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1996
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.

Gershunov, A, Michaelsen J.  1996.  Climatic-scale space-time variability of tropical precipitation. Journal of Geophysical Research-Atmospheres. 101:26297-26307.   10.1029/96jd01382   AbstractWebsite

More than 15 years of monthly microwave sounding unit rainfall data over the tropical oceans are analyzed to illustrate rainfall variability on various timescales and delineate its spatial patterns. The annual and semiannual components of the seasonal cycle are modeled with first and second annual harmonics at every 2.5 degrees x 2.5 degrees grid square. Regions of highest rainfall variability tend to be characterized by a powerful annual cycle. The semiannual cycle is generally a trivial component of the seasonal cycle, except in some regions where either the mean climatological precipitation is low or where the total seasonal cycle is weak. An interesting exception, in this respect, is a band of the southeastern tropical Pacific extending immediately to the south of the eastern equatorial Pacific cold tongue. Regions of highest climatological mean rainfall are characterized by weak seasonality but strong nonseasonal variability. After seasonality is described and removed from the data, nonseasonal variability is considered via principal component analysis in the time domain. The two dominant modes together describe precipitation variability associated with the El Nino-Southern Oscillation: they outline the evolution of warm- and cold-event precipitation anomalies and contrast the intense 1982-1983 warm event with the moderate events of 1986-1987 and 1992-1993. These two modes display oscillations with predominantly quasi-biennial and similar to 5-year periods. Another coherent mode summarizes intraseasonal variability which, although inadequately resolved by the monthly average rainfall data, displays typical signs of the 40- to 50-day oscillation. All coherent modes, despite having much of their energy concentrated around rather different frequencies, show signs of interaction.

1998
Gershunov, A.  1998.  ENSO influence on intraseasonal extreme rainfall and temperature frequencies in the contiguous United States: Implications for long-range predictability. Journal of Climate. 11:3192-3203.   10.1175/1520-0442(1998)011<3192:eioier>2.0.co;2   AbstractWebsite

Potential ENSO-related predictability of wintertime daily extreme precipitation and temperature frequencies is investigated. This is done empirically using six decades of daily data at 168 stations distributed over the contiguous United States. ENSO sensitivity in the extreme ranges of intraseasonal precipitation and temperature probability density functions is demonstrated via a compositing technique. Potential predictability of extremes is then investigated with a simple statistical model. Given a perfect forecast of ENSO, the frequency of intraseasonal extremes is specified as the average frequency of occurrence during similar-phased ENSO winters on record. Specification skill is assessed as the cross-validated proportion of local variance explained by this method. The skill depends on varying ENSO sensitivity in different geographic regions and quantile ranges and on consistency or variability from one like-phased ENSO event to another. ENSO sensitivity also varies according to the intensity of the tropical forcing; however, not always in the expected sense. Good predictability is likely for variables and in regions displaying a strong and consistent ENSO signal. This is found in some coherent regions of the United States for various combinations of frequency variable and ENSO phase. ENSO-based predictability of heavy and extreme precipitation frequency is potentially good along the Gulf Coast, central plains, Southwest, and in the Ohio River valley for El Nino winters and in the Southwest and Florida for La Nina winters. Not all large magnitude signals translate into significant specification skill. Extreme precipitation frequency in the Southwest is a good example of this. Extreme warm temperature frequency (EWF) is potentially predictable in the southern and eastern United States during Fl Nino winters and in the Midwest during the strongest events. La Nina winters exhibit potentially very good EWF predictability in a Large area of the southern United States centered on Texas. Despite showing coherent ENSO patterns, extreme cold temperature frequency (ECF) signals are mostly weak and inconsistent, especially during strong ENSO events. Curiously, specification skill improves in the northern United States, along the West Coast and in the southeast during weaker El Nino winters. An improvement in potential ECF predictability is also observed in the Midwest during weaker La Nina winters.

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.

Gershunov, A, Barnett TP.  1998.  ENSO influence on intraseasonal extreme rainfall and temperature frequencies in the contiguous United States: Observations and model results. Journal of Climate. 11:1575-1586.   10.1175/1520-0442(1998)011<1575:eioier>2.0.co;2   AbstractWebsite

The signature of ENSO in the wintertime frequencies of heavy precipitation and temperature extremes is derived from both observations and atmospheric general circulation model output for the contiguous United States. ENSO signals in the frequency of occurrence of heavy rainfall are found in the Southeast, Gulf Coast, central Rockies, and the general area of the Mississippi-Ohio River valleys. Strong, nonlinear signals in extreme warm temperature frequencies are found in the southern and eastern United States. Extreme cold temperature frequencies are found to be less sensitive to ENSO forcing than extreme warm temperature frequencies. Observed ENSO signals in extreme temperature frequencies are not simply manifestations of shifts in mean seasonal temperature. These signals in the wintertime frequency of extreme rainfall and temperature events appear strong enough to be useful in long-range regional statistical prediction. Comparisons of observational and model results show that the model climate is sensitive to ENSO on continental scales and provide some encouragement to modeling studies of intraseasonal sensitivity to low-frequency climatic forcing. However, large regional disagreements exist in all variables. Continental-scale El Nino signatures in intraseasonal temperature variability are not correctly modeled. Modeled signals in extreme temperature event frequencies are much more directly related to shifts in seasonal mean temperature than they are in nature.

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.

1999
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.

2000
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.

2001
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.

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.

2002
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.

2003
Gershunov, A, Cayan DR.  2003.  Heavy daily precipitation frequency over the contiguous United States: Sources of climatic variability and seasonal predictability. Journal of Climate. 16:2752-2765.   10.1175/1520-0442(2003)016<2752:hdpfot>2.0.co;2   AbstractWebsite

By matching large-scale patterns in climate fields with patterns in observed station precipitation, this work explores seasonal predictability of precipitation in the contiguous United States for all seasons. Although it is shown that total seasonal precipitation and frequencies of less-than-extreme daily precipitation events can be predicted with much higher skill, the focus of this study is on frequencies of daily precipitation above the seasonal 90th percentile (P90), a variable whose skillful prediction is more challenging. Frequency of heavy daily precipitation is shown to respond to ENSO as well as to non-ENSO interannual and interdecadal variability in the North Pacific. Specification skill achieved by a statistical model based on contemporaneous SST forcing with and without an explicit dynamical atmosphere is compared and contrasted. Statistical models relating the SST forcing patterns directly to observed station precipitation are shown to perform consistently better in all seasons than hybrid (dynamical-statistical) models where the SST forcing is first translated to atmospheric circulation via three separate general circulation models and the dynamically computed circulation anomalies are statistically related to observed precipitation. Skill is summarized for all seasons, but in detail for January-February-March, when it is shown that predictable patterns are spatially robust regardless of the approach used. Predictably, much of the skill is due to ENSO. While the U. S. average skill is modest, regional skill levels can be quite high. It is also found that non-ENSO-related skill is significant, especially for the extreme Southwest and that this is due mostly to non-ENSO interannual and decadal variability in the North Pacific SST forcing. Although useful specification skill is achieved by both approaches, hybrid predictability is not pursued further in this effort. Rather, prognostic analysis is carried out with the purely statistical approach to analyze P90 predictability based on antecedent SST forcing. Skill at various lead times is investigated and it is shown that significant regional skill can be achieved at lead times of several months even in the absence of strong ENSO forcing.

Westerling, AL, Gershunov A, Brown TJ, Cayan DR, Dettinger MD.  2003.  Climate and wildfire in the western United States. Bulletin of the American Meteorological Society. 84:595-+.   10.1175/bams-84-5-595   AbstractWebsite

A 21-yr gridded monthly fire-starts and acres-burned dataset from U.S. Forest Service, Bureau of Land Management, National Park Service, and Bureau of Indian Affairs fire reports recreates the seasonality and interannual variability of wildfire in the western United States. Despite pervasive human influence in western fire regimes, it is striking how strongly these data reveal a fire season responding to variations in climate. Correlating anomalous wildfire frequency and extent with the Palmer Drought Severity Index illustrates the importance of prior and accumulated precipitation anomalies for future wildfire season severity. This link to antecedent seasons' moisture conditions varies widely with differences in predominant fuel type. Furthermore, these data demonstrate that the relationship between wildfire season severity and observed moisture anomalies from antecedent seasons is strong enough to forecast fire season severity at lead times of one season to a year in advance.

2004
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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White, WB, Gershunov A, Annis JL, McKeon G, Syktus J.  2004.  Forecasting Australian drought using southern, hemisphere modes of sea-surface temperature variability. International Journal of Climatology. 24:1911-1927.   10.1002/joc.1091   AbstractWebsite

Drought of 3 to 7 years' duration has devastated the flora, fauna, and regional economies in rangeland grazing districts over eastern and central Australia every 15 to 25 years throughout the 20th century, in some cases degrading the land beyond recover. Recently, these drought and degradation episodes have been associated with a global interdecadal oscillation (IDO) of period 15 to 25 years. This IDO signal brought cooler sea-surface temperatures (SSTs) to the western extra-tropical South Pacific Ocean in association with reduced onshore transport of moisture over eastern/central Australia during the summer monsoon. Here, we utilize optimized canonical correlation analysis (CCA) to forecase principal components of summer precipitation (PCP) anomalies over Australia from the persistence of principal components that dominate spring SST anomalies across the Southern Hemisphere. These summer PCP forecasts are cross-validated with the CCA forecast model for each year independent of that year's variability. Resulting cross-validated forecasts are best over Queensland, correlating with those observed at >0.40 from 1890 through to 2001, significant at >99% confidence level. More importantly, 6 of 10 drought episodes (but only three of seven degradation episodes) observed in eastern/central Australia during the 20th century are forecast. Copyright (C) 2004 Royal Meteorological Society.

Gershunov, A, Roca R.  2004.  Coupling of latent heat flux and the greenhouse effect by large-scale tropical/subtropical dynamics diagnosed in a set of observations and model simulations. Climate Dynamics. 22:205-222.   10.1007/s00382-003-0376-7   AbstractWebsite

Coupled variability of the greenhouse effect (GH) and latent heat flux (LHF) over the tropical - subtropical oceans is described, summarized and compared in observations and a coupled ocean-atmosphere general circulation model (CGCM). Coupled seasonal and interannual modes account for much of the total variability in both GH and LHF. In both observations and model, seasonal coupled variability is locally 180degrees out-of-phase throughout the tropics. Moisture is brought into convergent/convective regions from remote source areas located partly in the opposite, non-convective hemisphere. On interannual time scales, the tropical Pacific GH in the ENSO region of largest interannual variance is 180degrees out of phase with local LHF in observations but in phase in the model. A local source of moisture is thus present in the model on interannual time scales while in observations, moisture is mostly advected from remote source regions. The latent cooling and radiative heating of the surface as manifested in the interplay of LHF and GH is an important determinant of the current climate. Moreover, the hydrodynamic processes involved in the GH-LHF interplay determine in large part the climate response to external perturbations mainly through influencing the water vapor feedback but also through their intimate connection to the hydrological cycle. The diagnostic process proposed here can be performed on other CGCMs. Similarly, it should be repeated using a number of observational latent heat flux datasets to account for the variability in the different satellite retrievals. A realistic CGCM could be used to further study these coupled dynamics in natural and anthropogenically altered climate conditions.

2005
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.

2006
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.

Favre, A, Gershunov A.  2006.  Extra-tropical cyclonic/anticyclonic activity in North-Eastern Pacific and air temperature extremes in Western North America. Climate Dynamics. 26:617-629.   10.1007/s00382-005-0101-9   AbstractWebsite

Synoptic extra-tropical cyclone and anticyclone trajectories have been constructed from mean daily sea level pressure (SLP) data using a new automated scheme. Frequency, intensity and trajectory characteristics of these transients have been summarized to form indices describing wintertime cyclonic and anticyclonic activity over the North-Eastern Pacific (east of 170 degrees W) during 1950-2001. During this period, the strength of anticyclones gradually diminished and their frequency became more variable, while cyclones intensified in a discrete shift with deeper lows and further southerly trajectories occurring since the mid-1970s. These changes in synoptic transients translate into anomalously low seasonal mean SLP in the Aleutian Low, a low-level circulation anomaly consistent with the positive phase of the North Pacific Decadal Oscillation, with positive sea surface temperature (SST) anomalies along the west coast of North America and negative in the central North Pacific Ocean. A link between cyclonic/anticyclonic activity and tropical SST anomalies also exists, but this link only becomes significant after the mid-1970s, a period that coincides with more southerly cyclone trajectories. Southward excursions of mid-latitude cyclones during El Ni (n) over tildeo/positive NPO winters accomplish the northward advection of tropical air and discourage the southward penetration of polar air masses associated with transient anticyclones. Naturally, these changes in cyclonic/anticyclonic activity directly impact surface air temperatures, especially at night. We document these profound impacts on observed wintertime minimum temperatures over Western North America.

2007
Panorska, AK, Gershunov A, Kozubowski TJ.  2007.  From diversity to volatility: probability of daily precipitation extremes. Nonlinear Dynamics in Geosciences. ( Tsonis AA, Elsner JB, Eds.).:465-484.: Springer New York   10.1007/978-0-387-34918-3_26   Abstract

A sensible stochastic model is required to correctly estimate the risk associated with daily precipitation extremes. The same requirement holds for studying high-frequency precipitation extremes in the context of climate variability and change. Results derived from probability theory were used to develop an efficient automated scheme to distinguish between heavy and exponential precipitation probability density function (PDF) tails in hundreds of daily station records spanning five decades over the North American continent. These results suggest that, at a vast majority of the stations, daily extreme precipitation probabilities do not decay exponentially, but more closely follow a power law. This means that statistical distributions traditionally used to model daily rainfall (e.g. exponential, Weibull, Gamma, lognormal) generally underestimate the probabilities of extremes. The degree of this distortion, i.e. volatility, depends on regional and seasonal climatic peculiarities. By examining geographical and seasonal patterns in extreme precipitation behavior, the authors show that the degree of volatility is determined regionally by the diversity in precipitation-producing mechanisms, or storm type diversity. Exponential tails are geographically limited to regions where precipitation falls almost exclusively from similar meteorological systems and where light probability tails are observed in all seasons. Topography plays an important role in flattening or fattening PDF tails by limiting the spatial extent of certain systems while orographically altering their precipitation amounts. Results presented here represent the first logical step towards choosing appropriate PDFs at various locations by specifying their regionally relevant family. Heavy tailed models are generally superior to those from the exponential family and can lead to more realistic estimates of extreme event probabilities, return periods, n-year events, and design limits. The correct choice of PDF is essential to safe engineering design, hazard assessment and other applications, as well as for fostering further investigations of hydrologic weather extremes and climate.

2008
Gershunov, A, Douville H.  2008.  Extensive summer hot and cold spells under current and possible future climatic conditions: Europe and North America. Climate extremes and society. ( Diaz HF, Murnane RJ, Eds.).:20., Cambridge: Cambridge University Press Abstract
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Ben Ari, T, Gershunov A, Gage KL, Snall T, Ettestad P, Kausrud KL, Stenseth NC.  2008.  Human plague in the USA: the importance of regional and local climate. Biology Letters. 4:737-740.   10.1098/rsbl.2008.0363   AbstractWebsite

A 56-year time series of human plague cases (Yersinia pestis) in the western United States was used to explore the effects of climatic patterns on plague levels. We found that the Pacific Decadal Oscillation (PDO), together with previous plague levels and above-normal temperatures, explained much of the plague variability. We propose that the PDO's impact on plague is conveyed via its effect on precipitation and temperature and the effect of precipitation and temperature on plague hosts and vectors: warmer and wetter climate leading to increased plague activity and thus an increased number of human cases. Our analysis furthermore provides insights into the consistency of plague mechanisms at larger scales.

White, WB, Gershunov A, Annis J.  2008.  Climatic influences on Midwest drought during the twentieth century. Journal of Climate. 21:517-531.   10.1175/2007jcli1465.1   AbstractWebsite

The Dustbowl Era drought in the 1930s was the principal Midwest drought of the twentieth century, occurring primarily in late spring-summer [April-August (AMJJA)] when > 70% of annual rainfall normally occurred. Another major Midwest drought occurred in the 1950s but primarily in fall-early winter [September-December (SOND)] when normal rainfall was similar to 1/2 as much. Optimized canonical correlation analysis (CCA) is applied to forecast AMJJA and SOND Midwest rainfall variability in cross-validated fashion from antecedent DJF and JJA sea surface temperature (SST) variability in the surrounding oceans. These CCA models simulate (i. e., hindcast, not forecast) the Dustbowl Era drought of the 1930s and four of seven secondary AMJJA droughts (>= 3-yr duration) during the twentieth century, and the principal Midwest drought of the 1950s and one of three secondary SOND droughts. Diagnosing the model canonical correlations finds the superposition of tropical Pacific cool phases of the quasi-decadal oscillation (QDO) and interdecadal oscillation (IDO) responsible for secondary droughts in AMJJA when ENSO was weak and finds the eastern equatorial Pacific cool phase of the ENSO responsible for secondary droughts during SOND when ENSO was strong. These explain why secondary droughts in AMJJA occurred more often (nearly every decade) and were of longer duration than secondary droughts in SOND when decadal drought tendencies were usually interrupted by ENSO. These diagnostics also find the AMJJA Dustbowl Era drought in the 1930s and the principal SOND drought in the 1950s driven primarily by different phases (i. e., in quadrature) of the pentadecadal signal in the Pacific decadal oscillation (PDO).

2009
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.

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.