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Jessica L. Conroy and Jonathan T. Overpeck
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Connie A. Woodhouse and Jonathan T. Overpeck

Droughts are one of the most devastating natural hazards faced by the United States today. Severe droughts of the twentieth century have had large impacts on economies, society, and the environment, especially in the Great Plains. However, the instrumental record of the last 100 years contains only a limited subset of drought realizations. One must turn to the paleoclimatic record to examine the full range of past drought variability, including the range of magnitude and duration, and thus gain the improved understanding needed for society to anticipate and plan for droughts of the future. Historical documents, tree rings, archaeological remains, lake sediment, and geomorphic data make it clear that the droughts of the twentieth century, including those of the 1930s and 1950s, were eclipsed several times by droughts earlier in the last 2000 years, and as recently as the late sixteenth century. In general, some droughts prior to 1600 appear to be characterized by longer duration (i.e., multidecadal) and greater spatial extent than those of the twentieth century. The authors' assessment of the full range of past natural drought variability, deduced from a comprehensive review of the paleoclimatic literature, suggests that droughts more severe than those of the 1930s and 1950s are likely to occur in the future, a likelihood that might be exacerbated by greenhouse warming in the next century. Persistence conditions that lead to decadal-scale drought may be related to low-frequency variations, or base-state shifts, in both the Pacific and Atlantic Oceans, although more research is needed to understand the mechanisms of severe drought.

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Jessica L. Conroy and Jonathan T. Overpeck

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The spatial domain of the Asian monsoon has been defined by the intensity, seasonal concentration, and annual range of precipitation. Monsoon subdomains, such as the Indian monsoon, East Asian monsoon, and western North Pacific monsoon, have also been identified based on seasonal wind reversals as well as the timing and source of monsoon moisture. However, precipitation across the Asian monsoon region is heterogeneous and spatially complex and may have influences farther north than commonly assumed, particularly if scientists consider records of past variability spanning the current interglacial period. This paper presents an additional means of identifying the Asian monsoon domain and monsoon subsystems using an empirical orthogonal function (EOF)-based regionalization of gridded precipitation values. Regions of unique precipitation variability for the Asian monsoon region are determined using monthly precipitation anomalies from the Climate Prediction Center Merged Analysis of Precipitation (CMAP) gridded precipitation dataset from 1979 to 2009. From these regions, an area of Asian monsoon influence extending from the Arabian Sea eastward to the western North Pacific Ocean is defined, similar to other studies. One key difference is that this region of monsoon influence penetrates farther north into the Tibetan Plateau and northern China. Thus, paleoclimate observations of wetter conditions in these northern arid regions may suggest an intensification of monsoon moisture, rather than a northward shift in the boundary of the monsoon. In contrast, the Arabian Peninsula, largely removed from monsoon precipitation today, likely saw a shift of monsoon influence inland earlier in the Holocene. Also identified are different subdomains of distinct precipitation variability in southeastern Asia, the western North Pacific, and the East Asian monsoon region of northeastern China that agree with previous studies. Not identified in the paper is a single Indian summer monsoon region. Instead, the Arabian Sea was found to have unique precipitation variability relative to the Indian subcontinent. Summers with enhanced precipitation over the Arabian Sea coincide with decreased summer precipitation in the western North Pacific. This relationship is likely a result of the El Niño–Southern Oscillation (ENSO)-induced development of the Philippine Sea anticyclone. Local and remote sea surface temperatures were generally found to covary with regional precipitation, but not all regions respond similarly to the remote climate variability associated with ENSO. There is some evidence that the EOF-defined regions were stable … through the Holocene, although additional regionalization analyses of paleorecords and model simulations of past precipitation variability are needed to reconstruct past regions of coherent precipitation variability.

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Jeremy L. Weiss, Christopher L. Castro, and Jonathan T. Overpeck

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Higher temperatures increase the moisture-holding capacity of the atmosphere and can lead to greater atmospheric demand for evapotranspiration, especially during warmer seasons of the year. Increases in precipitation or atmospheric humidity ameliorate this enhanced demand, whereas decreases exacerbate it. In the southwestern United States (Southwest), this means the greatest changes in evapotranspirational demand resulting from higher temperatures could occur during the hot–dry foresummer and hot–wet monsoon. Here seasonal differences in surface climate observations are examined to determine how temperature and moisture conditions affected evapotranspirational demand during the pronounced Southwest droughts of the 1950s and 2000s, the latter likely influenced by warmer temperatures now attributed mostly to the buildup of greenhouse gases. In the hot–dry foresummer during the 2000s drought, much of the Southwest experienced significantly warmer temperatures that largely drove greater evapotranspirational demand. Lower atmospheric humidity at this time of year over parts of the region also allowed evapotranspirational demand to increase. Significantly warmer temperatures in the hot–wet monsoon during the more recent drought also primarily drove greater evapotranspirational demand, but only for parts of the region outside of the core North American monsoon area. Had atmospheric humidity during the more recent drought been as low as during the 1950s drought in the core North American monsoon area at this time of year, greater evapotranspirational demand during the 2000s drought could have been more spatially extensive. With projections of future climate indicating continued warming in the region, evapotranspirational demand during the hot–dry and hot–wet seasons possibly will be more severe in future droughts and result in more extreme conditions in the Southwest, a disproportionate amount negatively impacting society.

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Luke A. Parsons, Sloan Coats, and Jonathan T. Overpeck

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Drought has severe consequences for humans and their environment, yet we have a limited understanding of the drivers of drought across the full range of time scales on which it occurs. Here, the atmosphere and ocean conditions that drive this continuum of drought variability in southwestern North America (SWNA) are studied using the latest observationally based products, paleoclimate reconstructions, and state-of-the-art Earth system model simulations of the last millennium. A novel application of the self-organizing maps (SOM) methodology allows for a visualization of the continuum of climate states coinciding with thousands of droughts of varying lengths in last millennium simulations from the Community Earth System Model (CESM), the Goddard Institute for Space Studies Model E2-R (GISS E2-R), and eight other members from phase 5 of the Coupled Model Intercomparison Project (CMIP5). It is found that most droughts are associated with a cool Pacific decadal oscillation (PDO) pattern, but persistent droughts can coincide with a variety of ocean–atmosphere states, including time periods showing a warm PDO or weak ocean–atmosphere anomalies. Many CMIP5 models simulate similar SWNA teleconnection patterns, but the SOM analysis demonstrates that models simulate different continuums of ocean–atmosphere states coinciding with droughts of different lengths, suggesting fundamental differences in their drought dynamics. These findings have important implications for our understanding and simulation of the drivers of persistent drought, and for their potential predictability.

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Toby R. Ault, Julia E. Cole, Jonathan T. Overpeck, Gregory T. Pederson, and David M. Meko

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Projected changes in global rainfall patterns will likely alter water supplies and ecosystems in semiarid regions during the coming century. Instrumental and paleoclimate data indicate that natural hydroclimate fluctuations tend to be more energetic at low (multidecadal to multicentury) than at high (interannual) frequencies. State-of-the-art global climate models do not capture this characteristic of hydroclimate variability, suggesting that the models underestimate the risk of future persistent droughts. Methods are developed here for assessing the risk of such events in the coming century using climate model projections as well as observational (paleoclimate) information. Where instrumental and paleoclimate data are reliable, these methods may provide a more complete view of prolonged drought risk. In the U.S. Southwest, for instance, state-of-the-art climate model projections suggest the risk of a decade-scale megadrought in the coming century is less than 50%; the analysis herein suggests that the risk is at least 80%, and may be higher than 90% in certain areas. The likelihood of longer-lived events (>35 yr) is between 20% and 50%, and the risk of an unprecedented 50-yr megadrought is nonnegligible under the most severe warming scenario (5%–10%). These findings are important to consider as adaptation and mitigation strategies are developed to cope with regional impacts of climate change, where population growth is high and multidecadal megadrought—worse than anything seen during the last 2000 years—would pose unprecedented challenges to water resources in the region.

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Luke A. Parsons, Garrison R. Loope, Jonathan T. Overpeck, Toby R. Ault, Ronald Stouffer, and Julia E. Cole

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Accurate assessments of future climate impacts require realistic simulation of interannual–century-scale temperature and precipitation variability. Here, well-constrained paleoclimate data and the latest generation of Earth system model data are used to evaluate the magnitude and spatial consistency of climate variance distributions across interannual to centennial frequencies. It is found that temperature variance generally increases with time scale in patterns that are spatially consistent among models, especially over the mid- and high-latitude oceans. However, precipitation is similar to white noise across much of the globe. When Earth system model variance is compared to variance generated by simple autocorrelation, it is found that tropical temperature variability in Earth system models is difficult to distinguish from variability generated by simple autocorrelation. By contrast, both forced and unforced Earth system models produce variability distinct from a simple autoregressive process over most high-latitude oceans. This new analysis of tropical paleoclimate records suggests that low-frequency variance dominates the temperature spectrum across the tropical Pacific and Indian Oceans, but in many Earth system models, interannual variance dominates the simulated central and eastern tropical Pacific temperature spectrum, regardless of forcing. Tropical Pacific model spectra are compared to spectra from the instrumental record, but the short instrumental record likely cannot provide accurate multidecadal–centennial-scale variance estimates. In the coming decades, both forced and natural patterns of decade–century-scale variability will determine climate-related risks. Underestimating low-frequency temperature and precipitation variability may significantly alter our understanding of the projections of these climate impacts.

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Michael E. Mann, Ed Gille, Jonathan Overpeck, Wendy Gross, Raymond S. Bradley, Frank T. Keimig, and Malcolm K. Hughes

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The recent availability of global networks of annual or seasonal resolution proxy data, combined with the few long instrumental and historical climate records available during the past few centuries, make it possible now to reconstruct annual and seasonal spatial patterns of temperature variation, as well as hemispheric, global-mean, and regional temperature trends, several centuries back in time.

Reconstructions of large-scale global or hemispheric trends during centuries past can place the instrumental assessments of climate during the twentieth century in a longer-term perspective and provide more robust evidence regarding the roles of potential climate forcings over time. The reconstructed spatial patterns lead to important inferences regarding ENSO-scale variability, the spatial influences of climatic forcings, and the regional patterns that underlie large-scale climate variations. Here proxy-based annual global temperature pattern reconstructions described recently by Mann et al. are expanded upon. For the first time seasonally resolved versions of the proxy-reconstructed surface temperature patterns are presented, and the seasonal differences between key climate indices and patterns of variations are diagnosed. The reader is enabled to interactively examine spatial as well as temporal details (and their uncertainties) of yearly temperatures back in time for both annual-mean and seasonal windows. Annual and seasonal time histories of reconstructed Northern Hemisphere, Southern Hemisphere, and global-mean temperature are made available, as are time histories of the Niño-3 index describing El Niño–related variations, time histories for particular regions of interest such as North America and Europe, and time series for temperature variations in different (e.g., tropical and extratropical) latitude bands. Time histories for specific grid points are available along with their estimated uncertainties. Time histories for the different eigenvectors [i.e., the reconstructed principal components (RPCs)] are also available, along with the raw instrumental series, which underlie the temperature pattern reconstructions. For both the annual-mean and seasonally resolved temperature reconstructions, the reader can directly compare reconstructed patterns for different years, as well as the raw and reconstructed patterns during calibration and verification intervals, and view animated year-by-year sequences of reconstructed global temperature patterns. The statistical relationships between climate forcings and temperature variations are also analyzed in more detail, taking into account potential lagged responses to climate forcings in empirical attribution analyses.

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Toby R. Ault, Julia E. Cole, Jonathan T. Overpeck, Gregory T. Pederson, Scott St. George, Bette Otto-Bliesner, Connie A. Woodhouse, and Clara Deser

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The distribution of climatic variance across the frequency spectrum has substantial importance for anticipating how climate will evolve in the future. Here power spectra and power laws (β) are estimated from instrumental, proxy, and climate model data to characterize the hydroclimate continuum in western North America (WNA). The significance of the estimates of spectral densities and β are tested against the null hypothesis that they reflect solely the effects of local (nonclimate) sources of autocorrelation at the monthly time scale. Although tree-ring-based hydroclimate reconstructions are generally consistent with this null hypothesis, values of β calculated from long moisture-sensitive chronologies (as opposed to reconstructions) and other types of hydroclimate proxies exceed null expectations. Therefore it may be argued that there is more low-frequency variability in hydroclimate than monthly autocorrelation alone can generate. Coupled model results archived as part of phase 5 of the Coupled Model Intercomparison Project (CMIP5) are consistent with the null hypothesis and appear unable to generate variance in hydroclimate commensurate with paleoclimate records. Consequently, at decadal-to-multidecadal time scales there is more variability in instrumental and proxy data than in the models, suggesting that the risk of prolonged droughts under climate change may be underestimated by CMIP5 simulations of the future.

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Samantha Stevenson, Jonathan T. Overpeck, John Fasullo, Sloan Coats, Luke Parsons, Bette Otto-Bliesner, Toby Ault, Garrison Loope, and Julia Cole

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Multidecadal hydroclimate variability has been expressed as “megadroughts” (dry periods more severe and prolonged than observed over the twentieth century) and corresponding “megapluvial” wet periods in many regions around the world. The risk of such events is strongly affected by modes of coupled atmosphere–ocean variability and by external impacts on climate. Accurately assessing the mechanisms for these interactions is difficult, since it requires large ensembles of millennial simulations as well as long proxy time series. Here, the Community Earth System Model (CESM) Last Millennium Ensemble is used to examine statistical associations among megaevents, coupled climate modes, and forcing from major volcanic eruptions. El Niño–Southern Oscillation (ENSO) strongly affects hydroclimate extremes: larger ENSO amplitude reduces megadrought risk and persistence in the southwestern United States, the Sahel, monsoon Asia, and Australia, with corresponding increases in Mexico and the Amazon. The Atlantic multidecadal oscillation (AMO) also alters megadrought risk, primarily in the Caribbean and the Amazon. Volcanic influences are felt primarily through enhancing AMO amplitude, as well as alterations in the structure of both ENSO and AMO teleconnections, which lead to differing manifestations of megadrought. These results indicate that characterizing hydroclimate variability requires an improved understanding of both volcanic climate impacts and variations in ENSO/AMO teleconnections.

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