Why Seasonal Prediction of California Winter Precipitation is Challenging

American Meteorological Society (AMS) | December 7th, 2022

Despite an urgent demand for reliable seasonal prediction of precipitation in California (CA) due to the recent recurrent and severe drought conditions, our predictive sk

Despite an urgent demand for reliable seasonal prediction of precipitation in California (CA) due to the recent recurrent and severe drought conditions, our predictive skill for CA winter precipitation remains limited. October hindcasts by the coupled dynamical models typically show a correlation skill of about 0.3 for CA winter (November–March) precipitation. In this study, an attempt is made to understand the underlying processes that limit seasonal prediction skill for CA winter precipitation. It is found that only about 25% of interannual variability of CA winter precipitation can be attributed to influences by El Niño–Southern Oscillation (ENSO). Instead, the year-to-year CA winter precipitation variability is primarily due to circulation anomalies independent from ENSO, featuring a circulation center over the west coast United States as a portion of a short Rossby wave train pattern over the North Pacific. Analyses suggest that dynamical models show nearly no skill in predicting these ENSO-independent circulation anomalies, thus leading to limited predictive skill for CA winter precipitation. Low predictability of these ENSO-independent circulation anomalies is further demonstrated by a large ensemble of atmospheric-only climate model simulations. While low predictability of the ENSO-independent circulation anomalies could be due to chaotic internal atmospheric processes over the mid- to high latitudes, possible underexploited predictability sources for CA precipitation in models are also discussed. This study pinpoints an urgent need for improved understanding of the formation mechanisms of ENSO-independent circulation anomalies over the U.S. West Coast for a breakthrough in seasonal prediction of CA winter precipitation.

Why the St. Francis Dam Collapsed

Scientific American | June 1st, 1928

St. Francis dam was constructed on the San Francisquito fault, a geological feature known for a number of years and made the subject of publication illustrated by map

St. Francis dam was constructed on the San Francisquito fault, a geological feature known for a number of years and made the subject of publication illustrated by maps, chief among these being that published several years ago by the Seismological Society of America, which shows the major faults of  California. This statement, made March 22nd by Harry R. Johnson, consulting geologist of Los Angeles, speaking in San Francisco before the delegates to the convention of the American Association of Petroleum Geologists, created a profound impression, accompanied as it was by an analysis of the condition of the fault at that particular place. Mr. Johnson prefaced his report by saying that the advisability of making it had been discussed with other delegates, and that it had been decided that such a public report was advisable in view of the fact that "the public mind had been deluged by the wildest assortment of theories as to the cause of the catastrophe."

Widening of the tropical belt in a changing climate

Nature Portfolio (Springer Nature) | December 2nd, 2007

Some of the earliest unequivocal signs of climate change have been the warming of the air and ocean, thawing of land and melting of ice in the Arctic. But recent studies

Some of the earliest unequivocal signs of climate change have been the warming of the air and ocean, thawing of land and melting of ice in the Arctic. But recent studies are showing that the tropics are also changing. Several lines of evidence show that over the past few decades the tropical belt has expanded. This expansion has potentially important implications for subtropical societies and may lead to profound changes in the global climate system. Most importantly, poleward movement of large-scale atmospheric circulation systems, such as jet streams and storm tracks, could result in shifts in precipitation patterns affecting natural ecosystems, agriculture, and water resources. The implications of the expansion for stratospheric circulation and the distribution of ozone in the atmosphere are as yet poorly understood. The observed recent rate of expansion is greater than climate model projections of expansion over the twenty-first century, which suggests that there is still much to be learned about this aspect of global climate change.

Widespread and increased drilling of wells into fossil aquifers in the USA

Nature Portfolio (Springer Nature) | April 19th, 2022

Most stored groundwater is ‘fossil’ in its age, having been under the ground for more than ~12 thousand years. Mapping where wells tap fossil aquifers is relevant for

Most stored groundwater is ‘fossil’ in its age, having been under the ground for more than ~12 thousand years. Mapping where wells tap fossil aquifers is relevant for water quality and quantity management. Nevertheless, the prevalence of wells that tap fossil aquifers is not known. Here we show that wells that are sufficiently deep to tap fossil aquifers are widespread, though they remain outnumbered by shallower wells in most areas. Moreover, the proportion of newly drilled wells that are deep enough to tap fossil aquifers has increased over recent decades. However, this widespread and increased drilling of wells into fossil aquifers is not necessarily associated with groundwater depletion, emphasizing that the presence of fossil groundwater does not necessarily indicate a non-renewable water supply. Our results highlight the importance of safeguarding fossil groundwater quality and quantity to meet present and future water demands.

Widespread aquifer depressurization after a century of intensive groundwater use in USA

American Association for the Advancement of Science (AAAS) | September 15th, 2023

Widespread deoxygenation in warming rivers

Nature Portfolio (Springer Nature) | September 14th, 2023

Deoxygenation is commonly observed in oceans and lakes but less expected in shallower, flowing rivers. Here we reconstructed daily water temperature and dissolved oxygen

Deoxygenation is commonly observed in oceans and lakes but less expected in shallower, flowing rivers. Here we reconstructed daily water temperature and dissolved oxygen in 580 rivers across the United States and 216 rivers in Central Europe by training a deep learning model using temporal weather and water quality data and static watershed attributes (for example, hydro-climate, topography, land use, soil). Results revealed persistent warming in 87% and deoxygenation in 70% of the rivers. Urban rivers demonstrated the most rapid warming, whereas agricultural rivers experienced the slowest warming but fastest deoxygenation. Mean deoxygenation rates (−0.038 ± 0.026 mg l−1 decade−1) were higher than those in oceans but lower than those in temperate lakes. These rates, however, may be underestimated, as training data are from grab samples collected during the day when photosynthesis peaks. Projected future rates are between 1.6 and 2.5 times higher than historical rates, indicating significant ramifications for water quality and aquatic ecosystems.

Widespread Increase in Atmospheric River Frequency and Impacts Over the 20th Century

American Geophysical Union (AGU) | December 11th, 2025

Atmospheric rivers (ARs) are long, narrow regions of atmospheric moisture transport that provide water resources to many regions, but can also cause hazards such as extre

Atmospheric rivers (ARs) are long, narrow regions of atmospheric moisture transport that provide water resources to many regions, but can also cause hazards such as extreme rainfall and moist heatwaves. Despite their importance, there is uncertainty about how the frequency and impacts of ARs may be changing with time. Our study extends the global record of AR activity back to 1940. We find that the frequency of ARs has increased broadly across the midlatitude regions, rather than shifting polewards as observed in recent decades. This increase in AR frequency leads to more rain and snow associated with these features in both the mid‐ and high‐latitudes. We also find that there is an increase in humid heatwaves associated with ARs over the same period

Widespread potential loss of streamflow into underlying aquifers across the USA

Nature Portfolio (Springer Nature) | March 17th, 2021

Most rivers exchange water with surrounding aquifers. Where groundwater levels lie below nearby streams, streamwater can infiltrate through the streambed, reducing stream

Most rivers exchange water with surrounding aquifers. Where groundwater levels lie below nearby streams, streamwater can infiltrate through the streambed, reducing streamflow and recharging the aquifer. These ‘losing’ streams have important implications for water availability, riparian ecosystems and environmental flows, but the prevalence of losing streams remains poorly constrained by continent-wide in situ observations. Here we analyse water levels in 4.2 million wells across the contiguous USA and show that nearly two-thirds (64 per cent) of them lie below nearby stream surfaces, implying that these streamwaters will seep into the subsurface if it is sufficiently permeable. A lack of adequate permeability data prevents us from quantifying the magnitudes of these subsurface flows, but our analysis nonetheless demonstrates widespread potential for streamwater losses into underlying aquifers. These potentially losing rivers are more common in drier climates, flatter landscapes and regions with extensive groundwater pumping. Our results thus imply that climatic factors, geological conditions and historic groundwater pumping jointly contribute to the widespread risk of streams losing flow into surrounding aquifers instead of gaining flow from them. Recent modelling studies have suggested that losing streams could become common in future decades, but our direct observations show that many rivers across the USA are already potentially losing flow, highlighting the importance of coordinating groundwater and surface water policy.

Wildfire and Drought: Impacts on wildfire planning, behavior, and effects

National Integrated Drought Information System | March 4th, 2016

The relationship between drought and wildfire can appear to be simple, but the drought and wildfire nexus encompasses a profusion of human and environmental impacts. Imag

The relationship between drought and wildfire can appear to be simple, but the drought and wildfire nexus encompasses a profusion of human and environmental impacts. Images of burning forests, neighborhoods, and “smoked out” communities are all now commonplace. The National Integrated Drought Information System, in partnership with NOAA’s Western Regional Climate Center (WRCC), the California Nevada Applications Program (CNAP, a NOAA Regional Integrated Science Applications team), the Desert Research Institute (DRI), and the National Drought Mitigation Center (NDMC), hosted the Integrating Drought Science and Information into Wildfire Management Workshop in Boise, Idaho on 21-22 October 2015.  The 22 participants in the workshop were invited as representatives of a broad spectrum of expertise including prescribed fire, wildland fire management, climatology, smoke and air quality, fuels management, fire behavior, drought, and policy. In this report, we present the opportunities and challenges the workshop participants identified and the impacts of drought on wildfire. The workshop also provided the opportunity to identify next steps in understanding and addressing drought impacts on wildfire.

Wildfire Impacts on Snowpack Phenology in a Changing Climate Within the Western U.S.

American Geophysical Union (AGU) | July 8th, 2022

Snowpack accumulation and melt are critical for water supply in the western United States. As the number and size of wildfires have increased, it is important to understa

Snowpack accumulation and melt are critical for water supply in the western United States. As the number and size of wildfires have increased, it is important to understand how these events impact the seasonal snowpack. Using data from the Snow Telemetery monitoring network, we quantify the impacts of wildfires on snowpack for various regions across the western U.S. and Alaska. The results indicate that decreases in snowpack tend to occur following fires. Also, the snowmelt period usually begins and finishes earlier in the spring due to the conditions in the burned forests. In regions of the western U.S. that are already experiencing earlier snowmelt due to climate changes, wildfires are further shifting the melt earlier, which could have substantial consequences for water supply and flood risks.