Changes in the western flank of the North Atlantic subtropical high since 1140 CE: Extremes, drivers, and hydroclimatic patterns.

Summer circulation and moisture patterns in the Southeast United States are controlled by the position of the North Atlantic subtropical high. In a warming climate, the subtropical high is projected to strengthen and expand west, but there remains uncertainty regarding its variability and linkages to natural drivers. Here, we use a tree-ring network across the Southeast United States to reconstruct the relative intensity of the pressure gradient across the subtropical high's western flank over the past 870 years. Variations in the flank's position and the pressure gradient have been a major driver of the hydroclimate-including creating a Southeast-Caribbean moisture dipole-since 1140 CE. We document a significant increase in flank positional variability since 1900 CE, with westward migrations becoming more extreme. Likewise, major volcanic eruptions cause a multiyear period of westward positioning, leading to distinct regional moisture gradients. Our record highlights important changes in flank behavior, which has important implications for water resource management in a warming world.