1. Introduction
In these relations we see the basic problem with RH. For any given RH, the VPD varies exponentially because of the Clausius–Clapeyron dependency of 
Despite the lack of popularity of VPD, it deserves a new lease on life because of its relationship to forest and grassland fire. Fire is an annual concern in many regions of the United States, particularly the western states. Though fire is a naturally occurring phenomenon to which ecosystems are adjusted and, in some cases, even dependent upon, it poses considerable problems for society. First, protection of life has become more difficult as the population of the Southwest has expanded and more people are living at the “urban–forest interface” (Pyne 2009). In addition, damage to property is a concern. Dealing with fire is one of the key problems of land management: How do we manage a process that is at the same time natural and essential and tremendously damaging? Now that western forests are experiencing drought and heat stress combined with outbreaks of bark beetles and unprecedented areas of burns, stresses that are expected to only get worse as human-induced climate change advances (Allen et al. 2010; Bentz et al. 2010; Williams et al. 2013), fire management is ever more important (Stephens et al. 2013). Hence, it is imperative to better understand the processes that control fire.
Many prior studies have sought relationships between climate and wildfire (e.g., Westerling et al. 2003, 2006; Westerling and Bryant 2008; Littell et al. 2009; Abatzoglou and Kolden 2013; Riley et al. 2013). In regard to links between climate and forest fire incidence in the southwestern United States, VPD explains more variance than precipitation, various drought indices, temperature, and wind individually can (Williams et al. 2015). Sedano and Randerson (2014) also found that VPD anomalies were closely related to fire ignition, fire growth, and burned area in Alaska. Potter (2012) provides a useful summary of the research relating fire to atmospheric moisture variables and notes that some early papers did consider aspects of the atmospheric water vapor deficit, though not VPD. For example, one of the very earliest studies of links between fire and weather (Munns 1921) noted a correlation between atmometer evaporation, which will depend strongly on the VPD, and the sizes of fires in southern California. Later, Gisborne (1928) invoked the importance of VPD (without directly mentioning it) when, referring to factors that lead to fires, he stated “A relative humidity of 21%, for example, does not always mean the same rate of drying of the fuel.” These combined works over many decades indicate that it is by drying of fuel that high VPD increases fire ignition and growth, as well as the burned area [for more discussion see Potter (2012)].
The importance of VPD is of course a confirmation of Anderson’s (1936) plea for the ecological relevance of VPD. It is not surprising that VPD is more successful in explaining burned forest fire area than are other meteorological variables. It is essentially a measure of the ability of the atmosphere to extract moisture from the surface vegetation, thus reflecting variations in the moisture content and flammability of forests. Because it accounts for the fact that it is the combination of low RH and high temperature that creates the most fire-prone conditions, VPD is more explanatory in this regard than RH. VPD is also more explanatory than temperature (e.g., Westerling et al. 2006) since it reflects the nonlinear dependence of 
Building on the work of Williams et al. (2014, 2015), we examined time histories of annual burned area of forest and grassland versus VPD for the southwestern United States (Fig. 1 shows this and other areas and locations referred to in the paper). The burned areas for 1984–2012 come from the Monitoring Trends in Burn Severity (MTBS) database (Eidenshink et al. 2007) and was extended beyond 2012 by using the MODIS burned area v5.1 dataset (Roy et al. 2008). The burned forest area was found to correlate best with the prior June–August VPD anomaly, while the burned grassland area correlated best with June VPD, reflecting the relative times needed to dry the fuels. These correlations are shown in Fig. 2. VPD is clearly a strong controlling influence on area burned of both vegetation types and an upward trend in both is clearly apparent over past decades (note the logarithmic scale).
Map showing the southwest (SW), CO, and ID–NV boxes used in this study as well as the locations of the Rodeo–Chediski and Hayman forest fires and the Murphy Complex rangeland fire.
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
Time series of annual burned area and VPD for two parts of the southwest United States during 1984–2014. (a) Forest area within parts of AZ, NM, CO, UT, and TX that are south of 38°N and west of 104°W and prior June–August VPD, (b) grassland area within the parts of NM, TX, and OK that are south of 38°N and east of 105°W and June VPD. For each region, VPD is shown for the months or month when mean VPD correlated most strongly with annual burned area.
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
VPD is only one fire-related meteorological variable and it is perhaps not always the one with the most explanatory power [e.g., see Winkler et al. (2007) for a discussion of the Haines index, which includes moisture deficit, measured by dewpoint depression, together with atmospheric stability as an index for the development of plume-dominated fires]. However, given the demonstrated importance of VPD to at least one topic of great ecological and social importance, it seems worthwhile to further explore the basic spatial and temporal variations of VPD across North America in terms of seasonal cycle, geographic variation, interannual variability, and long-term trends. To our knowledge, no such study has been conducted. Gaffen and Ross (1999) did conduct a study of climatology and trends of specific and relative humidity across the United States. Their maps of daytime RH show, in winter, high values along the West Coast and in the Southeast and low values in the Northeast and, in summer, a striking west–east lower–higher contrast.
To build on Gaffen and Ross (1999), the current study is motivated by the desire to develop a better understanding of the controls on moisture undersaturation in the atmosphere and also the need to improve our understanding of the outbreak and spread of wildland fires. As such, after providing a cross-U.S. analysis of the climatology and variability of VPD, we will examine the atmosphere–ocean–land causes of VPD variability in the Southwest, as well as the long-term trends in VPD. We will also provide case studies of the VPD anomalies, and their causes, leading up to June 2002 when two major southwest fires (the Rodeo-Chedeski fire in Arizona and Hayman fire in Colorado) occurred and to July 2007 when the Murphy Complex fire occurred in Idaho and Nevada.
2. Data and methods
To examine the atmospheric circulation variability associated with VPD variability, we examine geopotential heights and vertical pressure velocities from the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (Kalnay et al. 1996; Kistler et al. 2001). The NCEP–NCAR reanalysis was chosen as it is the only reanalysis that assimilates all available information that extends back before 1979 and hence overlaps the PRISM precipitation data. For surface sensible and latent heat fluxes, used to compute the Bowen ratio, we used data from version 2.0 of the Global Land Data Assimilation System (GLDAS), available online (http://disc.sci.gsfc.nasa.gov/gesNews/gldas_2_data_release). GLDAS uses a land surface model forced by observed meteorological conditions to estimate the land surface hydrology and surface fluxes of water and energy (Rodell et al. 2004; Sheffield and Wood 2006). All analyses cover the 1961–2012 period and anomalies, when used, are with respect to climatology over this period. The analysis period begins in 1961 because that is when PRISM dewpoint data (used to calculate VPD) are based on true measurements rather than estimated from temperature and precipitation (C. Daly 2014, personal communication).
3. Climatology of vapor pressure deficit across the United States
Figure 3 shows the VPD, 
The climatology of (left) VPD, (center) 
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
By spring the VPD has climbed above 8 mb across the majority of the United States except for most of the northern states. What is striking is the area of around 30-mb VPD in the interior southwest United States. This is driven by a sharp rise in 
In going from spring to summer, VPD increases modestly over the eastern United States, especially in the northern region but climbs strongly in the southwest and across the west. The highest monthly mean values that ever occur in the United States (above 40 mb) are found in summer in southeastern California, southern Nevada, and southwestern Arizona. This is related to high temperatures driving high 
4. Interannual variability of VPD across the United States
While the climatology of VPD is interesting, ecosystems are presumably largely evolved to deal with this. They will also be able to adapt, to some extent, to year-to-year variability. However, extreme high VPD years are expected to exert considerable water stress on vegation, leading to a risk of disease, fire, and mortality (Williams et al. 2013; Sedano and Randerson 2014). Hence, we next turn to examine the variability of VPD and its causes throughout the post-1961 period. To do this, we computed the variance of VPD, 
As in Fig. 3, but for variances (mb2).
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
In no season is the VPD variance simply proportional to the VPD climatology. In the fall and winter the VPD variance has a southwest-maximum–northeast-minimum pattern with lines of equal variance oriented in a roughly northwest-to-southeast manner. This is in contrast to the more zonal pattern of the VPD climatology. This VPD variance pattern is quite distinct from that of the 
In the spring, the Southwest region of climatological high VPD is also a region of high VPD variance and this is driven by high 
Since 
The clear and expected increase in the variance of vapor pressure quantities with the mean values suggests that normalized standard deviation may be a more informative measure. Hence, Fig. 5 shows the standard deviations normalized by their climatological values and expressed as a fraction. In this case large values show that the variance (the square of the standard deviation) is unusually large in comparison to the climatological value while small values show the opposite. The Southwest desert maximum of VPD variance does not appear on the maps of normalized standard deviation. Instead, the normalized standard deviation of VPD emphasizes the north-central United States in fall and winter and the plains and west other than the interior southwest in spring and summer. Hence, some areas of relatively low absolute VPD variance in the Pacific Northwest states appear as high areas of relative variability. In this regard, it is worth noting that Stavros et al. (2014) show that several measures of fire activity are greater in the northern parts of the western United States than the southern parts. The normalized standard deviations of 
As in Fig. 4, but for the standard deviation divided by the climatological values.
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
5. Relationship of VPD variability in the southwest United States to SST and circulation variability
The analysis above has shown that VPD variability is largest in the southwest United States at the California–Arizona border. However, this is a very arid region, with high climatological VPD, and not one with extensive fire occurrence as a result of the absence of extensive vegetation. Fire occurrence is more common in regions of lower climatological VPD that are less arid and can sustain vegetation that is nonetheless susceptible to burning. We have already shown that VPD variability is large in these intermediate aridity regions in the spring and summer seasons critical for fires and that this is influenced strongly by 
To look at this, we examine the correlation between VPD, 
The detrended correlations between (left) VPD, (center) 
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
High 
These relations are fairly easy to explain. During La Niña conditions in the fall, winter, and spring, high pressure develops and is centered over northwestern Mexico, which favors subsidence over the southwest United States, causing both high temperatures and high 
6. Relationship of variability of VPD to land surface conditions
While atmospheric circulation anomalies are expected to be able to influence VPD instantaneously via subsidence of warm, dry air, it is also expected that previous reductions in precipitation could dry out the soil and lead to an increase in VPD. As the soil dries out, incoming solar radiation needs to be increasingly balanced by sensible and longwave radiative heat loss, and less by evapotranspiration. This requires an increase in surface temperature and less moisture flux from the surface to the atmosphere, both effects that increase VPD. One measure of soil dryness is the Bowen ratio, 
The previous section showed that VPD increases as atmospheric circulation anomalies cause warming and/or drying. In the absence of a surface moisture anomaly, subsidence warming and drying would be expected to increase LH and reduce SH, surface flux changes that would offset the circulation-induced changes in VPD. This would cause a reduction in the Bowen ratio to accompany the increase in VPD.
Figure 7 shows the correlation across the United States between seasonal VPD and the Bowen ratio. In the western United States (except for the Pacific Northwest in spring), the Bowen ratio increases with VPD throughout the year. There are also positive correlations across the central and eastern United States in summer and fall. Areas of negative correlation develop in the south-central United States in winter and most of the eastern United States in spring. The strongest positive correlations are in the interior West and along the Gulf Coast in summer.
As in Fig. 6, but for correlations between Bowen ratio and (left) VPD, (center) 
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
The cause of these correlations can be understood in terms of the correlation of Bowen ratio with 
The winter negative Bowen ratio–
The correlation between VPD and Bowen ratio combines the influences of the correlations of Bowen ratio with 
Hence, it might be expected that VPD will rise following a period of reduced precipitation that dries the surface. We also computed the correlations with VPD, 
7. Relation of Southwest, Colorado, and Idaho–Nevada region VPD to the combined effects of land surface and atmospheric conditions
The actual VPD for (left) AMJ and (right) JAS and its reconstruction via linear regression based on Bowen ratio alone (
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
8. Trends in VPD across the United States
Next we consider whether there are long-term trends in VPD and its contributors. Trends are evaluated via a straightforward least squares regression of seasonal mean VPD, 
Linear trends in VPD, 
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
9. Changes in VPD up to and during the June 2002 Hayman and Rodeo–Chediski and July 2007 Murphy Complex fires
A main motivation of this paper is exploring the importance of VPD to the occurrence of fires in the western United States. Two important fires of the past decade are the Rodeo–Chediski fire in Arizona and the Hayman fire in Colorado, both of which began in June 2002, in the heart of a major multiyear western drought (Seager 2007; Weiss et al. 2009; Cayan et al. 2010). The Rodeo–Chediski fire burned from 18 June to 7 July 2002 and burned 189 095 ha of ponderosa pine and mixed conifers in northern Arizona, worse than any previous recorded Arizona fire (Schoennagel et al. 2004). The Hayman fire was smaller and burned 55 915 ha to the southwest of Denver beginning on 9 June 2002 (Schoennagel et al. 2004) and remains the worst fire in recorded Colorado history. Further, based on dendroecological records Williams et al. (2013) found 2002 to be the most severe year for forest drought stress in the Southwest since at least the year 1000. These facts motivate the presentation here of meteorological conditions and VPD anomalies in the months preceding the June 2002 fires. We also examine conditions leading up to the July 2007 Murphy Complex fire in southern Idaho and northern Nevada. Unlike the other two fires, the Murphy Complex fire was a rangeland fire that burned a sagebrush ecosystem (Launchbaugh et al. 2008). It began after six smaller fires ignited by lightning combined, and it burned 263 862 ha.
Figure 10 shows conditions during the previous winter, JFM 2002, in terms of standardized anomalies. Very high VPD was evident across the Southwest in JFM 2002 with maximum values in Arizona but not widespread in Colorado. Precipitation was below climatological normal across almost all of western North America. The Bowen ratio was high in the interior southwest in Arizona, New Mexico, and Colorado, consistent with a drier-than-normal land surface. Subsidence was also widespread across western North America occurring within northwesterly flow (as for the typical case of high Southwest VPD; Fig. 6). All of these prior winter conditions are conducive to elevating fire risk with both land surface and atmospheric drying being responsible. Figure 11 shows the same conditions for AMJ 2002. By spring high VPD anomalies had spread across the western United States centered on Arizona, New Mexico, Utah, and Colorado, reaching 3 standard deviations in most locations. Precipitation was also below normal by 2 or more standard deviations across the western United States and the Bowen ratio was elevated by 2 or more standard deviations across the Southwest. Unlike in the previous season and the typical AMJ case for high VPD (Fig. 6), a southwesterly flow anomaly was associated with anomalous ascent. The precipitation, land surface conditions, and VPD state remained conducive to elevated fire risk as in the previous season. Consistent with the regression results in Fig. 8, conditions conducive for fire were influenced by both the atmospheric circulation and the land surface state in the seasons before.
Conditions in the winter before the Rodeo–Chediski and Hayman fires of June 2002. Shown for JFM 2002 are the standardized anomalies of (a) VPD, (b) precipitation, (c) Bowen ratio, and (d) 700-mb vertical pressure velocity (colors) and geopotential heights (contours).
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
As in Fig. 10, but for AMJ 2002.
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
Turning to the conditions before the Murphy Complex fire, in the previous late winter to spring (February–April 2007; Fig. 12) there was already a high VPD and a widespread, but not universal, negative precipitation anomalies across the West. The vertical velocity and pressure patterns are not remarkable and it is not clear what caused the high VPD anomaly other than the precipitation reduction. By spring to summer (Fig. 13) the vast area of high VPD had become intense and coincided with a nearly equally expansive area of very negative (about 2 standard deviations) precipitation anomalies. There was also a widespread positive Bowen ratio anomaly indicating drying out of the surface. All these anomalies encompassed the area of the Murphy Complex fire. The circulation anomaly was from the east with strong descending (drying) motion upstream of the fire area.
As in Fig. 10, but for the Murphy Complex fire of July 2007 with February–April 2007 shown.
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
As in Fig. 12, but for May–July 2007.
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
These relations, within the context of two specific historic forest fires, and one very large rangeland fire, support the idea of VPD exerting an influence on fire and also the influence of contemporary and prior atmosphere and land surface conditions on the VPD.
10. Conclusions
To our knowledge, this is the first comprehensive study of vapor pressure deficit, which was recommended by Anderson (1936) as a more useful measure of the moisture state of the atmosphere than relative humidity. Unlike RH, for which the same value can be associated with very different moisture conditions depending on the air temperature, VPD is an absolute measure of the moisture deficit of the atmosphere. Hence, VPD is more closely related to the water stress on vegetation. Prior work (Williams et al. 2015) and results presented here have shown the relationship between VPD variability and burned area in the southwest United States. That relation is the prime motivation for this study since it makes clear that a better understanding of the climatology, variability, and trends of VPD is needed.
VPD follows a notable seasonal cycle with minimum values in the winter and maximum values in the summer. This is controlled by both the seasonal cycles of temperature and humidity. Because of the development of the subtropical anticyclones, which moisten the eastern United States and dry the western United States, actual vapor pressure has a summer maximum in the southeast but remains low in the west. In contrast, saturation vapor pressure in summer maximizes in the interior Southwest, southern and central plains, and the Southeast. Combining these influences, VPD in summer is far greater in the West than in the East. VPD reaches its all-U.S. maximum in summer at the California–Arizona border but more general maxima extend across the southwest United States.
The variance of VPD has a minimum in fall and then strengthens into winter and then to spring and to summer. The Southwest and the southern plains stand out as maxima of variance in spring and summer. The VPD variance quite closely tracks the saturation vapor pressure variance but the Southwest and the southern plains are also regions of relatively strong variance of actual vapor pressure. Hence, it appears that VPD variability can be influenced by both thermodynamic and dynamic processes.
High VPD in the interior southwest United States is associated with La Niña conditions in the tropical Pacific Ocean in fall, winter, and spring. This association works via ocean forcing of circulation anomalies that involve high pressure and northerly, subsiding flow over the Southwest. Such flow warms, increasing saturation vapor pressure, and dries, decreasing actual vapor pressure, and, hence, causes VPD to increase. Summer VPD anomalies in the Southwest are controlled by more local circulation anomalies that influence saturation vapor pressure.
High VPD in spring and summer can also be caused by an increase in Bowen ratio, that is an increase in sensible heat flux relative to latent heat flux, although the causes of this are distinct in the eastern and western United States. In the western United States, low surface moisture, following a drop in precipitation for example, can cause an increase in Bowen ratio and VPD.
Case studies of conditions in advance of the June 2002 Rodeo–Chediski and Hayman fires in Arizona and Colorado, respectively, and the July 2007 Murphy Complex fire in southern Idaho–northern Nevada show very high VPD that was caused by precipitation drops, an increase in Bowen ratio, and anomalous subsidence in the preceding months. This reveals the complexity of meteorological processes that can increase drying of the land surface and vegetation and set the stage for serious fires.
Since 1961, VPD has increased notably across the western United States with the strongest increases in the southwest. These trends have been primarily driven by warming that increases the saturation vapor pressure but have also been contributed to by a decrease in actual vapor pressure. Actual vapor pressure has increased elsewhere in the United States such that VPD has declined in the northern plains and the Midwest.
As an absolute measure of the difference between actual and potential water vapor holding capacity of the atmosphere, VPD is a useful indicator of the ability of the atmosphere to extract moisture from the land surface and, hence, is of relevance in studies of the links between meteorological conditions and wildland fires. Here, we have sought to achieve a basic understanding of the climatology and variability of VPD across the United States and have explained these in terms of atmospheric and land surface conditions. Future work will investigate closely the links between fires and VPD variability and the surface and atmospheric conditions that control them.
Monthly values of VPD, 
Acknowledgments
This work was supported by NSF Award AGS-1243204 (Linking Near-term Future Changes in Weather and Hydroclimate in Western North America to Adaptation for Ecosystem and Water Management). Author AH was supported by an Earth Institute at Columbia University undergraduate research internship. The GLDAS data used in this study were acquired as part of NASA’s Earth–Sun System Division and were archived and distributed by the Goddard Earth Sciences (GES) Data and Information Services Center (DISC) Distributed Active Archive Center (DAAC). We thank three reviewers for their helpful comments and criticisms.
APPENDIX
Evaluation of Error Introduced into Vapor Pressure Calculations by Use of Monthly Mean Data
To check the error involved in calculating 
Comparison of monthly mean (a)–(c) 
Citation: Journal of Applied Meteorology and Climatology 54, 6; 10.1175/JAMC-D-14-0321.1
REFERENCES
Abatzoglou, J. T., and C. A. Kolden, 2013: Relationships between climate and macroscale area burned in the western United States. Int. J. Wildland Fire, 22, 1003–1020, doi:10.1071/WF13019.
Adams, D., and A. Comrie, 1997: The North American monsoon. Bull. Amer. Meteor. Soc., 78, 2197–2213, doi:10.1175/1520-0477(1997)078<2197:TNAM>2.0.CO;2.
Allen, C. D., and Coauthors, 2010: A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. For. Ecol. Manage., 259, 660–684, doi:10.1016/j.foreco.2009.09.001.
Anderson, D. B., 1936: Relative humidity or vapor pressure deficit. Ecology, 17, 277–282, doi:10.2307/1931468.
Bentz, B. J., and Coauthors, 2010: Climate change and bark beetles of the western United States and Canada: Direct and indirect effects. BioScience, 60, 602–613, doi:10.1525/bio.2010.60.8.6.
Cayan, D., T. Das, D. Pierce, T. Barnett, M. Tyree, and A. Gershunova, 2010: Future dryness in the southwest United States and the hydrology of the early 21st century drought. Proc. Natl. Acad. Sci. USA, 107, 21 271–21 276, doi:10.1073/pnas.0912391107.
Daly, C., W. P. Gibson, G. H. Taylor, G. L. Johnson, and P. Pasteris, 2000: High quality spatial climate data sets for the United States and beyond. Trans. Amer. Soc. Agric. Biol. Eng., 43, 1957–1962, doi:10.13031/2013.3101.
Dennison, P. E., S. C. Brewer, J. D. Arnold, and M. A. Moritz, 2014: Large wildfire trends in the western United States, 1984–2011. Geophys. Res. Lett., 41, 2928–2933, doi:10.1002/2014GL059576.
Eidenshink, J., B. Schwind, K. Brewer, Z.-L. Zhu, B. Quayle, and S. Howard, 2007: A project for monitoring trends in burn severity. Fire Ecol., 3, 3–21, doi:10.4996/fireecology.0301003.
Gaffen, D. J., and R. J. Ross, 1999: Climatology and trends of U.S. surface humidity and temperature. J. Climate, 12, 811–828, doi:10.1175/1520-0442(1999)012<0811:CATOUS>2.0.CO;2.
Gisborne, H. T., 1928: Measuring forest-fire danger in northern Idaho. U.S. Dept. of Agriculture Misc. Publ. 29, 63 pp.
Isaac, V., and W. A. van Wijngaarden, 2012: Surface water vapor pressure and temperature trends in North America during 1948–2010. J. Climate, 25, 3599–3609, doi:10.1175/JCLI-D-11-00003.1.
Kalnay, E., and Coauthors, 1996: The NCEP/NCAR 40-Year Reanalysis Project. Bull. Amer. Meteor. Soc., 77, 437–471, doi:10.1175/1520-0477(1996)077<0437:TNYRP>2.0.CO;2.
Kistler, R., and Coauthors, 2001: The NCEP–NCAR 50-Year Reanalysis: Monthly means CD-ROM and documentation. Bull. Amer. Meteor. Soc., 82, 247–268, doi:10.1175/1520-0477(2001)082<0247:TNNYRM>2.3.CO;2.
Kumar, A., and M. P. Hoerling, 1998: Annual cycle of Pacific–North American seasonal predictability associated with different phases of ENSO. J. Climate, 11, 3295–3308, doi:10.1175/1520-0442(1998)011<3295:ACOPNA>2.0.CO;2.
Launchbaugh, K., and Coauthors, 2008: Interactions among livestock grazing, vegetation type, and fire behavior in the Murphy Wildland Fire Complex in Idaho and Nevada, July 2007. U.S. Geological Survey Open-File Rep. 2008-1215, 42 pp. [Available online at http://pubs.usgs.gov/of/2008/1214/.]
Littell, J. S., D. McKenzie, D. L. Peterson, and A. L. Westerling, 2009: Climate and wildfire area burned in western U.S. ecoprovinces, 1916–2003. Ecol. Appl., 19, 1003–1021, doi:10.1890/07-1183.1.
Mitchell, K. E., and Coauthors, 2004: The multi-institution North American Land Data Assimilation System (NLDAS): Utilizing multiple GCIP products and partners in a continental distributed hydrological modeling system. J. Geophys. Res.,109, D07S90, doi:10.1029/2003JD003823.
Munns, E. N., 1921: Evaporation and forest fires. Mon. Wea. Rev., 49, 149–152, doi:10.1175/1520-0493(1921)49<149:EAFF>2.0.CO;2.
Potter, B. E., 2012: Atmospheric interactions with wild land fire behavior—I. Basic surface interactions, vertical profiles and synoptic structures. Int. J. Wildland Fire, 21, 779–801, doi:10.1071/WF11128.
Pyne, S. J., 2009: Fire on the fringe. Environ. Res. Lett.,4, 031004, doi:10.1088/1748-9326/4/3/031004.
Riley, K. L., J. T. Abatzoglou, I. C. Grenfell, A. E. Klene, and F. A. Heinsch, 2013: The relationship of large fire occurrence with drought and fire danger indices in the western USA, 1984–2008: The role of temporal scale. Int. J. Wildland Fire, 22, 894–909, doi:10.1071/WF12149.
Rodell, M., and Coauthors, 2004: The Global Land Data Assimilation System. Bull. Amer. Meteor. Soc., 85, 381–394, doi:10.1175/BAMS-85-3-381.
Roy, D. P., L. Boschetti, C. Justice, and J. Ju, 2008: The Collection 5 MODIS Burned Area Product—Global evaluation by comparison with the MODIS Active Fire Product. Remote Sens. Environ., 112, 3690–3707, doi:10.1016/j.rse.2008.05.013.
Schoennagel, T., T. T. Veblen, and W. H. Rome, 2004: The interaction of fire, fuels and climate across Rocky Mountain forest. BioScience, 54, 661–676, doi:10.1641/0006-3568(2004)054[0661:TIOFFA]2.0.CO;2.
Seager, R., 2007: The turn-of-the-century North American drought: Dynamics, global context, and prior analogues. J. Climate, 20, 5527–5552, doi:10.1175/2007JCLI1529.1.
Seager, R., N. Harnik, Y. Kushnir, W. Robinson, and J. Miller, 2003a: Mechanisms of hemispherically symmetric climate variability. J. Climate, 16, 2960–2978, doi:10.1175/1520-0442(2003)016<2960:MOHSCV>2.0.CO;2.
Seager, R., R. Murtugudde, N. Naik, A. Clement, N. Gordon, and J. Miller, 2003b: Air–sea interaction and the seasonal cycle of the subtropical anticyclones. J. Climate, 16, 1948–1966, doi:10.1175/1520-0442(2003)016<1948:AIATSC>2.0.CO;2.
Seager, R., N. Harnik, W. A. Robinson, Y. Kushnir, M. Ting, H. P. Huang, and J. Velez, 2005: Mechanisms of ENSO-forcing of hemispherically symmetric precipitation variability. Quart. J. Roy. Meteor. Soc., 131, 1501–1527, doi:10.1256/qj.04.96.
Seager, R., L. Goddard, J. Nakamura, N. Naik, and D. Lee, 2014a: Dynamical causes of the 2010/11 Texas–northern Mexico drought. J. Hydrometeor., 15, 39–68, doi:10.1175/JHM-D-13-024.1.
Seager, R., D. Neelin, I. Simpson, H. Liu, N. Henderson, T. Shaw, Y. Kushnir, and M. Ting, 2014b: Dynamical and thermodynamical causes of large-scale changes in the hydrological cycle over North America in response to global warming. J. Climate, 27, 7921–7948, doi:10.1175/JCLI-D-14-00153.1.
Sedano, F., and J. T. Randerson, 2014: Vapor pressure deficit controls on fire ignition and fire spread in boreal forest ecosystems. Biogeosci. Discuss., 11, 1309–1353, doi:10.5194/bgd-11-1309-2014.
Sheffield, J., and G. G. E. Wood, 2006: Development of a 50-yr high-resolution global dataset of meteorological forcings for land surface modeling. J. Climate, 19, 3088–3111, doi:10.1175/JCLI3790.1.
Stavros, E. N., J. Abatzoglou, N. K. Larkin, D. McKenzie, and E. A. Steel, 2014: Climate and very large wildland fires in the contiguous western USA. Int. J. Wildland Fire, 23, 899–914, doi:10.1071/WF13169.
Stephens, S. L., J. K. Agee, P. Z. Fule, M. P. North, W. H. Romme, T. W. Swetnam, and M. G. Turner, 2013: Managing forests and fire in changing climates. Science, 342, 41–42, doi:10.1126/science.1240294.
Weiss, J. L., C. L. Castro, and J. T. Overpeck, 2009: Distinguishing pronounced droughts in the southwestern United States: Seasonality and effects of warmer temperatures. J. Climate, 22, 5918–5932, doi:10.1175/2009JCLI2905.1.
Westerling, A. L., and B. P. Bryant, 2008: Climate change and wildfire in California. Climatic Change, 87 (Suppl.), 231–249, doi:10.1007/s10584-007-9363-z.
Westerling, A. L., A. Gershunov, T. J. Brown, D. R. Cayan, and M. D. Dettinger, 2003: Climate and wildfire in the western United States. Bull. Amer. Meteor. Soc., 84, 595–604, doi:10.1175/BAMS-84-5-595.
Westerling, A. L., H. G. Hidalgo, D. R. Cayan, and T. W. Swetnam, 2006: Warming and earlier spring increase western U.S. forest wildfire activity. Science, 313, 940–943, doi:10.1126/science.1128834.
Williams, A. P., and Coauthors, 2013: Temperature as a potent driver of regional forest drought stress and tree mortality. Nat. Climate Change, 3, 292–297, doi:10.1038/nclimate1693.
Williams, A. P., and Coauthors, 2014: Causes and future implications of extreme 2011 atmospheric moisture demand and wildfire in the southwest United States. J. Appl. Meteor. Climatol., 53, 2671–2684, doi:10.1175/JAMC-D-14-0053.1.
Williams, A. P., and Coauthors, 2015: Correlations between components of the water balance and burned area reveal new insights for predicting forest-fire area in the southwest United States. Int. J. Wildland Fire, 24, 14–26, doi:10.1071/WF14023.
Winkler, J. A., B. E. Potter, D. Wilhelm, R. P. Shadbolt, K. Piromsopa, and X. Bian, 2007: Climatological and statistical characteristics of the Haines index for North America. Int. J. Wildland Fire, 16, 139–152, doi:10.1071/WF06086.
