Mexico has suffered a long history and prehistory of severe sustained drought.Drought over Mexico is modulated by ocean-atmospheric variability in the Atlantic and Pacific, raising the possibility for long-range seasonal climate forecasting, which could help mediate the economic and social impacts of future dry spells. The instrumental record of Mexican climate is very limited before 1920, but tree-ring chronologies developed from old-growth forests in Mexico can provide an excellent proxy representation of the spatial pattern and intensity of past moisture regimes useful for the analysis of climate dynamics and climate impacts. The Mexican Drought Atlas (MXDA) has been developed from an extensive network of 252 climate sensitive treering chronologies in and near Mexico.The MXDA reconstructionsextend from AD 1400 to 2012 and were calibrated with the instrumental summer (JJA) self-calibrating Palmer Drought Severity Index (scPDSI) on a 0.5° latitude/longitude grid extending over land areas from 14-34°N and 75-120°W using Ensemble Point-by-Point Regression (EPPR) for the 1944-1984 period. The grid point reconstructions were validated for the period 1920-1943 against instrumental gridded scPDSI values based on the fewer weather station observations available during that interval. The MXDA provides a new spatial perspective on the historical impacts of moisture extremes over Mexico during the past 600-years, including the Aztec Drought of One Rabbit in 1454, the drought of El Año de Hambre in 1785-1786, and the drought that preceded the Mexican Revolution of 1909-1910. The El Niño/Southern Oscillation (ENSO) is the most important oceanatmospheric forcing of moisture variability detected with the MXDA. In fact, the reconstructions suggest that the strongest central equatorial Pacific sea surface temperature (SST)teleconnection to the soil moisture balance over North America may reside in northern Mexico. This ENSO signal has stronger and more time-stable correlations than computed for either the Atlantic Multidecadal Oscillation or Pacific Decadal Oscillation. The extended Multivariate ENSO Index is most highly correlated with reconstructed scPDSI over northern Mexico, where warm events favor moist conditions during the winter, spring, and early summer. This ENSO teleconnection to northern Mexico has been strong over the past 150 years, but it has been comparatively weak and non-stationary in the MXDA over central and southern Mexico where eastern tropical Pacific and Caribbean/tropical Atlantic SSTs seem to be more important. The ENSO teleconnection to northern Mexico is weaker in the available instrumental PDSI, but analyses based on the millennium climate simulations with the Community Earth System Model suggest that the moisture balance during the winter, spring, and early summer over northern Mexico may indeed be particularly sensitive to ENSO forcing. Nationwide drought is predicted to become more common with anthropogenic climate change, but the MXDA reconstructions indicate that intense"All Mexico" droughts ha...

Ancient Montezuma baldcypress (Taxodium mucronatum) trees found in Barranca de Amealco, Queretaro, have been used to develop a 1,238‐year tree‐ring chronology that is correlated with precipitation, temperature, drought indices, and crop yields in central Mexico. This chronology has been used to reconstruct the spring‐early summer soil moisture balance over the heartland of the Mesoamerican cultural province, and is the first exactly dated, annually resolved paleoclimatic record for Mesoamerica spanning the Late Classic, Post Classic, Colonial, and modern eras. The reconstruction indicates that the Terminal Classic drought extended into central Mexico, supporting other sedimentary and speleothem evidence for this early 10th century drought in Mesoamerica. The reconstruction also documents severe and sustained drought during the decline of the Toltec state (1149–1167) and during the Spanish conquest of the Aztec state (1514–1539), providing a new precisely dated climate framework for Mesoamerican cultural change.

Precipitation over the southwestern United States exhibits distinctive seasonality, and contrasting oceanatmospheric dynamics are involved in the interannual variability of cool-and warm-season totals. Tree-ring chronologies based on annual-ring widths of conifers in the southwestern United States are well correlated with accumulated precipitation and have previously been used to reconstruct cool-season and annual precipitation totals. However, annual-ring-width chronologies cannot typically be used to derive a specific record of summer monsoon-season precipitation. Some southwestern conifers exhibit a clear anatomical transition from the earlywood and latewood components of the annual ring, and these exactly dated subannual ring components can be measured separately and used as unique proxies of cool-and warm-season precipitation and their associated large-scale ocean-atmospheric dynamics. Two 2139-yr-long reconstructions of cool-(November-May) and early-warm season (July) precipitation have been developed from ancient conifers and relict wood at El Malpais National Monument, New Mexico. Both reconstructions have been verified on independent precipitation data and reproduce the spatial correlation patterns detected in the large-scale SST and 500-mb height fields using instrumental precipitation data from New Mexico. Aboveaverage precipitation in the cool-season reconstruction is related to El Niñ o conditions and to the positive phase of the Pacific decadal oscillation. Above-average precipitation in July is related to the onset of the North American monsoon over New Mexico and with anomalies in the 500-mb height field favoring moisture advection into the Southwest from the North Pacific, the Gulf of California, and the Gulf of Mexico. Cooland warm-season precipitation totals are not correlated on an interannual basis in the 74-yr instrumental or 2139-yr reconstructed records, but wet winter-spring extremes tend to be followed by dry conditions in July and very dry winters tend to be followed by wet Julys in the reconstructions. This antiphasing of extremes could arise from the hypothesized cool-to early-warm-season change in the sign of large-scale oceanatmospheric forcing of southwestern precipitation, from the negative land surface feedback hypothesis in which winter-spring precipitation and snow cover reduce surface warming and delay the onset of the monsoon, or perhaps from an interaction of both large-scale and regional forcing. Episodes of simultaneous interseasonal drought (''perfect'' interseasonal drought) persisted for a decade or more during the 1950s drought of the instrumental era and during the eighth-and sixteenth-century droughts, which appear to have been two of the most profound droughts over the Southwest in the past 1400 yr. Simultaneous interseasonal drought is doubly detrimental to dry-land crop yields and is estimated to have occurred during the midseventeenth-century famines of colonial New Mexico but was less frequent during the late-thirteenth-century Great Drought among the Anasazi, w...

Cool- and warm-season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December–April totals and 547 different chronologies correlated with May–July totals. These discrete seasonal chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both the strong continent-wide El Niño–Southern Oscillation (ENSO) signal embedded in the cool-season reconstructions and the Arctic Oscillation signal registered by the warm-season estimates faithfully reproduce the sign, intensity, and spatial patterns of these ocean–atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide insight into decadal droughts and pluvials. They indicate that the sixteenth-century megadrought, the most severe and sustained North American drought of the past 500 years, was the combined result of three distinct seasonal droughts, each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic multidecadal oscillation. Significant 200–500-yr-long trends toward increased precipitation have been detected in the cool- and warm-season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

Prolonged drought conditions have persisted over western North America since at least 1999, affecting snowpack, stream discharge, reservoir levels, and wildfire activity [Mote et al., 2005; Westerling et al., 2006; MacDonald et al., 2008]. Instrumental precipitation, temperature, and Palmer Drought Severity Indices (PDSI) indicate that severe and sustained drought began in 1994 in Mexico, where it has continued with only limited relief for the past 15 years. This late twentieth‐and early 21st‐century Mexican drought (referred to below as the [early 21st‐century drought]) has equaled some aspects of the 1950s drought, which is the most severe drought evident in the instrumental climate record for Mexico (1900–2008). Large‐scale changes in ocean‐atmospheric circulation have contributed to the lower than normal precipitation that has led to the current drought [Seager, 2007], but global warming and the sharp regional warming across Mexico, which appears to have been aggravated by land cover changes [Englehart and Douglas, 2005], may have added an anthropogenic component to the early 21st‐century drought.

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Ancient blue oak trees are still widespread across the foothills of the Coast Ranges, Cascades, and Sierra Nevada in California. The most extensive tracts of intact old-growth blue oak woodland appear to survive on rugged and remote terrain in the southern Coast Ranges and on the foothills west and southwest of Mt. Lassen. In the authors' sampling of old-growth stands, most blue oak appear to have recruited to the canopy in the middle to late nineteenth century. The oldest living blue oak tree sampled was over 459 years old, and several dead blue oak logs had over 500 annual rings. Precipitation sensitive tree-ring chronologies up to 700 years long have been developed from old blue oak trees and logs. Annual ring-width chronologies of blue oak are strongly correlated with cool season precipitation totals, streamflow in the major rivers of California, and the estuarine water quality of San Francisco Bay. A new network of 36 blue oak chronologies records spatial anomalies in growth that arise from latitudinal changes in the mean storm track and location of landfalling atmospheric rivers. These long, climate-sensitive blue oak chronologies have been used to reconstruct hydroclimatic history in California and will help to better understand and manage water resources. The environmental history embedded in blue oak growth chronologies may help justify efforts to conserve these authentic old-growth native woodlands.

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