Publications

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1994
Ely, LL, Enzel Y, Cayan DR.  1994.  Anomalous North Pacific atmospheric circulation and large winter floods in the Southwestern United States. Journal of Climate. 7:977-987.   10.1175/1520-0442(1994)007<0977:anpaca>2.0.co;2   AbstractWebsite

Specific anomalous atmospheric circulation conditions over the North Pacific are conducive to the occurrence of the largest winter floods (greater-than-or-equal-to 10-yr return period) on rivers in six hydroclimatic subregions of Arizona and southern Utah, Nevada, and California. Composite maps of anomalies in daily 700-mb heights indicate that floods in all of the subregions are associated with a low pressure anomaly off the California coast and a high-pressure anomaly in the vicinity of either Alaska or the Aleutian Islands. Of these two major circulation features, the presence of the low is the controlling factor in determining whether large floods will occur. Shifts in the locations of the low and high pressure anomalies over the North Pacific appear to control which subregions experience floods, with high-elevation topographic features and proximity to air masses forming a major influence over the specific atmospheric circulation conditions that generate large floods in each hydroclimatic region. Concerning the interannual variability of flooding in the Southwest, there is an increased frequency of large winter floods during multiple-year periods dominated by negative SOI and a virtual absence of large floods during the intervening periods. This suggests that global-scale climatic anomalies exert a strong influence on the occurrence of severe regional winter floods.

1995
Dettinger, MD, Cayan DR.  1995.  Large-scale atmospheric forcing of recent trends toward early snowmelt runoff in California. Journal of Climate. 8:606-623.   10.1175/1520-0442(1995)008<0606:lsafor>2.0.co;2   AbstractWebsite

Since the late 1940s, snowmelt and runoff have come increasingly early in the water year in many basins in northern and central California. This subtle trend is most pronounced in moderate-altitude basins, which are sensitive to changes in mean winter temperatures. Such basins have broad areas in which winter temperatures are near enough to freezing that small increases result initially in the formation of less snow and eventually in early snowmelt. In moderate-altitude basins of California, a declining fraction of the annual runoff has come in April-June. This decline has been compensated by increased fractions of runoff at other, mostly earlier, times in the water year. Weather stations in central California, including the central Sierra Nevada, have shown trends toward warmer winters since the 1940s. A series of regression analyses indicate that runoff timing responds equally to the observed decadal-scale trends in winter temperature and interannual temperature variations of the same magnitude, suggesting that the temperature trend is sufficient to explain the runoff-timing trends. The immediate cause of the trend toward warmer winters in California is a concurrent, long-term fluctuation in winter atmospheric circulations over the North Pacific Ocean and North America that is not immediately distinguishable from natural atmospheric variability. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation of winter circulations shifted to a part of the central North Pacific Ocean that is teleconnected to California temperatures. Since the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the west coast of North America. These shifts in atmospheric circulations are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures.

1997
Weinheimer, AL, Cayan DR.  1997.  Radiolarian assemblages from Santa Barbara Basin sediments: Recent interdecadal variability. Paleoceanography. 12:658-670.   10.1029/97pa00986   AbstractWebsite

Santa Barbara Basin contains a sedimentary record ideal for high-resolution paleoclimate studies because of the annual varves and regional-to global-scale climate signals preserved in the sediments [Lange er al., 1990; Kennett and Ingram, 1995], even though it does not lie directly in the path of the California Current. A nearly 100-year annual time series (1909-1991) of polycystine radiolarian assemblages from Santa Barbara Basin (SBB) sediments was analyzed to the species level. Counts on a replicate SBB core, dated 1870-1987, indicate that results are reproducible and the flux of a few representative species can be extrapolated to estimate fluxes of environmentally sensitive groups. The frequency of species occurrences resembles a lognormal curve and year-by-year comparisons of species fluxes revealed only modest changes in the assemblages from 1909-1991, indicating that the assemblages represent a single oceanic province. For paleoceanographic analysis of the radiolarian record, species were combined into groups according to the water mass in which they occur. To test this method, temperature-sensitive species were identified using t-tests. This generated warm and cool classes exhibiting trends in relative flux similar to those of the water mass groups. Both total nux and relative fluxes of water mass groups relate to low-frequency, decadal-scale temperature fluctuations, but not strongly to El Nino-Southern Oscillation events. Generally, fluxes of species from different water masses covary suggesting changing carrying capacities and productivity through time, while the consistent inverse relationship in relative fluxes indicate variability in climate. The subtle, decadal-scale changes in assemblages, diversity, and increase in percent warm water-fauna are consistent with a spin-down of the California Current System suggested by other records.

2001
Nemani, RR, White MA, Cayan DR, Jones GV, Running SW, Coughlan JC, Peterson DL.  2001.  Asymmetric warming over coastal California and its impact on the premium wine industry. Climate Research. 19:25-34.   10.3354/cr019025   AbstractWebsite

Climatic changes over coastal California from 1951 to 1997 may have benefited the premium wine industry, as seen in higher quality wines and larger grape yields. Observed temperature warming trends were asymmetric, with greatest warming at night and during spring. Warming was associated with large increases in eastern Pacific sea surface temperatures (SST) and amounts of atmospheric water vapor. Although the average annual temperature warming trend was modest (1.13degreesC/47 yr), there was a 20 d reduction in frost occurrence and a 65 d increase in frost-free growing season length. In the Napa and Sonoma valleys, warmer winter and spring temperatures advanced the start of the growing season by 18 to 24 d, and enhanced atmospheric water vapor resulted in a 7% reduction in evaporative demand. Given the strong coupling between Pacific SSTs and the coastal California climate, and because regional-scale SSTs persist for 6 to 12 mo, additional research may allow the possibility of predicting vintage quantity and quality from previous winter conditions.

2003
White, WB, Dettinger MD, Cayan DR.  2003.  Sources of global warming of the upper ocean on decadal period scales. Journal of Geophysical Research-Oceans. 108   10.1029/2002jc001396   AbstractWebsite

[1] Recent studies find global climate variability in the upper ocean and lower atmosphere during the twentieth century dominated by quasi-biennial, interannual, quasi-decadal and interdecadal signals. The quasi-decadal signal in upper ocean temperature undergoes global warming/cooling of - 0.1degreesC, similar to that occurring with the interannual signal (i. e., El Nino-Southern Oscillation), both signals dominated by global warming/cooling in the tropics. From the National Centers for Environmental Prediction troposphere reanalysis and Scripps Institution of Oceanography upper ocean temperature reanalysis we examine the quasi-decadal global tropical diabatic heat storage (DHS) budget from 1975 to 2000. We find the anomalous DHS warming tendency of 0.3-0.9 W m(-2) driven principally by a downward global tropical latent-plus-sensible heat flux anomaly into the ocean, overwhelming the tendency by weaker upward shortwave-minus-longwave heat flux anomaly to drive an anomalous DHS cooling tendency. During the peak quasidecadal warming the estimated dissipation of DHS anomaly of 0.2-0.5 W m(-2) into the deep ocean and a similar loss to the overlying atmosphere through air-sea heat flux anomaly are balanced by a decrease in the net poleward Ekman heat advection out of the tropics of 0.4-0.7 W m(-2). This scenario is nearly the opposite of that accounting for global tropical warming during the El Nino. These diagnostics confirm that even though the global quasi-decadal signal is phase-locked to the 11-year signal in the Sun's surface radiative forcing of -0.1 W m(-2), the anomalous global tropical DHS tendency cannot be driven by it directly.

2005
Stewart, IT, Cayan DR, Dettinger MD.  2005.  Changes toward earlier streamflow timing across western North America. Journal of Climate. 18:1136-1155.   10.1175/jcli3321.1   AbstractWebsite

The highly variable timing of streamflow in snowmelt-dominated basins across western North America is an important consequence, and indicator, of climate fluctuations. Changes in the timing of snowmelt-derived streamflow from 1948 to 2002 were investigated in a network of 302 western North America gauges by examining the center of mass for flow, spring pulse onset dates, and seasonal fractional flows through trend and principal component analyses. Statistical analysis of the streamflow timing measures with Pacific climate indicators identified local and key large-scale processes that govern the regionally coherent parts of the changes and their relative importance. Widespread and regionally coherent trends toward earlier onsets of springtime snowmelt and streamflow have taken place across most of western North America, affecting an area that is much larger than previously recognized. These timing changes have resulted in increasing fractions of annual flow occurring earlier in the water year by 1-4 weeks. The immediate (or proximal) forcings for the spatially coherent parts of the year-to-year fluctuations and longer-term trends of streamflow timing have been higher winter and spring temperatures. Although these temperature changes are partly controlled by the decadal-scale Pacific climate mode [Pacific decadal oscillation (PDO)], a separate ani significant part of the variance is associated with a springtime warming trend that spans the PDO phases.

2006
Westerling, AL, Hidalgo HG, Cayan DR, Swetnam TW.  2006.  Warming and earlier spring increase western US forest wildfire activity. Science. 313:940-943.   10.1126/science.1128834   AbstractWebsite

Western United States forest wildfire activity is widely thought to have increased in recent decades, yet neither the extent of recent changes nor the degree to which climate may be driving regional changes in wildfire has been systematically documented. Much of the public and scientific discussion of changes in western United States wildfire has focused instead on the effects of 19th- and 20th-century land-use history. We compiled a comprehensive database of large wildfires in western United States forests since 1970 and compared it with hydroclimatic and land-surface data. Here, we show that large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons. The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.

2008
Pierce, DW, Barnett TP, Hidalgo HG, Das T, Bonfils C, Santer BD, Bala G, Dettinger MD, Cayan DR, Mirin A, Wood AW, Nozawa T.  2008.  Attribution of declining western US snowpack to human effects. Journal of Climate. 21:6425-6444.   10.1175/2008jcli2405.1   AbstractWebsite

Observations show snowpack has declined across much of the western United States over the period 1950-99. This reduction has important social and economic implications, as water retained in the snowpack from winter storms forms an important part of the hydrological cycle and water supply in the region. A formal model-based detection and attribution (D-A) study of these reductions is performed. The detection variable is the ratio of 1 April snow water equivalent (SWE) to water-year-to-date precipitation (P), chosen to reduce the effect of P variability on the results. Estimates of natural internal climate variability are obtained from 1600 years of two control simulations performed with fully coupled ocean-atmosphere climate models. Estimates of the SWE/P response to anthropogenic greenhouse gases, ozone, and some aerosols are taken from multiple-member ensembles of perturbation experiments run with two models. The D-A shows the observations and anthropogenically forced models have greater SWE/P reductions than can be explained by natural internal climate variability alone. Model-estimated effects of changes in solar and volcanic forcing likewise do not explain the SWE/P reductions. The mean model estimate is that about half of the SWE/P reductions observed in the west from 1950 to 1999 are the result of climate changes forced by anthropogenic greenhouse gases, ozone, and aerosols.

Bonfils, C, Santer BD, Pierce DW, Hidalgo HG, Bala G, Das T, Barnett TP, Cayan DR, Doutriaux C, Wood AW, Mirin A, Nozawa T.  2008.  Detection and attribution of temperature changes in the mountainous western United States. Journal of Climate. 21:6404-6424.   10.1175/2008jcli2397.1   AbstractWebsite

Large changes in the hydrology of the western United States have been observed since the mid-twentieth century. These include a reduction in the amount of precipitation arriving as snow, a decline in snowpack at low and midelevations, and a shift toward earlier arrival of both snowmelt and the centroid (center of mass) of streamflows. To project future water supply reliability, it is crucial to obtain a better understanding of the underlying cause or causes for these changes. A regional warming is often posited as the cause of these changes without formal testing of different competitive explanations for the warming. In this study, a rigorous detection and attribution analysis is performed to determine the causes of the late winter/early spring changes in hydrologically relevant temperature variables over mountain ranges of the western United States. Natural internal climate variability, as estimated from two long control climate model simulations, is insufficient to explain the rapid increase in daily minimum and maximum temperatures, the sharp decline in frost days, and the rise in degree-days above 0 degrees C (a simple proxy for temperature-driven snowmelt). These observed changes are also inconsistent with the model-predicted responses to variability in solar irradiance and volcanic activity. The observations are consistent with climate simulations that include the combined effects of anthropogenic greenhouse gases and aerosols. It is found that, for each temperature variable considered, an anthropogenic signal is identifiable in observational fields. The results are robust to uncertainties in model-estimated fingerprints and natural variability noise, to the choice of statistical down-scaling method, and to various processing options in the detection and attribution method.

Barnett, TP, Pierce DW, Hidalgo HG, Bonfils C, Santer BD, Das T, Bala G, Wood AW, Nozawa T, Mirin AA, Cayan DR, Dettinger MD.  2008.  Human-induced changes in the hydrology of the western United States. Science. 319:1080-1083.   10.1126/science.1152538   AbstractWebsite

Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. We present a regional, multivariable climate change detection and attribution study, using a high- resolution hydrologic model forced by global climate models, focusing on the changes that have already affected this primarily arid region with a large and growing population. The results show that up to 60% of the climate- related trends of river flow, winter air temperature, and snow pack between 1950 and 1999 are human- induced. These results are robust to perturbation of study variates and methods. They portend, in conjunction with previous work, a coming crisis in water supply for the western United States.

Kueppers, LM, Snyder MA, Sloan LC, Cayan D, Jin J, Kanamaru H, Kanamitsu M, Miller NL, Tyree M, Due H, Weare B.  2008.  Seasonal temperature responses to land-use change in the western United States. Global and Planetary Change. 60:250-264.   10.1016/j.gloplacha.2007.03.005   AbstractWebsite

In the western United States, more than 79 000 km 2 has been converted to irrigated agriculture and urban areas. These changes have the potential to alter surface temperature by modifying the energy budget at the land-atmosphere interface. This study reports the seasonally varying temperature responses of four regional climate models (RCMs) - RSM, RegCM3, MM5-CLM3, and DRCM - to conversion of potential natural vegetation to modern land-cover and land-use over a 1-year period. Three of the RCMs supplemented soil moisture, producing large decreases in the August mean (- 1.4 to -3.1 degrees C) and maximum (-2.9 to -6.1 degrees C) 2-m air temperatures where natural vegetation was converted to irrigated agriculture. Conversion to irrigated agriculture also resulted in large increases in relative humidity (9% to 36% absolute change). Modeled changes in the August minimum 2-m air temperature were not as pronounced or consistent across the models. Converting natural vegetation to urban land-cover produced less pronounced temperature effects in all models, with the magnitude of the effect dependent upon the preexisting vegetation type and urban parameterizations. Overall, the RCM results indicate that the temperature impacts of land-use change are most pronounced during the summer months, when surface heating is strongest and differences in surface soil moisture between irrigated land and natural vegetation are largest. (c) 2007 Elsevier B.V. All rights reserved.

Taylor, SV, Cayan DR, Graham NE, Georgakakos KP.  2008.  Northerly surface winds over the eastern North Pacific Ocean in spring and summer. Journal of Geophysical Research-Atmospheres. 113   10.1029/2006jd008053   AbstractWebsite

Persistent spring and summer northerly surface winds are the defining climatological feature of the western coast of North America, especially south of the Oregon coast. Northerly surface winds are important for upwelling and a vast array of other biological, oceanic, and atmospheric processes. Intermittence in northerly coastal surface wind is characterized and wind events are quantitatively defined using coastal buoy data south of Cape Mendocino on the northern California coast. The defined wind events are then used as a basis for composites in order to explain the spatial evolution of various atmospheric and oceanic processes. Wind events involve large-scale changes in the three-dimensional atmospheric circulation including the eastern North Pacific subtropical anticyclone and southeast trade winds. Composites of QSCAT satellite scatterometer wind estimates from 1999 to 2005 based on a single coastal buoy indicate that wind events typically last 72-96 h and result in anomalies in surface wind and Ekman pumping that extend over 1000 km from the west coast of North America. It may be useful to consider ocean circulation and dependent ecosystem dynamics and the distribution of temperature, moisture, and aerosols in the atmospheric boundary layer in the context of wind events defined herein.

2009
Das, T, Hidalgo HG, Dettinger MD, Cayan DR, Pierce DW, Bonfils C, Barnett TP, Bala G, Mirin A.  2009.  Structure and detectability of trends in hydrological measures over the western United States. Journal of Hydrometeorology. 10:871-892.   10.1175/2009jhm1095.1   AbstractWebsite

This study examines the geographic structure of observed trends in key hydrologically relevant variables across the western United States at (1)/(8)degrees spatial resolution during the period 1950-99. Geographical regions, latitude bands, and elevation classes where these trends are statistically significantly different from trends associated with natural climate variations are identified. Variables analyzed include late-winter and spring temperature, winter-total snowy days as a fraction of winter-total wet days, 1 April snow water equivalent (SWE) as a fraction of October-March (ONDJFM) precipitation total [precip(ONDJFM)], and seasonal [JFM] accumulated runoff as a fraction of water-year accumulated runoff. Observed changes were compared to natural internal climate variability simulated by an 850-yr control run of the finite volume version of the Community Climate System Model, version 3 (CCSM3-FV), statistically downscaled to a (1)/(8)degrees grid using the method of constructed analogs. Both observed and downscaled model temperature and precipitation data were then used to drive the Variable Infiltration Capacity (VIC) hydrological model to obtain the hydrological variables analyzed in this study. Large trends (magnitudes found less than 5% of the time in the long control run) are common in the observations and occupy a substantial part (37%-42%) of the mountainous western United States. These trends are strongly related to the large-scale warming that appears over 89% of the domain. The strongest changes in the hydrologic variables, unlikely to be associated with natural variability alone, have occurred at medium elevations [750-2500 m for JFM runoff fractions and 500-3000 m for SWE/Precip(ONDJFM)] where warming has pushed temperatures from slightly below to slightly above freezing. Further analysis using the data on selected catchments indicates that hydroclimatic variables must have changed significantly (at 95% confidence level) over at least 45% of the total catchment area to achieve a detectable trend in measures accumulated to the catchment scale.

Gershunov, A, Cayan DR, Iacobellis SF.  2009.  The great 2006 heat wave over California and Nevada: Signal of an increasing trend. Journal of Climate. 22:6181-6203.   10.1175/2009jcli2465.1   AbstractWebsite

Most of the great California-Nevada heat waves can be classified into primarily daytime or nighttime events depending on whether atmospheric conditions are dry or humid. A rash of nighttime-accentuated events in the last decade was punctuated by an unusually intense case in July 2006, which was the largest heat wave on record (1948-2006). Generally, there is a positive trend in heat wave activity over the entire region that is expressed most strongly and clearly in nighttime rather than daytime temperature extremes. This trend in nighttime heat wave activity has intensified markedly since the 1980s and especially since 2000. The two most recent nighttime heat waves were also strongly expressed in extreme daytime temperatures. Circulations associated with great regional heat waves advect hot air into the region. This air can be dry or moist, depending on whether a moisture source is available, causing heat waves to be expressed preferentially during day or night. A remote moisture source centered within a marine region west of Baja California has been increasing in prominence because of gradual sea surface warming and a related increase in atmospheric humidity. Adding to the very strong synoptic dynamics during the 2006 heat wave were a prolonged stream of moisture from this southwestern source and, despite the heightened humidity, an environment in which afternoon convection was suppressed, keeping cloudiness low and daytime temperatures high. The relative contributions of these factors and possible relations to global warming are discussed.

2010
Cayan, DR, Das T, Pierce DW, Barnett TP, Tyree M, Gershunov A.  2010.  Future dryness in the southwest US and the hydrology of the early 21st century drought. Proceedings of the National Academy of Sciences of the United States of America. 107:21271-21276.   10.1073/pnas.0912391107   AbstractWebsite

Recently the Southwest has experienced a spate of dryness, which presents a challenge to the sustainability of current water use by human and natural systems in the region. In the Colorado River Basin, the early 21st century drought has been the most extreme in over a century of Colorado River flows, and might occur in any given century with probability of only 60%. However, hydrological model runs from downscaled Intergovernmental Panel on Climate Change Fourth Assessment climate change simulations suggest that the region is likely to become drier and experience more severe droughts than this. In the latter half of the 21st century the models produced considerably greater drought activity, particularly in the Colorado River Basin, as judged from soil moisture anomalies and other hydrological measures. As in the historical record, most of the simulated extreme droughts build up and persist over many years. Durations of depleted soil moisture over the historical record ranged from 4 to 10 years, but in the 21st century simulations, some of the dry events persisted for 12 years or more. Summers during the observed early 21st century drought were remarkably warm, a feature also evident in many simulated droughts of the 21st century. These severe future droughts are aggravated by enhanced, globally warmed temperatures that reduce spring snowpack and late spring and summer soil moisture. As the climate continues to warm and soil moisture deficits accumulate beyond historical levels, the model simulations suggest that sustaining water supplies in parts of the Southwest will be a challenge.

2011
Das, T, Pierce DW, Cayan DR, Vano JA, Lettenmaier DP.  2011.  The importance of warm season warming to western US streamflow changes. Geophysical Research Letters. 38   10.1029/2011gl049660   AbstractWebsite

Warm season climate warming will be a key driver of annual streamflow changes in four major river basins of the western U.S., as shown by hydrological model simulations using fixed precipitation and idealized seasonal temperature changes based on climate projections with SRES A2 forcing. Warm season (April-September) warming reduces streamflow throughout the year; streamflow declines both immediately and in the subsequent cool season. Cool season (October-March) warming, by contrast, increases streamflow immediately, partially compensating for streamflow reductions during the subsequent warm season. A uniform warm season warming of 3 C drives a wide range of annual flow declines across the basins: 13.3%, 7.2%, 1.8%, and 3.6% in the Colorado, Columbia, Northern and Southern Sierra basins, respectively. The same warming applied during the cool season gives annual declines of only 3.5%, 1.7%, 2.1%, and 3.1%, respectively. Citation: Das, T., D. W. Pierce, D. R. Cayan, J. A. Vano, and D. P. Lettenmaier (2011), The importance of warm season warming to western U. S. streamflow changes, Geophys. Res. Lett., 38, L23403, doi: 10.1029/2011GL049660.

2013
White, AB, Anderson ML, Dettinger MD, Ralph FM, Hinojosa A, Cayan DR, Hartman RK, Reynolds DW, Johnson LE, Schneider TL, Cifelli R, Toth Z, Gutman SI, King CW, Gehrke F, Johnston PE, Walls C, Mann D, Gottas DJ, Coleman T.  2013.  A twenty-first-century California observing network for monitoring extreme weather events. Journal of Atmospheric and Oceanic Technology. 30:1585-1603.   10.1175/jtech-d-12-00217.1   AbstractWebsite

During Northern Hemisphere winters, the West Coast of North America is battered by extratropical storms. The impact of these storms is of paramount concern to California, where aging water supply and flood protection infrastructures are challenged by increased standards for urban flood protection, an unusually variable weather regime, and projections of climate change. Additionally, there are inherent conflicts between releasing water to provide flood protection and storing water to meet requirements for the water supply, water quality, hydropower generation, water temperature and flow for at-risk species, and recreation. To improve reservoir management and meet the increasing demands on water, improved forecasts of precipitation, especially during extreme events, are required. Here, the authors describe how California is addressing their most important and costliest environmental issue-water management-in part, by installing a state-of-the-art observing system to better track the area's most severe wintertime storms.

2014
Vano, JA, Udall B, Cayan DR, Overpeck JT, Brekke LD, Das T, Hartmann HC, Hidalgo HG, Hoerling M, McCabe GJ, Morino K, Webb RS, Werner K, Lettenmaier DP.  2014.  Understanding uncertainties in future Colorado River streamflow. Bulletin of the American Meteorological Society. 95:59-78.   10.1175/bams-d-12-00228.1   AbstractWebsite

The Colorado River is the primary water source for more than 30 million people in the United States and Mexico. Recent studies that project streamf low changes in the Colorado River all project annual declines, but the magnitude of the projected decreases range from less than 10% to 45% by the mid-twenty-first century. To understand these differences, we address the questions the management community has raised: Why is there such a wide range of projections of impacts of future climate change on Colorado River streamflow, and how should this uncertainty be interpreted? We identify four major sources of disparities among studies that arise from both methodological and model differences. In order of importance, these are differences in 1) the global climate models (GCMs) and emission scenarios used; 2) the ability of land surface and atmospheric models to simulate properly the high-elevation runoff source areas; 3) the sensitivities of land surface hydrology models to precipitation and temperature changes; and 4) the methods used to statistically downscale GCM scenarios. In accounting for these differences, there is substantial evidence across studies that future Colorado River streamflow will be reduced under the current trajectories of anthropogenic greenhouse gas emissions because of a combination of strong temperature-induced runoff curtailment and reduced annual precipitation. Reconstructions of preinstrumental streamflows provide additional insights; the greatest risk to Colorado River streamf lows is a multidecadal drought, like that observed in paleoreconstructions, exacerbated by a steady reduction in flows due to climate change. This could result in decades of sustained streamflows much lower than have been observed in the ~100 years of instrumental record.

2016
Clemesha, RES, Gershunov A, Iacobellis SF, Williams AP, Cayan DR.  2016.  The northward march of summer low cloudiness along the California coast. Geophysical Research Letters. 43:1287-1295.   10.1002/2015gl067081   AbstractWebsite

A new satellite-derived low cloud retrieval reveals rich spatial texture and coherent space-time propagation in summertime California coastal low cloudiness (CLC). Throughout the region, CLC is greatest during May-September but has considerable monthly variability within this summer season. On average, June is cloudiest along the coast of southern California and northern Baja, Mexico, while July is cloudiest along northern California's coast. Over the course of the summer, the core of peak CLC migrates northward along coastal California, reaching its northernmost extent in late July/early August, then recedes while weakening. The timing and movement of the CLC climatological structure is related to the summer evolution of lower tropospheric stability and both its component parts, sea surface temperature and potential temperature at 700hPa. The roughly coincident seasonal timing of peak CLC with peak summertime temperatures translates into the strongest heat-modulating capacity of CLC along California's north coast.

Pierce, DW, Cayan DR.  2016.  Downscaling humidity with Localized Constructed Analogs (LOCA) over the conterminous United States. Climate Dynamics. 47:411-431.   10.1007/s00382-015-2845-1   AbstractWebsite

Humidity is important to climate impacts in hydrology, agriculture, ecology, energy demand, and human health and comfort. Nonetheless humidity is not available in some widely-used archives of statistically downscaled climate projections for the western U.S. In this work the Localized Constructed Analogs (LOCA) statistical downscaling method is used to downscale specific humidity to a 1 degrees/16 degrees grid over the conterminous U.S. and the results compared to observations. LOCA reproduces observed monthly climatological values with a mean error of similar to 0.5 % and RMS error of similar to 2 %. Extreme (1-day in 1- and 20-years) maximum values (relevant to human health and energy demand) are within similar to 5 % of observed, while extreme minimum values (relevant to agriculture and wildfire) are within similar to 15 %. The asymmetry between extreme maximum and minimum errors is largely due to residual errors in the bias correction of extreme minimum values. The temporal standard deviations of downscaled daily specific humidity values have a mean error of similar to 1 % and RMS error of similar to 3 %. LOCA increases spatial coherence in the final downscaled field by similar to 13 %, but the downscaled coherence depends on the spatial coherence in the data being downscaled, which is not addressed by bias correction. Temporal correlations between daily, monthly, and annual time series of the original and downscaled data typically yield values >0.98. LOCA captures the observed correlations between temperature and specific humidity even when the two are downscaled independently.

2017
Sumargo, E, Cayan DR.  2017.  Variability of cloudiness over mountain terrain in the western United States. Journal of Hydrometeorology. 18:1227-1245.   10.1175/jhm-d-16-0194.1   AbstractWebsite

This study investigates the spatial and temporal variability of cloudiness across mountain zones in the western United States. Daily average cloud albedo is derived from a 19-yr series (1996-2014) of half-hourly Geostationary Operational Environmental Satellite (GOES) images. During springtime when incident radiation is active in driving snowmelt-runoff processes, the magnitude of daily cloud variations can exceed 50% of long-term averages. Even when aggregated over 3-month periods, cloud albedo varies by +/- 10% of long-term averages in many locations. Rotated empirical orthogonal functions (REOFs) of daily cloud albedo anomalies over high-elevation regions of the western conterminous United States identify distinct regional patterns, wherein the first five REOFs account for; similar to 67% of the total variance. REOF1 is centered over Northern California and Oregon and is pronounced between November and March. REOF2 is centered over the interior northwest and is accentuated between March and July. Each of the REOF/rotated principal components (RPC) modes associates with anomalous large-scale atmospheric circulation patterns and one or more large-scale teleconnection indices (Arctic Oscillation, Nino-3.4, and Pacific-North American), which helps to explain why anomalous cloudiness patterns take on regional spatial scales and contain substantial variability over seasonal time scales.

2018
Sumargo, E, Cayan DR.  2018.  The influence of cloudiness on hydrologic fluctuations in the mountains of the western United States. Water Resources Research. 54:8478-8499.   10.1029/2018wr022687   AbstractWebsite

This study investigates snowmelt and streamflow responses to cloudiness variability across the mountainous parts of the western United States. Twenty years (1996-2015) of Geostationary Operational Environmental Satellite-derived cloud cover indices (CC) with 4-km spatial and daily temporal resolutions are used as a proxy for cloudiness. The primary driver of nonseasonal fluctuations in daily mean solar insolation is the fluctuating cloudiness. We find that CC fluctuations are related to snowmelt and snow-fed streamflow fluctuations, to some extent (correlations of <0.5). Multivariate linear regression models of daily snowmelt (MELT) and streamflow (AQ) variations are constructed for each month from February to July, when snowmelt is most active. Predictors include CC from five antecedent days up to the current day. The CC-MELT and CC-AQ associations vary with time and location. The results show the dominance of negative correlations between CC and MELT, exemplifying the cloud-shading (or clear-sky) effect on snowmelt. The magnitude of the CC-MELT association (R-2) amounts to 5-61%, typically peaking in May. These associations fade earlier in summer during dry years than wet years, indicating the differing responses of thicker versus thinner snowpack. The CC-AQ association displays a less consistent pattern, with R-2 amounting to 2-47%. Nevertheless, MELT and AQ fluctuations exhibit spatially extensive patterns of correlations with daily cloudiness anomalies, indicating that the effects of cloudiness often operate over regional spatial scales. Plain Language Summary Much of the water supply in the western United States originates as mountain streams, which derive much of their water from snowmelt. The primary driver of mountain snowmelt is solar energy, and cloud cover regulates how much solar energy can reach the snow surface. Despite this fact, how snowmelt and streamflow respond to cloud cover (or its absence) has not been thoroughly studied. In our study, we describe snowmelt and streamflow responses to cloud cover using satellite images of cloud cover and surface records of snowmelt and streamflow. We find significant snowmelt and daily streamflow rate responses to cloud cover. Importantly, during the peak snowmelt season, snowmelt and streamflow decrease when cloud cover increases, and vice versa, confirming the cloud-shading effect on the snow surface. However, this cause-and-effect process is not so simple. We also find that cloud cover (or its absence) in the previous few days can affect how much snow melts and the streamflow rate is in a day. Snowmelt and streamflow responses to cloud cover are stronger, albeit shorter-lived, in dry years than in wet years, highlighting the relative importance of cloud cover in drier years.

2019
Gershunov, A, Shulgina T, Clemesha RES, Guirguis K, Pierce DW, Dettinger MD, Lavers DA, Cayan DR, Polade SD, Kalansky J, Ralph FM.  2019.  Precipitation regime change in Western North America: The role of atmospheric rivers. Scientific Reports. 9   10.1038/s41598-019-46169-w   AbstractWebsite

Daily precipitation in California has been projected to become less frequent even as precipitation extremes intensify, leading to uncertainty in the overall response to climate warming. Precipitation extremes are historically associated with Atmospheric Rivers (ARs). Sixteen global climate models are evaluated for realism in modeled historical AR behavior and contribution of the resulting daily precipitation to annual total precipitation over Western North America. The five most realistic models display consistent changes in future AR behavior, constraining the spread of the full ensemble. They, moreover, project increasing year-to-year variability of total annual precipitation, particularly over California, where change in total annual precipitation is not projected with confidence. Focusing on three representative river basins along the West Coast, we show that, while the decrease in precipitation frequency is mostly due to non-AR events, the increase in heavy and extreme precipitation is almost entirely due to ARs. This research demonstrates that examining meteorological causes of precipitation regime change can lead to better and more nuanced understanding of climate projections. It highlights the critical role of future changes in ARs to Western water resources, especially over California.