Dynamical and Thermodynamical Causes of Large-Scale Changes in the Hydrological Cycle over North America in Response to Global Warming

American Meteorological Society (AMS) | October 15th, 2014

The mechanisms of model-projected atmospheric moisture budget change across North America are exam- ined in simulations conducted with 22 models from phase 5 of t

The mechanisms of model-projected atmospheric moisture budget change across North America are exam- ined in simulations conducted with 22 models from phase 5 of the Coupled Model Intercomparison Project. Modern-day model budgets are validated against the European Centre for Medium-Range Weather Forecasts Interim Re-Analysis. In the winter half year transient eddies converge moisture across the continent while the mean flow wets the west from central California northward and dries the southwest. In the summer half year there is widespread mean flow moisture divergence across the west and convergence over the Great Plains that is offset by transient eddy divergence. In the winter half year the models project drying for the southwest and wetting to the north. Changes in the mean flow moisture convergence are largely responsible across the west but intensified transient eddy moisture convergence wets the northeast. In the summer half year widespread declines in precipitation minus evaporation (P 2 E) are supported by mean flow moisture divergence across the west and transient eddy divergence in the Great Plains. The changes in mean flow convergence are related to increases in specific humidity but also depend on changes in the mean flow including increased low-level divergence in the U.S. Southwest and a zonally varying wave that wets the North American west and east coasts in winter and dries the U.S. Southwest. Increased transient eddy fluxes occur even as low-level eddy activity weakens and arise from strengthened humidity gradients. A full explanation of North American hydroclimate changes will require ex- planation of mean and transient circulation changes and the coupling between the moisture and circulation fields.

Dynamical and thermodynamical modulations on future changes of landfalling atmospheric rivers over western North America

American Geophysical Union (AGU) | September 12th, 2015

This study examines future changes of landfalling atmospheric rivers (ARs) over western North America using outputs from the Coupled Model Intercomparison Project Phase

This study examines future changes of landfalling atmospheric rivers (ARs) over western North America using outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5). The result reveals a strikingly large increase of AR days by the end of the 21st century in the RCP8.5 scenario, with fractional increases between 50% and 600%, depending on the seasons and landfall locations. These increases are predominantly controlled by the super-Clausius-Clapeyron rate of increase of atmospheric water vapor with warming, while changes of winds that transport moisture in the ARs, or dynamical effect, mostly counter the thermodynamical effect of increasing water vapor, limiting the increase of AR events in the future. The consistent negative effect of wind changes on AR days during spring and fall can be linked to the robust poleward shift of the subtropical jet in the North Pacific basin.

El Niño-like teleconnection increases California precipitation in response to warming

Nature Portfolio (Springer Nature) | July 7th, 2017

Future California (CA) precipitation projections, including those from the most recent Climate Model Intercomparison Project (CMIP5), remain uncertain. This uncertainty i

Future California (CA) precipitation projections, including those from the most recent Climate Model Intercomparison Project (CMIP5), remain uncertain. This uncertainty is related to several factors, including relatively large internal climate variability, model shortcomings, and because CA lies within a transition zone, where mid-latitude regions are expected to become wetter and subtropical regions drier. Here, we use a multitude of models to show CA may receive more precipitation in the future under a business-as-usual scenario. The boreal winter season-when most of the CA precipitation increase occurs-is associated with robust changes in the mean circulation reminiscent of an El Niño teleconnection. Using idealized simulations with two different models, we further show that warming of tropical Pacific sea surface temperatures accounts for these changes. Models that better simulate the observed El Niño-CA precipitation teleconnection yield larger, and more consistent increases in CA precipitation through the twenty-first century.

Elevated Grand Canyon groundwater recharge during the warm Early Holocene

Nature Portfolio (Springer Nature) | October 2nd, 2023

Summer rainfall is an important contributor to water budgets in western North American deserts, where intense rainfall sustains ecosystems while also causing flash floods

Summer rainfall is an important contributor to water budgets in western North American deserts, where intense rainfall sustains ecosystems while also causing flash floods and damaging erosion. A better understanding of Grand Canyon palaeoclimate and the long-term history of the summer monsoon from summer-sensitive palaeoclimate records will improve our ability to project future hydroclimatic changes under warmer conditions. Here we show multi-proxy evidence for an intensification of the Early Holocene (11,700–8,200 years ago) hydrological cycle linked to a stronger and expanded summer North American Monsoon from calcite oxygen and uranium isotopes in a uranium-series precisely dated stalagmite from a Grand Canyon cave. Our results suggest that subsurface infiltration was greater in the Early Holocene than today at Grand Canyon. A data–model comparison with an isotope-enabled climate model suggests that enhanced infiltration was due to an Early Holocene monsoon intensification associated with rising atmospheric temperature. Projections of a future increase in precipitation intensity or more frequent and expanded North American monsoon rain events may paradoxically result in increased subsurface infiltration at Grand Canyon and other high-altitude plateaus, even within the context of western North American aridification in a hotter climate.

Emergent Trends Complicate the Interpretation of the United States Drought Monitor (USDM)

American Geophysical Union (AGU) | April 25th, 2024

Effective drought management must be informed by an understanding of whether and how current drought monitoring and assessment practices represent underlying nonstationar

Effective drought management must be informed by an understanding of whether and how current drought monitoring and assessment practices represent underlying nonstationary climate conditions, either naturally occurring or forced by climate change. Here we investigate the emerging climatology and associated trends in drought classes defined by the United States Drought Monitor (USDM), a weekly product that, since 2000, has been used to inform drought management in the United States. The USDM classifies drought intensity based in part on threshold percentiles in key hydroclimate quantities. Here we assess how those USDM‐defined drought threshold percentiles have changed over the last 23 years, examining precipitation, runoff, soil moisture (SM), terrestrial water storage (TWS), vapor pressure deficit (VPD), and near‐surface air temperature. We also assess underlying trends in the frequency of drought classifications across the U.S. Our analysis suggests that the frequency of drought class occurrence is exceeding the threshold percentiles defined by the USDM in a number of regions in the United States, particularly in the American West, where the last 23 years have emerged as a prolonged dry period. These trends are also reflected in percentile‐based thresholds in precipitation, runoff, SM, TWS, VPD, and temperature. Our results emphasize that while the USDM appears to be accurately reflecting observed nonstationarity in the physical climate, such trends raise critical questions about whether and how drought diagnosis, classification, and monitoring should address long‐term intervals of wet and dry periods or trends

Empirical Return Periods of the Most Intense Vapor Transports during Historical Atmospheric River Landfalls on the U.S. West Coast

American Meteorological Society (AMS) | August 21st, 2018

Atmospheric rivers (ARs) come in all intensities, and clear communication of risks posed by individual storms in observations and forecasts can be a challenge. Modest AR

Atmospheric rivers (ARs) come in all intensities, and clear communication of risks posed by individual storms in observations and forecasts can be a challenge. Modest ARs can be characterized by the percentile rank of their integrated water vapor transport (IVT) rates compared to past ARs. Stronger ARs can be categorized more clearly in terms of return periods or, equivalently, historical probabilities that at least one AR will exceed a given IVT threshold in any given year. Based on a 1980–2016 chronology ofAR landfalls on the U.S. West Coast from NASA’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), datasets, the largest instantaneous IVTs—greater than 1700 kgm21 s21—have occurred in ARs making landfall between 418 and 468Nwith return periods longer than 20 years. IVT values with similar return periods are smaller to the north and, especially, to the south (declining to ;750 kgm21 s21). The largest storm-sequence IVT totals have been centered near 42.58N, with scatter among the top few events, and these large storm-sequence totals depend more on sequence duration than on the instantaneous IVT that went into them. Maximum instantaneous IVTs are largest in the Pacific Northwest in autumn, with largest IVT values arriving farther south as winter and spring unfold, until maximum IVTs reach Northern California in spring.

Escalating Hydrological Extremes and Whiplashes in the Western U.S.: Challenges for Water Management and Frontline Communities

American Geophysical Union (AGU) | May 14th, 2025

Estimating Historical California Precipitation Phase Trends Using Gridded Precipitation, Precipitation Phase, and Elevation Data

California Department of Water Resources (DWR) | July 18th, 2014

Climate change projections for California indicate reduction in the percentage of precipitation that falls as snow, and increase in the percentage that falls as rain, due

Climate change projections for California indicate reduction in the percentage of precipitation that falls as snow, and increase in the percentage that falls as rain, due to warmer temperatures in the future. Previous work has shown overall decreases in rainfall / snowfall ratios for the western US over the last 60 years. Of interest in this study is the quantification of snow/rain ratios for smaller regions, specifically in California. Estimating cumulative precipitation phase ratios for specific regions is difficult due to large differences in local precipitation. In California, the high relief of the surface topography makes such estimates particularly difficult. The low spatial resolution of suitable precipitation/snow water equivalent monitoring stations contributes to the difficulty in quantifying the trends for sub-state sized regions of interest in this study. The present exploratory study develops and describes a methodology that uses readily available research data sets to produce gridded estimates of historical rainfall as a fraction of total precipitation for areas comprising the major water-supply watersheds of California. Using this methodology, statistically significant increases in the ratio of annual liquid to total precipitation are seen for large areas in the northern part of the State and northern Sierra over the water year. No significant annual trends are seen for regions in the central and southern portions of the Sierra. Future work extending the analysis to distinct elevation ranges and seasonality would provide more refined conclusions.

Estuary News, April 1998

San Francisco Estuary Partnership (SFEP) | April 15th, 1998

Estuary News, April 2001

San Francisco Estuary Partnership (SFEP) | April 15th, 2001