weather stations, we still have much to learn.
Sources
Two media stories about the prediction for a dry winter are:
https://www.nytimes.com/2022/09/29/us/california-drought-weather-forecast.html
https://ktla.com/news/local-news/noaa-releases-california-winter-weather-predictions/
Long-range forecasts from NOAA are from their Climate Prediction Center (which actually has lots of very accurate and useful information)
https://www.cpc.ncep.noaa.gov/
Here is the original NOAA press release
https://www.noaa.gov/news-release/us-winter-outlook-warmer-drier-south-with-ongoing-la-nina
The NOAA image of sea surface temperatures in a La Niña event is from:
https://www.noaa.gov/news/double-dip-la-nina-emerges
Recent articles on flooding in the central valley include:
https://www.nytimes.com/interactive/2023/03/27/us/california-flood-photos.html
Precipitation on the California Data Exchange Center site for precipitation is here:
https://cdec.water.ca.gov/reportapp/javareports?name=PRECIPSUM
Snow accumulation data are here:
https://cdec.water.ca.gov/snowapp/sweq.action
And the reservoir data are here:
https://cdec.water.ca.gov/resapp/RescondMain
Stories on diverting this year's water excess in California to groundwater and other management options include:
The data on predicted water supply for the Colorado and Great Basin watersheds are from:
https://www.cbrfc.noaa.gov/wsup/pub2/discussion/current.pdf
A description of "the Blob" and the image used can be found here:
https://en.wikipedia.org/wiki/The_Blob_(Pacific_Ocean)
There are many, many different versions of the quotation from Enrico Fermi to be found.
Two media stories on the breakdown of the El Niño-rainfall relationship and possible role of MJO are:
https://www.nytimes.com/interactive/2023/03/09/us/track-california-storm.html
The interaction between MJO and El Niño is described clearly here:
https://www.climate.gov/news-features/blogs/enso/what-mjo-and-why-do-we-care
The figure in the text is from this site
A detailed and recently updated discussion of the Madden-Julian oscillation is here:
https://en.wikipedia.org/wiki/Madden%E2%80%93Julian_oscillation
And a recent scientific review article that describes the challenges in predicting the MJO cycle is here:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019JD030911
The NOAA Climate Prediction Center on MJO is here:
https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_mjo_index/mjo_index.shtml
One reference on the impact of the ENSO oscillation on global average temperature is:
https://eos.org/features/simpler-presentations-of-climate-change