Evidence of an atmospheric CO2 fertilization effect on radial growth rates was uncovered by examining climate–growth relationships for seven western juniper tree‐ring chronologies in central Oregon using multiple regression models. Consistent upward trends of the residuals from dendroclimatic models indicated a decreased ability for climate parameters to predict growth with time. Additionally, an assessment was made of whether enhanced growth was detectable under drought conditions, because a major benefit of elevated atmospheric CO2 is the reduction of water stress. Mean ring indices were compared between ecologically comparable drought years, when atmospheric CO2 was lower (1896–1949), and more recent drought years that occurred under higher atmospheric CO2 concentrations (1950–96/98). The results presented herein show that: (i) residuals from climate/growth models had a significant positive trend at six of seven sites, suggesting the presence of a nonclimatic factor causing increased growth during recent decades; (ii) overall growth was 23% greater in the latter half of the 20th century; (iii) growth indices during matched drought and matched wet years were 63% and 30% greater, respectively, in the later 20th century than the earlier 20th century; and (iv) harsher sites had greater responses during drought periods between early and late periods. While it is not possible to rule out other factors, these results are consistent with expectations for CO2 fertilization effects.

Summary• The primary objective of this study was to determine if gradually increasing levels of atmospheric CO 2 , as opposed to 'step' increases commonly employed in controlled studies, have a positive impact on radial growth rates of ponderosa pine ( Pinus ponderosa ) in natural environments, and to determine the spatial extent and variability of this growth enhancement.• We developed a series of tree-ring chronologies from minimally disturbed sites across a spectrum of environmental conditions. A series of difference of means tests were used to compare radial growth post-1950, when the impacts of rising atmospheric CO 2 are best expressed, with that pre-1950. Spearman's correlation was used to relate site stress to growth-rate changes.• Significant increases in radial growth rates occurred post-1950, especially during drought years, with the greatest increases generally found at the most water-limited sites. Site harshness is positively related to enhanced radial growth rates.• Atmospheric CO 2 fertilization is probably operative, having a positive effect on radial growth rates of ponderosa pine through increasing water-use efficiency. A CO 2 -driven growth enhancement may affect ponderosa pine growing under both natural and controlled conditions.

This study examines patterns of drought frequency and duration in the contiguous USA based on multiple definitions of drought events. Patterns are examined using data from the three Palmer indices: monthly moisture anomaly index (ZINX), drought severity index (PDSI), and hydrologic drought index (PHDI). The data span a 94-year period and are spatially aggregated to the climatic division level. The stimulus for this study is the lack of systematic examination of the possibility of varying spatial patterns of drought frequency and duration among the drought types represented by these indices.Isoline maps of mean drought frequency indicate that the index used to define a drought has a major impact on the spatial patterns. Patterns identified using the monthly moisture anomaly index are nearly inverse of those for the drought severity and hydrologic drought indices. Patterns of drought duration also vary among the indices, although previous findings of greater drought persistence in the interior regions of the USA were upheld by all three indices. Results also indicate that varying the parameters (intensity, minimum duration) of an index-specific drought event definition has a minor impact only on the spatial patterns.

From the 344 state climate divisions in the conterminous United States, nine distinct regions of warm-season drought variability are identified using principal component analysis. The drought metric used is the Palmer hydrological drought index for the period 1895-2008. The focus of this paper is multi-decadal drought variability in the Southeast (SEUS) and eastern Gulf South (EGS) regions of the United States, areas in which the lowfrequency forcing mechanisms of warm-season drought are still poorly understood. Low-frequency drought variability in the SEUS and EGS is associated with smoothed indexed time series of major ocean-atmosphere circulation features, including two indices of spatiotemporal variability in the North Atlantic subtropical anticyclone (Bermuda high). Long-term warm-season drought conditions are significantly out of phase between the two regions. Multi-decadal regimes of above-and below-average moisture in the SEUS and EGS are closely associated with slow variability in sea surface temperatures in the North Atlantic Ocean and with the summer mean position and mean strength of the Bermuda high. Multivariate linear regression indicates that 82%-92% of the low-frequency variability in warm-season moisture is explained by two of the three leading principal components of low-frequency variability in the climate indices. The findings are important for water resource managers and water-intensive industries in the SEUS and EGS. The associations identified in the paper are valuable for enhanced drought preparedness and forecasting in the study area and potentially for global models of coupled ocean-atmosphere variability.

Precipitation from land-falling tropical cyclones (TCs) has a significant hydroclimatic influence in the southeastern United States, particularly during drought years. The frequency with which TCs ended drought conditions was examined for southeastern coastal states from Texas to North Carolina during 1895-2011. The region was divided into the Gulf Coast states (GCS) and the southeastern Atlantic coast states (ACS). The spatiotemporal patterns of tropical cyclone drought busters (TCDBs) were analyzed. Larger-scale ocean-atmosphere influences on TCDBs were examined using chi-squared analysis. The ACS experienced TCDBs more frequently and farther inland compared to the GCS. The number of TCDBs has significantly increased with time in the ACS. TCDBs numbers in the GCS did not exhibit significant increases, but the area alleviated of drought conditions increased significantly in the last 117 years. The dominant larger-scale oceanatmosphere forcing of TCDBs was a combination of a warm Atlantic Ocean ABSTRACTPrecipitation from land-falling tropical cyclones (TCs) has a significant hydroclimatic influence in the southeastern United States, particularly during drought years. The frequency with which TCs ended drought conditions was examined for southeastern coastal states from Texas to North Carolina during 1895-2011. The region was divided into the Gulf Coast states (GCS) and the southeastern Atlantic coast states (ACS). The spatiotemporal patterns of tropical cyclone drought busters (TCDBs) were analyzed. Larger-scale oceanatmosphere influences on TCDBs were examined using chi-squared analysis. The ACS experienced TCDBs more frequently and farther inland compared to the GCS. The number of TCDBs has significantly increased with time in the ACS. TCDBs numbers in the GCS did not exhibit significant increases, but the area alleviated of drought conditions increased significantly in the last 117 years. The dominant larger-scale ocean-atmosphere forcing of TCDBs was a combination of a warm Atlantic Ocean [positive Atlantic multidecadal oscillation index (AMO1)] and weak westerlies [negative North Atlantic Oscillation index (NAO2)]. AMO1 leads to an increase in the number of TCs throughout the North Atlantic basin, and NAO2 increases the likelihood of TC landfall by controlling the steering of TCs toward the southeastern United States.