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1. Water Year 2021 Compound Precipitation and Temperature Extremes in California and Nevada

   https://doi.org/10.1175/BAMS-D-22-0112.1  

   Hoell, Andrew; Quan, Xiao-Wei; Hoerling, Martin; Diaz, Henry F.; Fu, Rong; He, Cenlin; Lisonbee, Joel R.; Mankin, Justin S.; Seager, Richard; Sheffield, Amanda; et al (December 2022, Bulletin of the American Meteorological Society)

   Anthropogenically forced-warming and La Niña forced-precipitation deficits caused at least a sixfold risk increase for compound extreme low precipitation and high temperature in California–Nevada from October 2020 to September 2021. 

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2. Ocean-forcing of cool season precipitation drives ongoing and future decadal drought in southwestern North America

   https://doi.org/10.1038/s41612-023-00461-9  

   Seager, Richard; Ting, Mingfang; Alexander, Patrick; Liu, Haibo; Nakamura, Jennifer; Li, Cuihua; Newman, Matthew (September 2023, npj Climate and Atmospheric Science)

   Abstract The US Southwest is in a drought crisis that has been developing over the past two decades, contributing to marked increases in burned forest areas and unprecedented efforts to reduce water consumption. Climate change has contributed to this ongoing decadal drought via warming that has increased evaporative demand and reduced snowpack and streamflows. However, on the supply side, precipitation has been low during the 21st century. Here, using simulations with an atmosphere model forced by imposed sea surface temperatures, we show that the 21st century shift to cooler tropical Pacific sea surface temperatures forced a decline in cool season precipitation that in turn drove a decline in spring to summer soil moisture in the southwest. We then project the near-term future out to 2040, accounting for plausible and realistic natural decadal variability of the Pacific and Atlantic Oceans and radiatively-forced change. The future evolution of decadal variability in the Pacific and Atlantic will strongly influence how wet or dry the southwest is in coming decades as a result of the influence on cool season precipitation. The worst-case scenario involves a continued cold state of the tropical Pacific and the development of a warm state of the Atlantic while the best case scenario would be a transition to a warm state of the tropical Pacific and the development of a cold state of the Atlantic. Radiatively-forced cool season precipitation reduction is strongest if future forced SST change continues the observed pattern of no warming in the equatorial Pacific cold tongue. Although this is a weaker influence on summer soil moisture than natural decadal variability, no combination of natural decadal variability and forced change ensures a return to winter precipitation or summer soil moisture levels as high as those in the final two decades of the 20th century. 

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3. Gross discrepancies between observed and simulated 20th-21st-century precipitation trends in Southeastern South America

   https://doi.org/10.1175/JCLI-D-20-0746.1  

   Varuolo-Clarke, Arianna M.; Smerdon, Jason E.; Williams, A. Park; Seager, Richard (May 2021, Journal of Climate)

   null (Ed.)

   Abstract Southeastern South America (SESA; encompassing Paraguay, Southern Brazil, Uruguay, and northern Argentina) experienced a 27% increase in austral summer precipitation from 1902-2019, one of the largest observed trends in seasonal precipitation globally. Previous research identifies Atlantic Multidecadal Variability and anthropogenic forcing from stratospheric ozone depletion and greenhouse gas emissions as key factors contributing to the positive precipitation trends in SESA. We analyze multi-model ensemble simulations from Phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP) and find that not only do Earth System Models simulate positive SESA precipitation trends that are much weaker over the historical interval, but some models persistently simulate negative SESA precipitation trends under historical forcings. Similarly, 16-member ensembles from two atmospheric models forced with observed historical sea surface temperatures never simulate precipitation trends that even reach the lower bound of the observed trend’s range of uncertainty. Moreover, while future 21 st -century projections from CMIP6 yield positive ensemble mean precipitation trends over SESA that grow with increasing greenhouse-gas emissions, the mean forced response never exceeds the observed historical trend. Pre-industrial control runs from CMIP6 indicate that some models do occasionally simulate centennial-scale trends in SESA that fall within the observational range, but most models do not. Results point to significant uncertainties in the attribution of anthropogenically forced influences on the observed increases in precipitation over SESA, while also suggesting that internal decadal-to-centennial variability of unknown origin and not present in state-of-the-art models may have also played a large role in generating the 20 th -21 st -century SESA precipitation trend. 

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4. Investigating the Causes of Increased Twentieth-Century Fall Precipitation over the Southeastern United States

   https://doi.org/10.1175/JCLI-D-18-0244.1  

   Bishop, Daniel A.; Williams, A. Park; Seager, Richard; Fiore, Arlene M.; Cook, Benjamin I.; Mankin, Justin S.; Singh, Deepti; Smerdon, Jason E.; Rao, Mukund P. (January 2019, Journal of Climate)

   Much of the eastern United States experienced increased precipitation over the twentieth century. Characterizing these trends and their causes is critical for assessing future hydroclimate risks. Here, U.S. precipitation trends are analyzed for 1895–2016, revealing that fall precipitation in the southeastern region north of the Gulf of Mexico (SE-Gulf) increased by nearly 40%, primarily increasing after the mid-1900s. Because fall is the climatological dry season in the SE-Gulf and precipitation in other seasons changed insignificantly, the seasonal precipitation cycle diminished substantially. The increase in SE-Gulf fall precipitation was caused by increased southerly moisture transport from the Gulf of Mexico, which was almost entirely driven by stronger winds associated with enhanced anticyclonic circulation west of the North Atlantic subtropical high (NASH) and not by increases in specific humidity. Atmospheric models forced by observed SSTs and fully coupled models forced by historical anthropogenic forcing do not robustly simulate twentieth-century fall wetting in the SE-Gulf. SST-forced atmospheric models do simulate an intensified anticyclonic low-level circulation around the NASH, but the modeled intensification occurred farther west than observed. CMIP5 analyses suggest an increased likelihood of positive SE-Gulf fall precipitation trends given historical and future GHG forcing. Nevertheless, individual model simulations (both SST forced and fully coupled) only very rarely produce the observed magnitude of the SE-Gulf fall precipitation trend. Further research into model representation of the western ridge of the fall NASH is needed, which will help us to better predict whether twentieth-century increases in SE-Gulf fall precipitation will persist into the future. 

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5. Dynamics of Future Soil Moisture Drought in Southwest North America: Linkages across Seasons in the Ocean–Atmosphere–Land System

   https://doi.org/10.1175/JCLI-D-24-0235.1  

   Seager, Richard; Ting, Mingfang; Alexander, Patrick; Liu, Haibo; Li, Cuihua; Nakamura, Jennifer (May 2025, Journal of Climate)

   Abstract Southwest North America is projected by models to aridify, defined as declining summer soil moisture, under the influence of rising greenhouse gases. Here, we investigate the driving mechanisms of aridification that connect the oceans, atmosphere, and land surface across seasons. The analysis is based on atmosphere model simulations forced by imposed sea surface temperatures (SSTs). For the historical period, these are the observed ones, and the model is run to 2041 using SSTs that account for realistic and plausible evolutions of Pacific Ocean and Atlantic Ocean interannual to decadal variability imposed on estimates of radiatively forced SST change. The results emphasize the importance of changes in precipitation throughout the year for declines in summer soil moisture. In the worst-case scenario, a cool tropical Pacific and warm North Atlantic lead to reduced cool season precipitation and soil moisture. Drier soils then persist into summer such that evapotranspiration reduces and soil moisture partially recovers. In the best-case scenario, the opposite states of the oceans lead to increased cool season precipitation but higher evapotranspiration prevents this from increasing summer soil moisture. Across the scenarios, atmospheric humidity is primarily controlled by soil moisture: drier soils lead to reduced evapotranspiration, lower air humidity, and higher vapor pressure deficit (VPD). Radiatively forced change reduces fall precipitation via anomalous transient eddy moisture flux divergence. Fall drying causes soils to enter winter dry such that, even in the best-case scenario of cool season precipitation increase, soil moisture remains dry. Radiative forcing reduces summer precipitation aided by reduced evapotranspiration from drier soils. Significance StatementSouthwest North America has long been projected to undergo aridification under rising greenhouse gases. In this model-based paper, we examine how coupling across seasons between the atmosphere and land system moves the region toward reduced summer soil moisture. The results show the dominant control on summer soil moisture by precipitation throughout the year. It also shows that even in best-case scenarios when changes in decadal modes of ocean variability lead to increases in cool season precipitation, rising spring and summer evapotranspiration means this does not translate into increased summer soil moisture. The work places projections of regional aridification on a firmer basis of understanding of the ocean driving of the atmosphere and its coupling to the land system. 

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