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Gershunov, A, Michaelsen J.  1996.  Vertical variability of water vapor in the midlatitude upper troposphere. Contributions to Atmospheric Physics [Beitraege zur Physik der Atmosphaere.], Wiesbaden, Germany. 69:205-214. AbstractWebsite

Radiative, dynamical and phase-change considerations of tropospheric moisture, all point to moisture in the upper troposphere as a major determinant of the global climate. Hemispheric-scale vertical variability of upper-tropospheric moisture is observed through a multivariate statistical analysis of three years of monthly mean SAGE-II data for 1986-88. Midlatitude zonally averaged vertical variability of moisture in the upper troposphere is separated into coherent modes using principal components analysis. Bulk vertical variations in the upper troposphere are separated from a dynamical mode of variability representing vertical moisture gradient and horizontal advection. The procedure is repeated for the northern and southern midlatitudes. We discuss the vertical structure and temporal variability of the meaningful modes and observe a north-south hemispheric asymmetry in the characteristics of the vertical moisture variability. It is hypothesized that midlatitude wave cyclones are responsible for the poleward and vertical transport of water vapor to and in the midlatitude upper troposphere.

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Kozubowski, TJ, Panorska AK, Qeadan F, Gershunov A, Rominger D.  2009.  Testing Exponentiality Versus Pareto Distribution via Likelihood Ratio. Communications in Statistics-Simulation and Computation. 38:118-139.   10.1080/03610910802439121   AbstractWebsite

We consider the problem of maximum likelihood estimation of the parameters of the Pareto Type II (Lomax) distribution. We show that in certain parametrization and after modification of the parameter space to include exponential distribution as a special case, the MLEs of parameters always exist. Moreover, the MLEs have a non standard asymptotic distribution in the exponential case due to the lack of regularity. Further, we develop a likelihood ratio test for exponentiality versus Pareto II distribution. We emphasize that this problem is non standard, and the limiting null distribution of the deviance statistic in not chi-square. We derive relevant asymptotic theory as well as a convenient computational formula for the critical values for the test. An empirical power study and power comparisons with other tests are also provided. A problem from climatology involving precipitation data from hundreds of meteorological stations across North America provides a motivation for and an illustration of the new test.

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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.

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Alfaro, EJ, Pierce DW, Steinemann AC, Gershunov A.  2005.  Relationships between the irrigation-pumping electrical loads and the local climate in Climate Division 9, Idaho. Journal of Applied Meteorology. 44:1972-1978.   10.1175/jam2315.1   AbstractWebsite

The electrical load from irrigation pumps is an important part of the overall electricity demand in many agricultural areas of the U.S. west. The date the pumps turn on and the total electrical load they present over the summer varies from year to year, partly because of climate fluctuations. Predicting this variability would be useful to electricity producers that supply the region. This work presents a contingency analysis and linear regression scheme for forecasting summertime irrigation pump loads in southeastern Idaho. The basis of the predictability is the persistence of spring soil moisture conditions into summer, and the effect it has on summer temperatures. There is a strong contemporaneous relationship between soil moisture and temperature in the summer and total summer pump electrical loads so that a reasonable prediction of summer pump electrical loads based on spring soil moisture conditions can be obtained in the region. If one assumes that decision makers will take appropriate actions based on the forecast output, the net economic benefit of forecast information is approximately $2.5 million per year, making this prediction problem an important seasonal summer forecasting issue with significant economic implications.

Guirguis, K, Gershunov A, Schwartz R, Bennett S.  2011.  Recent warm and cold daily winter temperature extremes in the Northern Hemisphere. Geophysical Research Letters. 38   10.1029/2011gl048762   AbstractWebsite

The winters of 2009-2010 and 2010-2011 brought frigid temperatures to parts of Europe, Russia, and the U. S. We analyzed regional and Northern Hemispheric (NH) daily temperature extremes for these two consecutive winters in the historical context of the past 63 years. While some parts clearly experienced very cold temperatures, the NH was not anomalously cold. Extreme warm events were much more prevalent in both magnitude and spatial extent. Importantly, the persistent negative state of the North Atlantic Oscillation (NAO) explained the bulk of the observed cold anomalies, however the warm extremes were anomalous even accounting for the NAO and also considering the states of the Pacific Decadal Oscillation (PDO) and El Nino Southern Oscillation (ENSO). These winters' widespread and intense warm extremes together with a continuing hemispheric decline in cold snap activity was a pattern fully consistent with a continuation of the warming trend observed in recent decades. Citation: Guirguis, K., A. Gershunov, R. Schwartz, and S. Bennett (2011), Recent warm and cold daily winter temperature extremes in the Northern Hemisphere, Geophys. Res. Lett., 38, L17701, doi:10.1029/2011GL048762.

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Cavanaugh, NR, Gershunov A, Panorska AK, Kozubowski TJ.  2015.  The probability distribution of intense daily precipitation. Geophysical Research Letters. 42:1560-1567.   10.1002/2015gl063238   AbstractWebsite

The probability tail structure of over 22,000 weather stations globally is examined in order to identify the physically and mathematically consistent distribution type for modeling the probability of intense daily precipitation and extremes. Results indicate that when aggregating data annually, most locations are to be considered heavy tailed with statistical significance. When aggregating data by season, it becomes evident that the thickness of the probability tail is related to the variability in precipitation causing events and thus that the fundamental cause of precipitation volatility is weather diversity. These results have both theoretical and practical implications for the modeling of high-frequency climate variability worldwide.

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.

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.

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.

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.

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Vashishtha, D, Sieber W, Hailey B, Guirguis K, Gershunov A, Al-Delaimy WK.  2018.  Outpatient clinic visits during heat waves: findings from a large family medicine clinical database. Family Practice. 35:567-570.   10.1093/fampra/cmy013   AbstractWebsite

Introduction. The purpose of this study was to determine whether heat waves are associated with increased frequency of clinic visits for ICD-9 codes of illnesses traditionally associated with heat waves. Methods. During 4 years of family medicine clinic data between 2012 and 2016, we identified six heat wave events in San Diego County. For each heat wave event, we selected a control period in the same season that was twice as long. Scheduling a visit on a heat wave day (versus a non-heat wave day) was the primary predictor, and receiving a primary ICD-9 disease code related to heat waves was the outcome. Analyses were adjusted for age, gender, race/ethnicity and marital status. Results. Of the 5448 visits across the heat wave and control periods, 6.4% of visits (n = 346) were for heat wave-related diagnoses. Scheduling a visit on heat wave day was not associated with receiving a heat wave-related ICD code as compared with the control period (adjusted odds ratio: 1.35; 95% confidence interval: 0.86-1.36; P = 0.51). Discussion. We show that in a relatively large and demographically diverse population, patients who schedule appointments during heat waves are not being more frequently seen for diagnoses typically associated with heat waves in the acute setting. Given that heat waves are increasing in frequency due to climate change, there is an opportunity to increase utilization of primary care clinics during heat waves.

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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.

Gershunov, A, Barnett T, Cayan D.  1999.  North Pacific interdecadal oscillation seen as factor in ENSO-related North American climate anomalies. EOS Trans. AGU. 80:25-30.   10.1029/99EO00019   Abstract

The North Pacific Oscillation (NPO) may be a significant factor in how El Niño and La Niña affect North American weather. A cold NPO phase indeed may have been partially responsible for the uncommon strength and stability of the El Niño-induced North American climate anomalies of early 1998. On the other hand, the latest La Niña excursion, if NPO persists in its cold phase, would likely produce weaker, less stable, and less predictable climate anomalies.It is well known that the effects of interannual tropical forcing, or El Niño-Southern Oscillation (ENSO) influences, penetrate into middle latitudes to produce particular forms of climate anomalies, such as the relatively well-predicted temperature and precipitation patterns over North America during the great El Niño of 1997–1998. Evidence is now mounting that this ENSO effect accentuates certain types of synoptic scale events, so that the likelihood of extreme events is biased above or below its climatological normal over broad regions. Observations show that these ENSO effects over the United States are also affected by the phases of decadal-scale climate states such as the North Pacific Oscillation (NPO).The NPO influence can be seen in both the seasonal aggregate of various ENSO patterns and the distribution of extreme daily events.

Biondi, F, Gershunov A, Cayan DR.  2001.  North Pacific decadal climate variability since 1661. Journal of Climate. 14:5-10.   10.1175/1520-0442(2001)014<0005:npdcvs>2.0.co;2   AbstractWebsite

Climate in the North Pacific and North American sectors has experienced interdecadal shifts during the twentieth century. A network of recently developed tree-ring chronologies for Southern and Baja California extends the instrumental record and reveals decadal-scale variability back to 1661. The Pacific decadal oscillation (PDO) is closely matched by the dominant mode of tree-ring variability that provides a preliminary view of multiannual climate fluctuations spanning the past four centuries. The reconstructed PDO index features a prominent bidecadal oscillation, whose amplitude weakened in the late 1700s to mid-1800s. A comparison with proxy records of ENSO suggests that the greatest decadal-scale oscillations in Pacific climate between 1706 and 1977 occurred around 1750, 1905, and 1947.

Favre, A, Gershunov A.  2009.  North Pacific cyclonic and anticyclonic transients in a global warming context: possible consequences for Western North American daily precipitation and temperature extremes. Climate Dynamics. 32:969-987.   10.1007/s00382-008-0417-3   AbstractWebsite

Trajectories of surface cyclones and anticyclones were constructed using an automated scheme by tracking local minima and maxima of mean daily sea level pressure data in the NCEP-NCAR reanalysis and the Centre National de Recherches M,t,orologiques coupled global climate Model (CNRM-CM3) SRES A2 integration. Mid-latitude lows and highs traveling in the North Pacific were tracked and daily frequencies were gridded. Transient activity in the CNRM-CM3 historical simulation (1950-1999) was validated against reanalysis. The GCM correctly reproduces winter trajectories as well as mean geographical distributions of cyclones and anticyclones over the North Pacific in spite of a general under-estimation of cyclones' frequency. On inter-annual time scales, frequencies of cyclones and anticyclones vary in accordance with the Aleutian Low (AL) strength. When the AL is stronger (weaker), cyclones are more (less) numerous over the central and eastern North Pacific, while anticyclones are significantly less (more) numerous over this region. The action of transient cyclones and anticyclones over the central and eastern North Pacific determines seasonal climate over the West Coast of North America, and specifically, winter weather over California. Relationships between winter cyclone/anticyclone behavior and daily precipitation/cold temperature extremes over Western North America (the West) were examined and yielded two simple indices summarizing North Pacific transient activity relevant to regional climates. These indices are strongly related to the observed inter-annual variability of daily precipitation and cold temperature extremes over the West as well as to large scale seasonally averaged near surface climate conditions (e.g., air temperature at 2 m and wind at 10 m). In fact, they represent the synoptic links that accomplish the teleconnections. Comparison of patterns derived from NCEP-NCAR and CNRM-CM3 revealed that the model reproduces links between cyclone/anticyclone frequencies over the Northeastern Pacific and extra-tropical climate conditions but is deficient in relation to tropical climate variability. The connections between these synoptic indices and Western weather are well reproduced by the model. Under advanced global warming conditions, that is, the last half of the century, the model predicts a significant reduction of cyclonic transients throughout the mid-latitude North Pacific with the exception of the far northern and northeastern domains. Anticyclonic transients respond somewhat more regionally but consistently to strong greenhouse forcing, with notably fewer anticyclones over the Okhotsk/Kamchatka sector and generally more anticyclones in the Northeastern Pacific. These modifications of synoptic weather result in regional feedbacks, that is, regional synoptic alterations of the anthropogenic warming signal around the North Pacific. In the eastern Pacific, for example, synoptic feedbacks, having to do especially with the northward shift of the eastern Pacific storm-track (responding, in turn, to a weaker equator-to-pole temperature gradient), are favorable to more anticyclonic conditions off the American mid-latitude west coast and more cyclonic conditions at higher latitudes. These circulation feedbacks further reduce the equator-to-pole temperature gradient by favoring high-latitude mean winter warming especially over a broad wedge of the Arctic north of the Bering Sea and moderating the warming along the mid-latitude west coast of north America while also reducing precipitation frequencies from California to Northern Mexio.

Schwartz, RE, Gershunov A, Iacobellis SF, Cayan DR.  2014.  North American west coast summer low cloudiness: Broadscale variability associated with sea surface temperature. Geophysical Research Letters. 41:3307-3314.   10.1002/2014gl059825   AbstractWebsite

Six decades of observations at 20 coastal airports, from Alaska to southern California, reveal coherent interannual to interdecadal variation of coastal low cloudiness (CLC) from summer to summer over this broad region. The leading mode of CLC variability represents coherent variation, accounting for nearly 40% of the total CLC variance spanning 1950-2012. This leading mode and the majority of individual airports exhibit decreased low cloudiness from the earlier to the later part of the record. Exploring climatic controls on CLC, we identify North Pacific Sea Surface Temperature anomalies, largely in the form of the Pacific Decadal Oscillation (PDO) as well correlated with, and evidently helping to organize, the coherent patterns of summer coastal cloud variability. Links from the PDO to summer CLC appear a few months in advance of the summer. These associations hold up consistently in interannual and interdecadal frequencies.

Grotjahn, R, Black R, Leung R, Wehner MF, Barlow M, Bosilovich M, Gershunov A, Gutowski WJ, Gyakum JR, Katz RW, Lee YY, Lim YK, Prabhat.  2016.  North American extreme temperature events and related large scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends. Climate Dynamics. 46:1151-1184.   10.1007/s00382-015-2638-6   AbstractWebsite

The objective of this paper is to review statistical methods, dynamics, modeling efforts, and trends related to temperature extremes, with a focus upon extreme events of short duration that affect parts of North America. These events are associated with large scale meteorological patterns (LSMPs). The statistics, dynamics, and modeling sections of this paper are written to be autonomous and so can be read separately. Methods to define extreme events statistics and to identify and connect LSMPs to extreme temperature events are presented. Recent advances in statistical techniques connect LSMPs to extreme temperatures through appropriately defined covariates that supplement more straightforward analyses. Various LSMPs, ranging from synoptic to planetary scale structures, are associated with extreme temperature events. Current knowledge about the synoptics and the dynamical mechanisms leading to the associated LSMPs is incomplete. Systematic studies of: the physics of LSMP life cycles, comprehensive model assessment of LSMP-extreme temperature event linkages, and LSMP properties are needed. Generally, climate models capture observed properties of heat waves and cold air outbreaks with some fidelity. However they overestimate warm wave frequency and underestimate cold air outbreak frequency, and underestimate the collective influence of low-frequency modes on temperature extremes. Modeling studies have identified the impact of large-scale circulation anomalies and land-atmosphere interactions on changes in extreme temperatures. However, few studies have examined changes in LSMPs to more specifically understand the role of LSMPs on past and future extreme temperature changes. Even though LSMPs are resolvable by global and regional climate models, they are not necessarily well simulated. The paper concludes with unresolved issues and research questions.

OrtizBevia, MJ, Perez-Gonzalez I, Alvarez-Garcia FJ, Gershunov A.  2010.  Nonlinear estimation of El Nino impact on the North Atlantic winter. Journal of Geophysical Research-Atmospheres. 115   10.1029/2009jd013387   AbstractWebsite

The differences in the teleconnections forced by different El Nino events (Ninos) can be partly explained by the intrinsic nonlinearity of the atmospheric response. In the present study, we segregate the responses of the North Atlantic to strong from those to moderate Ninos and compare nonlinear and linear estimates. El Nino forcing is represented by the tropical Pacific sea surface temperature anomalies, and the North Atlantic atmospheric response is represented by sea level pressure anomalies in the region. To gain insight into the evolution of El Nino teleconnections in a future climate, linear and nonlinear analyses are carried out on the corresponding data fields in the control and scenario simulations of a climate model experiment. This experiment presents, in its control version, realistic teleconnections. In the observational analysis, the nonlinear method performs only slightly better than the linear one. However, in the analysis of the interannual variability by a long control experiment of a realistic climate simulation, the nonlinear estimate improves significantly with respect to the linear one. The analysis of the corresponding scenario experiment points to an intensification of the (negative) surface pressure anomalies associated with the Ninos in the west European sector in a future climate. This feature is related to the important stratospheric anomalies in the same region, revealed by previous studies.

Polade, SD, Gershunov A, Cayan DR, Dettinger MD, Pierce DW.  2013.  Natural climate variability and teleconnections to precipitation over the Pacific-North American region in CMIP3 and CMIP5 models. Geophysical Research Letters. 40:2296-2301.   10.1002/grl.50491   AbstractWebsite

Natural climate variability will continue to be an important aspect of future regional climate even in the midst of long-term secular changes. Consequently, the ability of climate models to simulate major natural modes of variability and their teleconnections provides important context for the interpretation and use of climate change projections. Comparisons reported here indicate that the CMIP5 generation of global climate models shows significant improvements in simulations of key Pacific climate mode and their teleconnections to North America compared to earlier CMIP3 simulations. The performance of 14 models with simulations in both the CMIP3 and CMIP5 archives are assessed using singular value decomposition analysis of simulated and observed winter Pacific sea surface temperatures (SSTs) and concurrent precipitation over the contiguous United States and northwestern Mexico. Most of the models reproduce basic features of the key natural mode and their teleconnections, albeit with notable regional deviations from observations in both SST and precipitation. Increasing horizontal resolution in the CMIP5 simulations is an important, but not a necessary, factor in the improvement from CMIP3 to CMIP5.

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Alfaro, EJ, Gershunov A, Cayan DR.  2004.  A method for prediction of California summer air surface temperature. EOS Trans. AGU. 85:553,557-558.   10.1029/2004EO510001   Abstract
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Gershunov, A, Schneider N, Barnett T.  2001.  Low-frequency modulation of the ENSO-Indian monsoon rainfall relationship: Signal or noise? Journal of Climate. 14:2486-2492.   10.1175/1520-0442(2001)014<2486:lfmote>2.0.co;2   AbstractWebsite

Running correlations between pairs of stochastic time series are typically characterized by low-frequency evolution. This simple result of sampling variability holds for climate time series but is not often recognized for being merely noise. As an example, this paper discusses the historical connection between El Nino-Southern Oscillation (ENSO) and average Indian rainfall (AIR). Decades of strong correlation (similar to -0.8) alternate with decades of insignificant correlation, and it is shown that this decadal modulation could be due solely to stochastic processes. In fact, the specific relationship between ENSO and AIR is significantly less variable on decadal timescales than should be expected from sampling variability alone.

Westerling, AL, Gershunov A, Cayan DR, Barnett TP.  2002.  Long lead statistical forecasts of area burned in western US wildfires by ecosystem province. International Journal of Wildland Fire. 11:257-266.   10.1071/wf02009   AbstractWebsite

A statistical forecast methodology exploits large-scale patterns in monthly U.S. Climatological Division Palmer Drought Severity Index (PDSI) values over a wide region and several seasons to predict area burned in western US. wildfires by ecosystem province a season in advance. The forecast model, which is based on canonical correlations, indicates that a few characteristic patterns determine predicted wildfire season area burned. Strong negative associations between anomalous soil moisture (inferred from PDSI) immediately prior to the fire season and area burned dominate in most higher elevation forested provinces, while strong positive associations between anomalous soil moisture a year prior to the fire season and area burned dominate in desert and shrub and grassland provinces. In much of the western US., above- and below-normal fire season forecasts were successful 57% of the time or better, as compared with a 33% skill for a random guess, and with a low probability of being surprised by a fire season at the opposite extreme of that forecast.

Gershunov, A, Michaelsen J, Gautier C.  1998.  Large-scale coupling between the tropical greenhouse effect and latent heat flux via atmospheric dynamics. Journal of Geophysical Research-Atmospheres. 103:6017-6031.   10.1029/97jd03520   AbstractWebsite

The clear-sky greenhouse effect (GE) is determined primarily by the amount and vertical distribution of water vapor in the atmospheric column. GE hampers surface radiative cooling and is maintained through surface evaporative cooling. This paper examines the intimate space-time relationships between the patterns of radiative heating of the atmosphere and sui-face evaporative cooling. We use data derived from satellite and in situ observations to show that tropical maritime GE is decoupled in space and time from latent heat flux (LHF), its source of water vapor. Large scale transport of atmospheric water vapor responsible for the observed relationships between GE and LHF is discussed. The spatial patterns of average GE and LHF are imbedded in the Walker and Hadley circulations and reinforce these circulations with strong evaporative cooling in the subtropical highs and greenhouse warning in the equatorial trough zones. Throughout tropical areas characterized by strong seasonality, the seasonal cycles of GE and LHF are out of phase. Much of the moisture that feeds GE in these off equatorial regions is advected by the Hadley circulation from tropical moisture Source regions of the opposite hemisphere. An out-of-phase relationship between GE and LHF also turns up on El Nino-Southern Oscillation timescales, most notably in the central tropical Pacific. The "super" greenhouse effect (SGE), a situation when GE absorption increases more than colocated surface longwave emission, is a seasonal feature of extensive tropical off-equatorial areas where it is maintained by moisture convergence and convection. On interannual timescales, the same dynamical processes appear to assert the SGE in the central equatorial Pacific. GE and LHF regimes are also described for the equatorial cold tongue and warm pool regions.

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Polade, SD, Pierce DW, Cayan DR, Gershunov A, Dettinger MD.  2014.  The key role of dry days in changing regional climate and precipitation regimes. Scientific Reports. 4   10.1038/srep04364   AbstractWebsite

Future changes in the number of dry days per year can either reinforce or counteract projected increases in daily precipitation intensity as the climate warms. We analyze climate model projected changes in the number of dry days using 28 coupled global climate models from the Coupled Model Intercomparison Project, version 5 (CMIP5). We find that the Mediterranean Sea region, parts of Central and South America, and western Indonesia could experience up to 30 more dry days per year by the end of this century. We illustrate how changes in the number of dry days and the precipitation intensity on precipitating days combine to produce changes in annual precipitation, and show that over much of the subtropics the change in number of dry days dominates the annual changes in precipitation and accounts for a large part of the change in interannual precipitation variability.

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Gershunov, A, Barnett TP.  1998.  Interdecadal modulation of ENSO teleconnections. Bulletin of the American Meteorological Society. 79:2715-2725.   10.1175/1520-0477(1998)079<2715:imoet>2.0.co;2   AbstractWebsite

Seasonal climate anomalies over North America exhibit rather large variability between years characterized by the same ENSO phase. This lack of consistency reduces potential statistically based ENSO-related climate predictability. The authors show that the North Pacific oscillation (NPO) exerts a modulating effect on ENSO teleconnections. Sea lever pressure (SLP) data over the North Pacific, North America, and the North Atlantic and daily rainfall records in the contiguous United States are used to demonstrate that typical ENSO signals tend to be stronger and more stable during preferred phases of the NPO. Typical El Nino patterns (e.g., low pressure over the northeastern Pacific, dry northwest, and wet southwest, etc.) are strong and consistent only during the high phase of the NPO, which is associated with an anomalously cold northwestern Pacific. The generally reversed SLP and precipitation patterns during La Nina winters are consistent only during the low NPO phase. Climatic anomalies tend to be weak and spatially incoherent during low NPO-El Nino and high NPO-La Nina winters. These results suggest that confidence in ENSO-based long-range climate forecasts for North America should reflect interdecadal climatic anomalies in the North Pacific.