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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.

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The synoptic processes that end droughts are poorly understood, yet have significant climatological implications. Here we examined the spatiotemporal patterns of rapid drought cessation (RDC) in the southeastern United States during the1979-2013 warm season (April-November) for three storm types: Frontal, Tropical, and Air mass. We defined RDC as a 1 month shift in soil moisture sufficient to alleviate an existing drought. We found that 73% of all warm-season droughts were ended by RDC events and the three storm-type groups ended droughts over similar spatial areas. Frontal storms were the most frequent mechanism for RDC events, yet their occurrences significantly decreased and were negatively related to increases in Northern Hemisphere air temperatures. Projected future warming in the Northern Hemisphere suggests a continued decline in the frequency and relative contribution of Frontal storms as RDC events, potentially influencing the timing and spatial scale of drought cessation in the southeastern U.S.

Droughts and landfalling tropical cyclones (TCs; tropical depressions, tropical storms, and hurricanes) are important features of the hydroclimate of the southeastern USA at seasonal, interannual, and interdecadal scales. The societal impacts and climatological aspects of both droughts and Atlantic TCs have been widely addressed in the scientific literature. However, in general, previous research has assessed the two phenomena separately. Recently, the spatiotemporal patterns and hydroclimatic impacts of drought amelioration by landfalling TCs have been analyzed for the southeastern USA, as well as the large‐scale dynamic forcing mechanisms that enhance or suppress drought‐TC co‐occurrence. At multidecadal time scales, both droughts and TCs in this region vary in association with several leading modes of basin‐wide and regional climate variability. These climate modes appear to be coherently linked to a background oceanic–atmospheric pattern that either promotes or suppresses the likelihood of both droughts and TC landfalls. The relative frequency of TC landfalls in drought‐stricken areas, the importance of these events in the regional moisture budget, and the potential for future changes in the large‐scale forcing environment raise fundamental questions about possible changes in the hydroclimate of the southeastern USA, where population growth and rising water demand already place strain on freshwater resources. In this article, we provide a review and synthesis of the recent research on variability in drought, landfalling TCs, the characteristics of the space–time association between these two phenomena in the southeastern USA, and the coherent large‐scale oceanic–atmospheric environment that either promotes or suppresses their co‐occurrence. Further, we review Global Climate Model projections related to these factors, and we identify avenues for future research on this important topic.

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Tropical cyclones (TCs) are an important source of precipitation for much of the eastern United States. However, our understanding of the spatiotemporal variability of tropical cyclone precipitation (TCP) and the connections to large-scale atmospheric circulation is limited by irregularly distributed rain gauges and short records of satellite measurements. To address this, we developed a new gridded (0.25° × 0.25°) publicly available dataset of TCP (1948–2015; Tropical Cyclone Precipitation Dataset, or TCPDat) using TC tracks to identify TCP within an existing gridded precipitation dataset. TCPDat was used to characterize total June–November TCP and percentage contribution to total June–November precipitation. TCP totals and contributions had maxima on the Louisiana, North Carolina, and Texas coasts, substantially decreasing farther inland at rates of approximately 6.2–6.7 mm km−1. Few statistically significant trends were discovered in either TCP totals or percentage contribution. TCP is positively related to an index of the position and strength of the western flank of the North Atlantic subtropical high (NASH), with the strongest correlations concentrated in the southeastern United States. Weaker inverse correlations between TCP and El Niño–Southern Oscillation are seen throughout the study site. Ultimately, spatial variations of TCP are more closely linked to variations in the NASH flank position or strength than to the ENSO index. The TCP dataset developed in this study is an important step in understanding hurricane–climate interactions and the impacts of TCs on communities, water resources, and ecosystems in the eastern United States.