Deglacial reconstruction of the spatial extent and intensity of the North Atlantic Subtropical High

David Fastovich, Tripti Bhattacharya , Stephen T. Jackson, Teresa R. Krause, James M. Russell, Timothy M. Shanahan, Chijun Sun, John Warren Williams 

Rainfall-related hazards are growing in southeastern North America (SENA) but well-documented climate-model biases cast doubt on projected rainfall changes. The North Atlantic Subtropical High (NASH) supplies the moisture that drives extreme rainfall in SENA, so reconstructing its past behavior offers a test of model fidelity. Using the hydrogen-isotope composition of leaf-wax biomarkers (δDwax) at two sites positioned across an isotopic gradient (White Pond, SC and Hall’s Cave, TX) and a suite of isotope-enabled climate simulations, we develop a new index for the intensity and spatial extent of the NASH. With this index we reconstruct NASH variability across the last deglaciation (21 to 10 ka). The zonal gradient in δDwax identifies two distinct NASH responses to deglacial climate forcings. During the Younger Dryas, when Atlantic Ocean heat transport weakened, the NASH expanded westward. In contrast, the NASH gradually contracted eastward and weakened as deglacial greenhouse-gas concentrations increased. An isotope-enabled simulation captures the first response but severely underestimates the second. Because the NASH delivers moisture that drives extreme rainfall in SENA, this underestimation suggests that models cannot yet produce confident projections of regional rainfall extremes, or alternatively that those extremes will exceed current projections. Rainfall hazards facing SENA may be larger or less certain than current models suggest.