Abstract
Summer circulation patterns in the Southeast United States are controlled by the position of the North Atlantic subtropical high, which can direct moisture and tropical cyclones across the region. In a warming climate, the subtropical high is projected to strengthen and its mean position to expand westward, but there is much uncertainty regarding interannual variability and link-ages to natural drivers. Here, we use a multispecies tree-ring network across the Southeast to reconstruct relative intensity of the pressure gradient across the subtropical high's western flank over the past 870 years. Variations in the anticyclone's position and the strength of the pressure gradient across the flank have been a major driver of the hydroclimate-including forming a moisture dipole between the Southeast and Caribbean/Central America-since 1140 CE. We document a significant increase in flank position variability since 1900 CE and significant trends in flank position, indicating more extreme westward migrations over time. Flank variability is, in part, externally forced by major volcanic eruptions, which cause a multiyear period of westward positioning that may develop distinct moisture gradients in the region. Our record can help uncover regional hydroclimatic patterns over different timescales while also informing water resource management strategies