TREE-RING OXYGEN ISOTOPE RECORDS OF CLIMATE AND ONTOGENY OF NORTH ATLANTIC HURRICANES

Many characteristics of hurricanes that are important to their potential impact on U. S. coastal regions, including their frequency and track, are influenced by their ontogeny. Two types of North American hurricanes can be distinguished on the basis of their ontogeny: tropical-only (TO) and baroclinically-enhanced (BE) hurricanes. The former group develop at low latitudes, unenhanced by middle latitude baroclinic influences. A greater understanding of the climate modes that control hurricane formation will improve long-term predictive modeling of hurricane development and help to clarify the relationship between hurricane frequency and global climate change. Elsner and Kara (1999) noted the occurrence of a reversal in the proportion of tropical-only (TO) versus baroclinically-enhanced North Atlantic hurricanes (BE) ~ mid-1960s, where TO hurricanes comprised the majority of annual storms between 1886 and 1965, and BE hurricanes dominated from 1966 to 1994. Data since the mid-1990s suggest that we may again be in a period of TO dominance. Climatic conditions that favor the formation of TO hurricanes tend to inhibit the formation of BE hurricanes. We have examined the correlation between a seasonally-resolved (earlywood-EW, latewood-LW) oxygen isotope time series of tree-ring alpha-cellulose from long-leaf pines in Valdosta, Georgia, to various climate modes. From 1875 to 1950, EW d¹⁸O values show a significant negative correlation with SST anomalies in the North Atlantic (AMO indices). From 1960 to 1990, EW isotopic compositions show a weaker, and positive, correlation to AMO indices. The change in the isotope-AMO relationship is approximately concurrent with the change from TO- to BE-dominated hurricane occurrence. The seasonality of the isotope record (i.e., the difference between EW and LW d¹⁸O values) also appears to respond to climate modes. Values of Δ(EW-LW) are ~ 2 ‰ over most of the period 1770 to 1930. In the decades 1930-1950 and 1970-1990, periods that roughly correspond to warm phases of the PDO, Δ(EW-LW) are only a few tenths permil. These results suggest that tree-ring oxygen isotope compositions are sensitive to changes in climate modes that control hurricane frequency and may yield a rich archive of climate information.