Water Year 2021 Characterized by Persistent Dry Weather and Worsening Drought in California

• Total precipitation has been well below normal throughout much of California during water year (WY) 2021
• In some regions, drier than normal conditions extend back to the start of WY 2020
• Drought has expanded and intensified across the state, and current water storage levels are below normal in many
• Below-normal snowpack in the Sierra Nevada may limit water resource availability as summer approaches
• The abnormally dry conditions were driven by a lack of landfalling atmospheric rivers (ARs) and persistent
ridging/blocking over the Northeast Pacific Ocean
• Drought is expected to continue through spring 2021, thereby increasing the threat of significant wildfire activity in
summer 2021

Opinion: SB 1 Provides Critical Protection for California’s Beloved Coastline

I didn’t see the ocean until I was 18 years old. That late start didn’t stop me from falling in love with the sea, a love I have pursued in earnest ever since I moved to San Diego.

Here in our community, and in communities throughout California, warming waters and rising sea levels threaten both the coast we love and the people and businesses that are located there. Predictions are, that left unchecked, sea level rise will cause billions of dollars in damage in California and disrupt countless lives.

Phillips Station-Mountain Snowpack-Climate Change-DWR Snow Survey

New Study Identifies Mountain Snowpack Most “At-Risk” from Climate Change

As the planet warms, scientists expect that mountain snowpack should melt progressively earlier in the year. However, observations in the U.S. show that as temperatures have risen, snowpack melt is relatively unaffected in some regions while others can experience snowpack melt a month earlier in the year.

This discrepancy in the timing of snowpack disappearance—the date in the spring when all the winter snow has melted—is the focus of new research by scientists at Scripps Institution of Oceanography at the University of California San Diego.

In a new study published March 1 in the journal Nature Climate ChangeScripps Oceanography climate scientists Amato Evan and Ian Eisenman identify regional variations in snowpack melt as temperatures increase, and they present a theory that explains which mountain snowpacks worldwide are most “at-risk” from climate change. The study was funded by NOAA’s Climate Program Office.

Mountain snowpack changing rapidly in coastal regions

Looking at nearly four decades of observations in the Western U.S., the researchers found that as temperatures rise, the timing of snowpack disappearance is changing most rapidly in coastal regions and the south, with smaller changes in the northern interior of the country. This means that snowpack in the Sierra Nevada, the Cascades, and the mountains of southern Arizona is much more vulnerable to rising temperatures than snowpack found in places like the Rockies or the mountains of Utah.

The scientists used these historical observations to create a new model for understanding why the timing of snowpack disappearance differs widely across mountain regions. They theorize that changes in the amount of time that snow can accumulate and the amount of time the surface is covered with snow during the year are the critical reasons why some regions are more vulnerable to snowpack melt than others.

Mountain Snowpack-Climate Change-Scripps Institution of Oceanography

Using a new model, the Scripps researchers theorize that snowpack in coastal regions, the Arctic, and the Western U.S. may be among the most at-risk for premature melt from rising temperatures. Graphic: Courtesy Scripps Institution of Oceanography

Snowpack vulnerable due to increasing temperature

“Global warming isn’t affecting everywhere the same. As you get closer to the ocean or further south in the U.S., the snowpack is more vulnerable, or more at-risk, due to increasing temperature, whereas in the interior of the continent, the snowpack seems much more impervious, or resilient to rising temperatures,” said Evan, lead author of the study. “Our theory tells us why that’s happening, and it’s basically showing that spring is coming a lot earlier in the year if you’re in Oregon, California, Washington, and down south, but not if you’re in Colorado or Utah.”

Applying this theory globally, the researchers found that increasing temperatures would affect the timing of snowpack melt most prominently in the Arctic, the Alps of Europe, and the southern region of South America, with much smaller changes in the northern interiors of Europe and Asia, including the central region of Russia.

Climate Change and snowmelt

To devise the model that led to these findings, Evan and Eisenman analyzed daily snowpack measurements from nearly 400 sites across the Western U.S managed by the Natural Resources Conservation Service Snowpack Telemetry (SNOTEL) network. They looked at SNOTEL data each year from 1982 to 2018 and focused on changes in the date of snowpack disappearance in the spring. They also examined data from the North American Regional Reanalysis (NARR) showing the daily mean surface air temperature and precipitation over the same years for each of these stations.

Using an approach based on physics and mathematics, the model simulates the timing of snowpack accumulation and snowpack melting as a function of temperature. The scientists could then use the model to solve for the key factor that was causing the differences in snowpack warming: time. Specifically, they looked at the amount of time snow can accumulate and the amount of time the surface is covered with snow.

“I was excited by the simplicity of the explanation that we ultimately arrived at,” said Eisenman. “Our theoretical model provides a mechanism to explain why the observed snowmelt dates change so much more at some locations than at others, and it also predicts how snowmelt dates will change in the future under further warming.”

A “shrinking winter” and longer fire season

The model shows that regions with very large swings in temperature between the winter and summer are less susceptible to warming than those where the change in temperature from winter to summer is smaller. The model also shows that regions where the annual mean temperature is closest to 0˚C are less susceptible to early melt. The most susceptible regions are ones where the differences between wintertime and summertime temperatures are small, and where the average temperature is either far above, or even far below 0˚C.

For example, in an interior mountain region of the U.S. like the Colorado Rockies, where the temperature dips below 0°C for about half the year, an increase of 1°C can lead to a quicker melt by a couple of days—not a huge difference.

However, in a coastal region like the Pacific Northwest, the influence of the ocean and thermal regulation helps keep the winter temperatures a bit warmer, meaning there are fewer days below 0°C in which snow can accumulate. The researchers hypothesize that in the region’s Cascade Mountains, a 1°C increase in temperature could result in the snow melting about a month earlier in the season—a dramatic difference.

Arctic “at risk”

One of the most “at-risk” regions is the Arctic, where snow accumulates for nine months each year and takes about three months to melt. The model suggests that 1°C warming there would result in a faster melt by about a week—a significant period of time for one of the fastest warming places on Earth.

This study builds upon previous work done by Scripps scientists since the mid-1990s to map out changes in snowmelt timing and snowpacks across the Western U.S. The authors said that a “shrinking” winter—one that is shorter, warmer, and with less overall precipitation—has adverse societal effects because it contributes to a longer fire season. This could have devastating impacts on already fire-prone regions. In California, faster snowpack melt rates have already made forest management more difficult and provided prime conditions for invasive species like the bark beetle to thrive.

Funding for this work was provided by a NOAA/CPO grant to the University of California.

Landfalling Atmospheric Rivers Increase in First Quarter of Water Year 2021 Over 2020

A Scripps Center for Western Weather and Water Extremes report shows that more atmospheric rivers have made landfall over the U.S. West Coast in the first four months of Water Year 2021 compared to the previous period in 2020.

Atmospheric Rivers-CW3E-Scripps-Rain

Landfalling Atmospheric Rivers Increase in First Quarter of Water Year 2021 Over 2020

A Scripps Center for Western Weather and Water Extremes report shows that more atmospheric rivers have made landfall over the U.S. West Coast in the first four months of Water Year 2021 compared to the previous period in 2020.

The four months of Water Year 2021 experienced a total of 35 landfalling ARs over the U.S. West Coast, six more than the first four months of Water Year 2020, according to the CW3E quarter year summary released February 11.

Key takeaways from the CW3E update on atmospheric rivers

• The first four months of Water Year 2021 experienced a total of 35 landfalling ARs over the U.S. West Coast, 6 more than the first four months of Water Year 2020.
• Water Year 2021 also experienced more than twice as many strong or greater ARs (14) compared to Water Year 2020 (6).
• While Water Year 2021 saw more ARs in its first 4 months compared to Water Year 2020, a large majority of those ARs were
strongest over the Pacific Northwest (Oregon and Washington).
• The average landfall latitude thus far in Water Year 2021 was 45.9°N compared to 43.9°N during the first half of Water Year 2020.
• The lack of landfalling ARs over Southern California and Baja California has resulted in much drier conditions over the southwestern U.S. compared to WY 2020.

CW3E-Atmospheric Rivers-Water Year 2021

The four months of Water Year 2021 experienced a total of 35 landfalling ARs over the U.S. West Coast, six more than the first four months of Water Year 2020. Graphic: Center for Western Weather and Water Extremes

Distribution of Landfalling Atmospheric Rivers over the U.S. West Coast During Water Year 2021: Quarter Year Summary

Atmospheric River-NOAA-WNN-CW3E

A partnership with the San Diego County Water Authority and the Scripps Institution of Oceanography at UC San Diego seeks to optimize water management to better predict atmospheric rivers before, during, and after those seasonal storms.

In August 2020, Scripps Center for Western Weather and Water Extremes (CW3E) launched the Water Affiliates Group, which brings together cutting-edge science and hands-on water industry experience to enhance reservoir operations in light of the changing climate. The Water Authority has a long-running alliance with Scripps and is among six founding water agencies statewide.

CW3E and its partners will share and support best practices in forecast-informed reservoir operations, increase research around atmospheric rivers and droughts, and develop strategies for mitigating flood risk and increasing water supply reliability.

Atmospheric Rivers-CW3E-water supply

The above image, from the EOS article, is a depiction of an atmospheric river, interacting with West Coast mountains and a midlatitude cyclone over the northeast Pacific on 5 February 2015. This image provides an example of approximate locations of associated tropical moisture exports and a warm conveyor belt (WCB). Graphic: Adapted from NOAA/ESRL Physical Sciences Division via CW3E

Scripps Oceanography Gets Share of California Energy Commission Contract to Simulate Climate Change

The California Energy Commission has awarded a $1.5 million contract to three University of California campuses, including UC San Diego, that will work in tandem to better simulate climate change scenarios that can be used by utilities and others to anticipate the effects.

Scripps Researcher Finds Ozone-Eating Chemical Level Falling

A San Diego scientist says a surge in outlawed ozone-depleting gasses appears to be easing. Scripps Institution of Oceanography researcher Ray Weiss says levels of chlorofluorocarbons (CFCs) in the atmosphere began rising a few years ago, even though they had been outlawed by the Montreal Protocol. The chemical was once used as a refrigerant and in the manufacture of foam, but CFCs were outlawed more than a decade ago.

Researchers Exploring How San Diego County Wetlands Can Be Part of Climate-Saving Strategies

Buried in San Diego County’s lagoons are centuries worth of carbon, cached in muddy stockpiles that scientists say could help combat climate change. Recently, scientists with the conservation organization Wildcoast and Scripps Institution of Oceanography started studying how much carbon coastal wetlands can capture, and how to restore these environments to boost that capacity.

San Diego Scientist Gets Closer to Understanding Why the Coast Collapses

Adam Young spent the last three years firing a laser from the back of his truck at Del Mar’s cliffs which are crumbling into the Pacific Ocean.

Cliff collapses along the California coast killed three Encinitas beachgoers in 2019. That same year, another bluff collapse in Del Mar destabilized a set of train tracks regularly carrying passengers between Los Angeles and San Diego. Policymakers need to make big decisions about how best to reckon with earth that seems to fall at random, but scientists still don’t understand what truly causes them to fall.

That’s what Young, a coastal geomorphologist (the study of how the earth’s surface formed and changes) at Scripps Institution of Oceanography, wants to know: If we know how ocean waves and winter rains eat away at a cliff face, can we eventually predict where and when it will collapse?

Scientists Use New Methods to Better Forecast Atmospheric Rivers

Earlier this year, the Center for Western Weather and Water Extremes launched a new sub-seasonal to seasonal forecast product to better predict the influence atmospheric rivers will have on the Western United States. Better and more accurate forecasting tools for atmospheric rivers are critical for a number of community uses, including water management, agriculture, insurance and commodities trading, to name a few.

The demand for better atmospheric forecasting tools has facilitated the development of the new S2S forecasting products launched by CW3E this year.