Publications

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2016
Guzman-Morales, J, Gershunov A, Theiss J, Li HQ, Cayan D.  2016.  Santa Ana Winds of Southern California: Their climatology, extremes, and behavior spanning six and a half decades. Geophysical Research Letters. 43:2827-2834.   10.1002/2016gl067887   AbstractWebsite

Santa Ana Winds (SAWs) are an integral feature of the regional climate of Southern California/Northern Baja California region, but their climate-scale behavior is poorly understood. In the present work, we identify SAWs in mesoscale dynamical downscaling of a global reanalysis from 1948 to 2012. Model winds are validated with anemometer observations. SAWs exhibit an organized pattern with strongest easterly winds on westward facing downwind slopes and muted magnitudes at sea and over desert lowlands. We construct hourly local and regional SAW indices and analyze elements of their behavior on daily, annual, and multidecadal timescales. SAWs occurrences peak in winter, but some of the strongest winds have occurred in fall. Finally, we observe that SAW intensity is influenced by prominent large-scale low-frequency modes of climate variability rooted in the tropical and north Pacific ocean-atmosphere system.

2015
Shukla, S, Steinemann A, Iacobellis SF, Cayan DR.  2015.  Annual drought in California: Association with monthly precipitation and climate phases. Journal of Applied Meteorology and Climatology. 54:2273-2281.   10.1175/jamc-d-15-0167.1   AbstractWebsite

Annual precipitation in California is more variable than in any other state and is highly influenced by precipitation in winter months. A primary question among stakeholders is whether low precipitation in certain months is a harbinger of annual drought in California. Historical precipitation data from 1895 to 2013 are investigated to identify leading monthly indicators of annual drought in each of the seven climate divisions (CDs) as well as statewide. For this study, drought conditions are defined as monthly/annual (October-September) precipitation below the 20th/30th percentile, and a leading indicator is defined as a monthly drought preceding or during an annual drought that has the strongest association (i.e., joint probability of occurrence) with a statewide annual drought. Monthly precipitation variability and contributions to annual precipitation, along with joint probabilities of drought among the winter months, are first analyzed. Then the probabilities of annual drought and the variability in leading indicators are analyzed according to different climate phases and CDs. This study identified December within a water year as being the leading indicator that is most frequently associated with annual drought statewide (56%) and in most of the CDs (the highest was CD2 at 65%). Associated with its leading-indicator status, December drought was most frequently associated with drought in other winter months (joint probability > 30%). Results from this study can help stakeholders to understand and assess the likelihood of annual drought events given monthly precipitation preceding or early in the water year.

2013
DeFlorio, MJ, Pierce DW, Cayan DR, Miller AJ.  2013.  Western US extreme precipitation events and their relation to ENSO and PDO in CCSM4. Journal of Climate. 26:4231-4243.   10.1175/jcli-d-12-00257.1   AbstractWebsite

Water resources and management over the western United States are heavily impacted by both local climate variability and the teleconnected responses of precipitation to the El Nino-Southern Oscillation (ENSO) and Pacific decadal oscillation (PDO). In this work, regional precipitation patterns over the western United States and linkages to ENSO and the PDO are analyzed using output from a Community Climate System Model version 4 (CCSM4) preindustrial control run and observations, with emphasis on extreme precipitation events. CCSM4 produces realistic zonal gradients in precipitation intensity and duration over the western United States, with higher values on the windward side of the Cascade Mountains and Sierra Nevada and lower values on the leeward. Compared to its predecessor CCSM3, CCSM4 shows an improved teleconnected signal of both ENSO and the PDO to large-scale circulation patterns over the Pacific-North America region and also to the spatial pattern and other aspects of western U.S. precipitation. The so-called drizzle problem persists in CCSM4 but is significantly improved compared to CCSM3. In particular, it is found that CCSM4 has substantially less precipitation duration bias than is present in CCSM3. Both the overall and extreme intensity of wintertime precipitation over the western United States show statistically significant linkages with ENSO and PDO in CCSM4. This analysis provides a basis for future studies using greenhouse gas (GHG)-forced CCSM4 runs.

2003
Bromirski, PD, Flick RE, Cayan DR.  2003.  Storminess variability along the California coast: 1858-2000. Journal of Climate. 16:982-993.   10.1175/1520-0442(2003)016<0982:svatcc>2.0.co;2   AbstractWebsite

The longest available hourly tide gauge record along the West Coast (U. S.) at San Francisco yields meteorologically forced nontide residuals (NTR), providing an estimate of the variation in "storminess'' from 1858 to 2000. Mean monthly positive NTR (associated with low sea level pressure) show no substantial change along the central California coast since 1858 or over the last 50 years. However, in contrast, the highest 2% of extreme winter NTR levels exhibit a significant increasing trend since about 1950. Extreme winter NTR also show pronounced quasi-periodic decadal-scale variability that is relatively consistent over the last 140 years. Atmospheric sea level pressure anomalies (associated with years having high winter NTR) take the form of a distinct, large-scale atmospheric circulation pattern, with intense storminess associated with a broad, southeasterly displaced, deep Aleutian low that directs storm tracks toward the California coast.

2001
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.

1998
Cayan, DR, Dettinger MD, Diaz HF, Graham NE.  1998.  Decadal variability of precipitation over western North America. Journal of Climate. 11:3148-3166.   10.1175/1520-0442(1998)011<3148:dvopow>2.0.co;2   AbstractWebsite

Decadal (>7- yr period) variations of precipitation over western North America account for 20%-50% of the variance of annual precipitation. Spatially, the decadal variability is broken into several regional [O(1000 km)] components. These decadal variations are contributed by fluctuations in precipitation from seasons of the year that vary from region to region and that are not necessarily concentrated in the wettest season(s) alone. The precipitation variations are linked to various decadal atmospheric circulation and SST anomaly patterns where scales range from regional to global scales and that emphasize tropical or extratropical connections, depending upon which precipitation region is considered. Further, wet or dry decades are associated with changes in frequency of at least a few shea-period circulation "modes" such as the pacific-North American pattern. precipitation fluctuations over the southwestern United States and the Saskatchewan region of western Canada are associated with extensive shifts of sea level pressure and SST anomalies, suggesting that they are components of low-frequency precipitation variability from global-scale climate processes. Consistent with the global scale of its pressure and SST connection, the Southwest decadal precipitation is aligned with opposing precipitation fluctuations in northern Africa.

Miller, AJ, Cayan DR, White WB.  1998.  A westward-intensified decadal change in the North Pacific thermocline and gyre-scale circulation. Journal of Climate. 11:3112-3127.   10.1175/1520-0442(1998)011<3112:awidci>2.0.co;2   AbstractWebsite

From the early 1970s to the mid-1980s, the main thermocline of the subarctic gyre of the North Pacific Ocean shoaled with temperatures at 200-400-m depth cooling by 1 degrees-4 degrees C over the region. The gyre-scale structure of the shoaling is quasi-stationary and intensified in the western part of the basin north of 30 degrees N, suggesting concurrent changes in gyre-scale transport. A similar quasi-stationary cooling in the subtropical gyre south of 25 degrees N is also observed but lags the subpolar change by several years. To explore the physics of these changes, the authors examine an ocean model forced by observed wind stress and heat flux anomalies from 1970-88 in which they find similar changes in gyre-scale thermocline structure. The model current fields reveal that the North Pacific subpolar and subtropical gyres strengthened by roughly 10% from the 1970s to the 1980s. The bulk of the eastward Row of the model Kuroshio-Oyashio Extension returned westward via the subpolar gyre circuit, while the subtropical gyre return flow along 20 degrees N lags the subpolar changes by several years. The authors demonstrate that the model thermocline cooling and increased transport occurred in response to decadal-scale changes in basin-scale wind stress curl with the quasi-stationary oceanic response being in a time-dependent quasi-Sverdrup balance over much of the basin east of the date line. This wind stress curl driven response is quasi-stationary but occurs in conjunction with a propagating temperature anomaly associated with subduction in the central North Pacific that links the subpolar and subtropical gyre stationary changes and gives the appearance of circumgyre propagation. Different physics evidently controls the decadal subsurface temperature signal in different parts of the extratropical North Pacific.

1997
White, WB, Lean J, Cayan DR, Dettinger MD.  1997.  Response of global upper ocean temperature to changing solar irradiance. Journal of Geophysical Research-Oceans. 102:3255-3266.   10.1029/96jc03549   AbstractWebsite

By focusing on time sequences of basin-average and global-average upper ocean temperature (i.e., from 40 degrees S to 60 degrees N) we find temperatures responding to changing solar irradiance in three separate frequency bands with periods of >100 years, 18-25 years, and 9-13 years. Moreover, we find them in two different data sets, that is, surface marine weather observations from 1990 to 1991 and bathythermograph (BT) upper ocean temperature profiles from 1955 to 1994. Band-passing basin-average find each frequency component in phase across the Indian, Pacific, and Atlantic Oceans, yielding global-average records with maximum amplitudes of 0.04 degrees +/- 0.01 degrees K and 0.07 degrees 0.01 degrees K on decadal and interdecadal scales, respectively. These achieve maximum correlation with solar irradiance records (i.e., with maximum amplitude 0.5 W m(-2) at the top of the atmosphere) al phase lags ranging from 30 degrees to 50 degrees. From the BT data set, solar signals in global-average temperature penetrate to 80-160 m, confined to the upper layer above the main pycnocline. Operating a global-average heat budget for the upper ocean yields sea surface temperature responses of 0.01 degrees-0.03 degrees K and 0.02 degrees-0.05 degrees K on decadal and interdecadal scales, respectively, from the 0.1 W m(-2) Penetration of solar irradiance to the sea surface. Since this is of the same order as that observed (i.e., 0.04 degrees-0.07 degrees K), we can infer that anomalous heat from changing solar irradiance is stored in the upper layer of the ocean.