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Auad, G, Miller AJ, Roads JO, Cayan D.  2001.  Pacific Ocean wind stress and surface heat flux anomalies from NCEP reanalysis and observations: Cross-statistics and ocean model responses. Journal of Geophysical Research-Oceans. 106:22249-22265.   10.1029/2000jc000264   AbstractWebsite

Wind stresses and surface heat fluxes over the Pacific Ocean from the National Center for Environmental Prediction (NCEP) reanalysis and the comprehensive Ocean-Atmosphere Data Set (COADS) (blended with FSU tropical wind stresses) are compared over a common time interval (1958-1997) in their statistics anal in the responses that they induce in sea surface temperature (SST) and heat storage when used to force an ocean model. Wind stress anomalies from the two data sets are well correlated in the midlatitude extratropics, especially in the highly sampled North Pacific. In the tropics and subtropics, low correlations were found between the two wind stress data sets. The amplitudes of the stress variations of the two data sets are similar in midlatitudes, but in the tropics NCEP wind stresses are weaker than the LOADS/FSU stresses, especially on interannual timescales. Surface heat flux anomalies from the two data sets are well correlated on interannual and shorter timescales in the North Pacific Ocean poleward of 20 degreesN, but they are poorly correlated elsewhere and on decadal timescales. In the extratropics the amplitudes of the heat flux variations of the two data sets are comparable, but in the tropics the NCEP heat fluxes are weaker than those of CORDS. Ocean model hindcasts driven by bath data sets are also compared: The midlatitude SST hindcasts were superior when using the NCEP flux anomalies while tropical SST hindcasts were equally skillful for the two hindcasts when considering all climatic timescales. The spatial and temporal sampling rates of the LOADS observations and their consequent impacts on constraining the NCEP reanalysis appear to be the main factors controlling the results found here.

Namias, J, Yuan XJ, Cayan DR.  1988.  Persistence of North Pacific sea surface temperature and atmospheric flow patterns. Journal of Climate. 1:682-703.   10.1175/1520-0442(1988)001<0682:ponpss>2.0.co;2   AbstractWebsite

North Pacific monthly sea surface temperature (SST) anomalies are more persistent than a first-order Markov process, often lasting for more than 5 months. Sea surface temperature persistence undergoes an annual cycle that is attributable to the depth of the surface mixed layer and to the annual cycle of forcing. For a given lag, the pattern correlation is minimum when it involves SST during the summer months and maximum when it involves SST during the winter months. Average winter SST anomalies that have exhibited greatest persistence during the last four decades have been negative in the central North Pacific and positive along the West Coast, but antipersistent SST anomalies have not conformed to a repeatable pattern. The atmospheric 700 mb height anomalies associated with high persistence SST cases indicate that strong SST persistence is associated with long-lasting atmospheric anomaly patterns. For highly persistent January SST anomalies, 700 mb anomalies often last from December through February. The high persistence 700 mb anomalies tend lobe negative over the east-central North Pacific and positive over North America, with strong teleconnections. This pattern translates to strengthened westerlies over the subtropics and weakened westerlies in middle latitudes across the North Pacific-a zonal wind profile that is nearly opposite to that which appeared in low persistency SST cases. Over the four decades since 1947, North Pacific SST persistence has undergone substantial multiyear variability, and has increased significantly since the beginning of this record. Related low-frequency fluctuations, as well as linear trends, have occurred in the zonal mean subtropical westerlies across the North Pacific and in related large-scale atmospheric indices, the PNA pattern and the Southern Oscillation Index.

Knowles, N, Cayan DR.  2002.  Potential effects of global warming on the Sacramento/San Joaquin watershed and the San Francisco estuary. Geophysical Research Letters. 29   10.1029/2001gl014339   AbstractWebsite

[1] California's primary hydrologic system, the San Francisco estuary and its upstream watershed, is vulnerable to the regional hydrologic consequences of projected global climate change. Projected temperature anomalies from a global climate model are used to drive a combined model of watershed hydrology and estuarine dynamics. By 2090, a projected temperature increase of 2.1degreesC results in a loss of about half of the average April snowpack storage, with greatest losses in the northern headwaters. Consequently, spring runoff is reduced by 5.6 km(3) (similar to20% of historical annual runoff), with associated increases in winter flood peaks. The smaller spring flows yield spring/summer salinity increases of up to 9 psu, with larger increases in wet years.

Das, T, Dettinger MD, Cayan DR, Hidalgo HG.  2011.  Potential increase in floods in California's Sierra Nevada under future climate projections. Climatic Change. 109:71-94.   10.1007/s10584-011-0298-z   AbstractWebsite

California's mountainous topography, exposure to occasional heavily moisture-laden storm systems, and varied communities and infrastructures in low lying areas make it highly vulnerable to floods. An important question facing the state-in terms of protecting the public and formulating water management responses to climate change-is "how might future climate changes affect flood characteristics in California?" To help address this, we simulate floods on the western slopes of the Sierra Nevada Mountains, the state's primary catchment, based on downscaled daily precipitation and temperature projections from three General Circulation Models (GCMs). These climate projections are fed into the Variable Infiltration Capacity (VIC) hydrologic model, and the VIC-simulated streamflows and hydrologic conditions, from historical and from projected climate change runs, allow us to evaluate possible changes in annual maximum 3-day flood magnitudes and frequencies of floods. By the end of the 21st Century, all projections yield larger-than-historical floods, for both the Northern Sierra Nevada (NSN) and for the Southern Sierra Nevada (SSN). The increases in flood magnitude are statistically significant (at p <= 0.01) for all the three GCMs in the period 2051-2099. The frequency of flood events above selected historical thresholds also increases under projections from CNRM CM3 and NCAR PCM1 climate models, while under the third scenario, GFDL CM2.1, frequencies remain constant or decline slightly, owing to an overall drying trend. These increases appear to derive jointly from increases in heavy precipitation amount, storm frequencies, and days with more precipitation falling as rain and less as snow. Increases in antecedent winter soil moisture also play a role in some areas. Thus, a complex, as-yet unpredictable interplay of several different climatic influences threatens to cause increased flood hazards in California's complex western Sierra landscapes.

Means, JD, Cayan D.  2013.  Precipitable Water from GPS Zenith Delays Using North American Regional Reanalysis Meteorology. Journal of Atmospheric and Oceanic Technology. 30:485-495.   10.1175/jtech-d-12-00064.1   AbstractWebsite

Precipitable water or integrated water vapor can be obtained from zenith travel-time delays from global positioning system (GPS) signals if the atmospheric pressure and temperature at the GPS site is known. There have been more than 10 000 GPS receivers deployed as part of geophysics research programs around the world; but, unfortunately, most of these receivers do not have collocated barometers. This paper describes a new technique to use North American Regional Reanalysis pressure, temperature, and geopotential height data to calculate station pressures and surface temperature at the GPS sites. This enables precipitable water to be calculated at those sites using archived zenith delays. The technique has been evaluated by calculating altimeter readings at aviation routine weather report (METAR) sites and comparing them with reported altimeter readings. Additionally, the precipitable water values calculated using this method have been found to agree with SuomiNet GPS precipitable water, with RMS differences of 2 mm or less, and are also generally in agreement with radiosonde measurements of precipitable water. Applications of this technique are shown and are explored for different synoptic situations, including atmospheric-river-type baroclinic storms and the North American monsoon.

Polade, SD, Gershunov A, Cayan DR, Dettinger MD, Pierce DW.  2017.  Precipitation in a warming world: Assessing projected hydro-climate changes in California and other Mediterranean climate regions. Scientific Reports. 7   10.1038/s41598-017-11285-y   AbstractWebsite

In most Mediterranean climate (MedClim) regions around the world, global climate models (GCMs) consistently project drier futures. In California, however, projections of changes in annual precipitation are inconsistent. Analysis of daily precipitation in 30 GCMs reveals patterns in projected hydrometeorology over each of the five MedClm regions globally and helps disentangle their causes. MedClim regions, except California, are expected to dry via decreased frequency of winter precipitation. Frequencies of extreme precipitation, however, are projected to increase over the two MedClim regions of the Northern Hemisphere where projected warming is strongest. The increase in heavy and extreme precipitation is particularly robust over California, where it is only partially offset by projected decreases in low-medium intensity precipitation. Over the Mediterranean Basin, however, losses from decreasing frequency of low-medium-intensity precipitation are projected to dominate gains from intensifying projected extreme precipitation. MedClim regions are projected to become more sub-tropical, i.e. made dryer via pole-ward expanding subtropical subsidence. California's more nuanced hydrological future reflects a precarious balance between the expanding subtropical high from the south and the south-eastward extending Aleutian low from the north-west. These dynamical mechanisms and thermodynamic moistening of the warming atmosphere result in increased horizontal water vapor transport, bolstering extreme precipitation events.

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.

Pandey, GR, Cayan DR, Georgakakos KP.  1999.  Precipitation structure in the Sierra Nevada of California during winter. Journal of Geophysical Research-Atmospheres. 104:12019-12030.   10.1029/1999jd900103   AbstractWebsite

Influences of upper air characteristics along the coast of California upon wintertime (November-April) precipitation in the Sierra Nevada are investigated. Precipitation events in the Sierra Nevada region occur mostly during wintertime, irrespective of station location (leeside or windside) and elevation. Most precipitation episodes in the region are associated with moist southwesterly winds (coming from the southwest direction) and also tend to occur when the 700-mbar temperature at the upwind direction is close to -2 degrees C. This favored wind direction and temperature signify the importance of both moisture transport and orographic lifting in augmenting precipitation in the region. By utilizing the observed dependency of the precipitation upon the upper air conditions, a linear model is formulated to quantify the precipitation observed at different sites as a function of moisture transport. The skill of the model increases with timescale of aggregation, reaching more than 50% variance explained at an aggregation period of 5-7 days. This indicates that upstream air moisture transport can be used to estimate the precipitation totals in the Sierra Nevada region.

Gershunov, A, Barnett TP, Cayan DR, Tubbs T, Goddard L.  2000.  Predicting and downscaling ENSO impacts on intraseasonal precipitation statistics in California: The 1997/98 event. Journal of Hydrometeorology. 1:201-210.   10.1175/1525-7541(2000)001<0201:padeio>2.0.co;2   AbstractWebsite

Three long-range forecasting methods have been evaluated for prediction and downscaling of seasonal and intraseasonal precipitation statistics in California. Full-statistical, hybrid-dynamical-statistical and full-dynamical approaches have been used to forecast Fl Nino-Southern Oscillation (ENSO)-related total precipitation, daily precipitation frequency, and average intensity anomalies during the January-March season. For El Nino winters, the hybrid approach emerges as the best performer, while La Nina forecasting skill is poor. The full-statistical forecasting method features reasonable forecasting skill for both La Nina and El Nino winters. The performance of the full-dynamical approach could not be evaluated as rigorously as that of the other two forecasting schemes. Although the full-dynamical forecasting approach is expected to outperform simpler forecasting schemes in the long run, evidence is presented to conclude that, at present, the full-dynamical forecasting approach is the least viable of the three, at least in California. The authors suggest that operational forecasting of any intraseasonal temperature, precipitation, or streamflow statistic derivable from the available-records is possible now for ENSO-extreme years.

Alfaro, EJ, Gershunov A, Cayan D.  2006.  Prediction of summer maximum and minimum temperature over the central and western United States: The roles of soil moisture and sea surface temperature. Journal of Climate. 19:1407-1421.   10.1175/jcli3665.1   AbstractWebsite

A statistical model based on canonical correlation analysis (CCA) was used to explore climatic associations and predictability of June-August (JJA) maximum and minimum surface air temperatures (Tmax and Tmin) as well as the frequency of Tmax daily extremes (Tmax90) in the central and western United States (west of 90 degrees W). Explanatory variables are monthly and seasonal Pacific Ocean SST (PSST) and the Climate Division Palmer Drought Severity Index (PDSI) during 1950-2001. Although there is a positive correlation between Tmax and Tmin, the two variables exhibit somewhat different patterns and dynamics. Both exhibit their lowest levels of variability in summer, but that of Tmax is greater than Tmin. The predictability of Tmax is mainly associated with local effects related to previous soil moisture conditions at short range (one month to one season), with PSST providing a secondary influence. Predictability of Tmin is more strongly influenced by large-scale (PSST) patterns, with PDSI acting as a short-range predictive influence. For both predictand variables (Tmax and Tmin), the PDSI influence falls off markedly at time leads beyond a few months, but a PSST influence remains for at least two seasons. The maximum predictive skill for JJA Tmin, Tmax, and Tmax90 is from May PSST and PDSI. Importantly. skills evaluated for various seasons and time leads undergo a seasonal cycle that has maximum levels in summer. At the seasonal time frame, summer Tmax prediction skills are greatest in the Midwest, northern and central California, Arizona, and Utah. Similar results were found for Tmax90. In contrast, Tmin skill is spread over most of the western region, except for clusters of low skill in the northern Midwest and southern Montana, Idaho, and northern Arizona.

Pierce, DW, Das T, Cayan DR, Maurer EP, Miller NL, Bao Y, Kanamitsu M, Yoshimura K, Snyder MA, Sloan LC, Franco G, Tyree M.  2013.  Probabilistic estimates of future changes in California temperature and precipitation using statistical and dynamical downscaling. Climate Dynamics. 40:839-856.   10.1007/s00382-012-1337-9   AbstractWebsite

Sixteen global general circulation models were used to develop probabilistic projections of temperature (T) and precipitation (P) changes over California by the 2060s. The global models were downscaled with two statistical techniques and three nested dynamical regional climate models, although not all global models were downscaled with all techniques. Both monthly and daily timescale changes in T and P are addressed, the latter being important for a range of applications in energy use, water management, and agriculture. The T changes tend to agree more across downscaling techniques than the P changes. Year-to-year natural internal climate variability is roughly of similar magnitude to the projected T changes. In the monthly average, July temperatures shift enough that that the hottest July found in any simulation over the historical period becomes a modestly cool July in the future period. Januarys as cold as any found in the historical period are still found in the 2060s, but the median and maximum monthly average temperatures increase notably. Annual and seasonal P changes are small compared to interannual or intermodel variability. However, the annual change is composed of seasonally varying changes that are themselves much larger, but tend to cancel in the annual mean. Winters show modestly wetter conditions in the North of the state, while spring and autumn show less precipitation. The dynamical downscaling techniques project increasing precipitation in the Southeastern part of the state, which is influenced by the North American monsoon, a feature that is not captured by the statistical downscaling.

Li, HQ, Kanamitsu M, Hong SY, Yoshimura K, Cayan DR, Misra V, Sun LQ.  2014.  Projected climate change scenario over California by a regional ocean-atmosphere coupled model system. Climatic Change. 122:609-619.   10.1007/s10584-013-1025-8   AbstractWebsite

This study examines a future climate change scenario over California in a 10-km coupled regional downscaling system of the Regional Spectral Model for the atmosphere and the Regional Ocean Modeling System for the ocean forced by the global Community Climate System Model version 3.0 (CCSM3). In summer, the coupled and uncoupled downscaled experiments capture the warming trend of surface air temperature, consistent with the driving CCSM3 forcing. However, the surface warming change along the California coast is weaker in the coupled downscaled experiment than it is in the uncoupled downscaling. Atmospheric cooling due to upwelling along the coast commonly appears in both the present and future climates, but the effect of upwelling is not fully compensated for by the projected large-scale warming in the coupled downscaling experiment. The projected change of extreme warm events is quite different between the coupled and uncoupled downscaling experiments, with the former projecting a more moderate change. The projected future change in precipitation is not significantly different between coupled and uncoupled downscaling. Both the coupled and uncoupled downscaling integrations predict increased onshore sea breeze change in summer daytime and reduced offshore land breeze change in summer nighttime along the coast from the Bay area to Point Conception. Compared to the simulation of present climate, the coupled and uncoupled downscaling experiments predict 17.5 % and 27.5 % fewer Catalina eddy hours in future climate respectively.

Cloern, JE, Knowles N, Brown LR, Cayan D, Dettinger MD, Morgan TL, Schoellhamer DH, Stacey MT, van der Wegen M, Wagner RW, Jassby AD.  2011.  Projected evolution of California's San Francisco Bay-Delta-River System in a century of climate change. Plos One. 6   10.1371/journal.pone.0024465   AbstractWebsite

Background: Accumulating evidence shows that the planet is warming as a response to human emissions of greenhouse gases. Strategies of adaptation to climate change will require quantitative projections of how altered regional patterns of temperature, precipitation and sea level could cascade to provoke local impacts such as modified water supplies, increasing risks of coastal flooding, and growing challenges to sustainability of native species. Methodology/Principal Findings: We linked a series of models to investigate responses of California's San Francisco Estuary-Watershed (SFEW) system to two contrasting scenarios of climate change. Model outputs for scenarios of fast and moderate warming are presented as 2010-2099 projections of nine indicators of changing climate, hydrology and habitat quality. Trends of these indicators measure rates of: increasing air and water temperatures, salinity and sea level; decreasing precipitation, runoff, snowmelt contribution to runoff, and suspended sediment concentrations; and increasing frequency of extreme environmental conditions such as water temperatures and sea level beyond the ranges of historical observations. Conclusions/Significance: Most of these environmental indicators change substantially over the 21(st) century, and many would present challenges to natural and managed systems. Adaptations to these changes will require flexible planning to cope with growing risks to humans and the challenges of meeting demands for fresh water and sustaining native biota. Programs of ecosystem rehabilitation and biodiversity conservation in coastal landscapes will be most likely to meet their objectives if they are designed from considerations that include: (1) an integrated perspective that river-estuary systems are influenced by effects of climate change operating on both watersheds and oceans; (2) varying sensitivity among environmental indicators to the uncertainty of future climates; (3) inevitability of biological community changes as responses to cumulative effects of climate change and other drivers of habitat transformations; and (4) anticipation and adaptation to the growing probability of ecosystem regime shifts.

Feng, DM, Beighley E, Raoufi R, Melack J, Zhao YH, Iacobellis S, Cayan D.  2019.  Propagation of future climate conditions into hydrologic response from coastal southern California watersheds. Climatic Change. 153:199-218.   10.1007/s10584-019-02371-3   AbstractWebsite

As a biodiverse region under a Mediterranean climate with a mix of highly developed and natural watersheds, coastal Santa Barbara County (SB), located in southern California, is susceptible to the hydrologic impacts of climate change. This study investigates the potential changes in hydro-meteorological variables in this region as well as their societal and ecological implications for projected climate conditions during the twenty-first century. Daily streamflow ensembles from 135 coastal watersheds for the period 2021-2100 are developed using the Hillslope River Routing (HRR) model forced with downscaled precipitation and temperature projections derived from 10 climate models in the Coupled Model Inter-Comparison Project, Phase 5, and two emission scenarios (Representative Concentration Pathways, RCP, 4.5 and 8.5). Analysis of the projected ensemble precipitation and streamflow series relative to historical conditions (1961-2000) shows (i) minimal change in annual precipitation (median change within +/- 3%); (ii) an altered seasonal rainfall distribution with a decrease in rainfall at the beginning of the rainy season (Oct-Dec), an increase during the Jan-Mar period, and a decrease at the end of the season (Apr-Jun); (iii) increases in the magnitude and frequency of large storms (>36mm/day) which combined with a shorter rainy season, lead to increases in annual peak flows; and (iv) the propagation of the altered precipitation characteristics resulting in nonlinear changes in the magnitude and variability of annual maximum discharges (i.e., mean, standard deviation, skew) impacting estimated return period discharges (e.g., estimated 100-year flood discharges for the period 2061-2100 under 8.5 increase by up to 185%). While these results are specific to southern coastal California, the nature of nonlinear hydrologic response to altered precipitation characteristics underscores the value of regional studies investigating potential impacts of climate projections on streamflow dynamics.