The development of a 2Њ lat ϫ 3Њ long grid of summer drought reconstructions for the continental United States estimated from a dense network of annual tree-ring chronologies is described. The drought metric used is the Palmer Drought Severity Index (PDSI). The number of grid points is 154 and the reconstructions cover the common period 1700-1978. In producing this grid, an automated gridpoint regression method called ''pointby-point regression'' was developed and tested. In so doing, a near-optimal global solution was found for its implementation. The reconstructions have been thoroughly tested for validity using PDSI data not used in regression modeling. In general, most of the gridpoint estimates of drought pass the verification tests used. In addition, the spatial features of drought in the United States have been faithfully recorded in the reconstructions even though the method of reconstruction is not explicitly spatial in its design. The drought reconstructions show that the 1930s ''Dust Bowl'' drought was the most severe such event to strike the United States since 1700. Other more local droughts are also revealed in the regional patterns of drought obtained by rotated principal component analysis. These reconstructions are located on a NOAA Web site at the World Data Center-A in Boulder, Colorado, and can be freely downloaded from there.

Exactly dated tree-ring chronologies from ENSO-sensitive regions in subtropical North America and Indonesia together register the strongest ENSO signal yet detected in tree-ring data worldwide and have been used to reconstruct the winter Southern Oscillation index (SOI) from 1706 to 1977. This reconstruction explains 53% of the variance in the instrumental winter SOI during the boreal cool season (December-February) and was verified in the time, space, and frequency domains by comparisons with independent instrumental SOI and sea surface temperature (SST) data. The large-scale SST anomaly patterns associated with ENSO in the equatorial and North Pacific during the 1879-1977 calibration period are reproduced in detail by this reconstruction. Cross-spectral analyses indicate that the reconstruction reproduces over 70% of the instrumental winter SOI variance at periods between 3.5 and 5.6 yr, and over 88% in the 4-yr frequency band. Oscillatory modes of variance identified with singular spectrum analysis at ~3.5, 4.0, and 5.8 yr in both the instrumental and reconstructed series exhibit regimelike behavior over the 272-yr reconstruction. The tree-ring estimates also suggest a statistically significant increase in the interannual variability of winter SOI, more frequent cold events, and a slightly stronger sea level pressure gradient across the equatorial Pacific from the mid-nineteenth to twentieth centuries. Some of the variability in this reconstruction must be associated with background climate influences affecting the ENSO teleconnection to subtropical North America and may not arise solely from equatorial ENSO forcing. However, there is some limited independent support for the nineteenth to twentieth century changes in tropical Pacific climate identified in this reconstruction and, if substantiated, it will have important implications to the low-frequency dynamics of ENSO.

The two most severe, sustained droughts in the continental United States during the 20th century occurred in the 1930s and 1950s. The 1950s drought was most extreme over the southwest and southern Great Plains, where ecological consequences are still evident on the landscape [Swetnam and Betancourt], 1998].The Dust Bowl,vividly recounted in John Steinbeck's The Grapes of Wrath, was the nations most severe, sustained,and widespread drought of the past 300 years, according to tree‐ring reconstructions of the Palmer drought severity index (PDSI) across the continental United States [Cook et al., 1999] (http://www.ngdc.noaa.gov/paleo/pdsiyear.html. Droughts during the 1750s, 1820s, and 1850s–1860s estimated from tree rings were similar to the 1950s drought in terms of magnitude, persistence, and spatial coverage, but these earlier episodes do not appear to have surpassed the severity or extent of the Dust Bowl drought. However, longer tree‐ring reconstructions of PDSI for the United States and precipitation for northwestern Mexico and western Canada indicate that the “megadrought” of the 16th century far exceeded any drought of the 20th century (Figure 1) [also see Wood‐house and Overpeck, 1998], and is considered to be the most severe prolonged drought over much of North America for at least the last 500 years [Meko et al., 1995].

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.

Tree-ring chronologies can provide surprisingly accurate estimates of the natural variability of important climate parameters such as precipitation and temperature during the centuries prior to the Industrial Revolution. Bald cypress tree-ring chronologies have been used to reconstruct spring rainfall for the past 1000 years in North Carolina, South Carolina, and Georgia. These rainfall reconstructions explain from 54% to 68% of the spring rainfall variance in each state, and are well verified against independent rainfall measurements. In fact, these tree-ring data explain only 6% to 13% less statewide rainfall variance than is explained by the same number of instrumental raingage records. The reconstructions indicate thatthe spring rainfall extremes and decade-long regimes witnessed during the past century of instrumental observation have been a prominent feature of southeastern United States climate over the past millennium. These spring rainfall regimes are linked in part to anomalies in the seasonal expansion and migration of the subtropical anticyclone over the North Atlantic. The western sector of the Bermuda high often ridges strongly westward into the southeastern United States during dry springs, but during wet springs it is usually located east of its mean position and well offshore. Similar anomalies in the western sector of the Bermuda high occurred during multidecadal regimes of spring rainfall over the Southeast. During the relatively dry springs from 1901 to 1939, the high often ridged into the Southeast, but the western periphery of the high was more frequently located offshore during the relatively wet period from 1940 to 1980. Spring and summer rainfall extremes and decade-long regimes over the Southeast are frequently out of phase, and the tendency for wet (dry) springs to be followed by dry (wet) summers also appears to reflect anomalies in the zonal position of the Bermuda high during spring and summer.