A tree-ring oxygen isotope record of tropical cyclone activity, moisture stress, and long-term climate oscillations for the Southeastern U.S.

by 1974- Miller, Dana Lynette

Geological proxies are needed to extend the record of hurricane occurrence beyond historical observations. Tree rings preserve uniquely high resolution and precisely dated records of past environmental conditions. Oxygen isotopic compositions of alpha cellulose in seasonally-resolved components (earlywood (EW) and latewood (LW)) of tree rings of southeastern coastal plain pines predominantly reflect precipitation source and/or temperature providing a snapshot of climate activity for the region. Tropical cyclones produce large amounts of precipitation with distinctly lower oxygen isotope ratios than typical low-latitude thunderstorms. Evidence of isotopically depleted precipitation may persist in surface and soil waters for several weeks after a large event, and will be incorporated into cellulose during tree growth, capturing an isotopic record of tropical cyclone activity. A 227-year record of EW and LW oxygen isotope compositions of alpha cellulose in slash and longleaf pine (Pinus palustris Mill. and Pinus elliottii Engelm.) tree rings record evidence of past tropical cyclone activity, seasonal moisture stress, and multidecadal climate oscillations. The isotopic values for EW and LW are overprinted on systematic, decadal to multi-decadal-scale variations. Negative isotopic anomalies in the time series, interpreted as hurricane events, were identified using a one-year autoregression modeling technique. Hurricane occurrence inferred from the oxygen isotope proxy compare well with the instrumental record of hurricanes over the period 1940-1997. The proxy record further supports historical records back to 1770 and suggests a number of possible tropical cyclone events not captured by documentary evidence. The results suggest the potential for a tree-ring oxygen isotope proxy record, iii extending back many centuries, of long-term trends in hurricane occurrence. Records of seasonal moisture stress, inferred from positive isotopic anomalies in the isotopic time series are similarly tested and yield a robust record of moisture stress in the study area. Long-term variations in the oxygen isotope compositions of tree-ring alpha cellulose are governed by the influence of long-term climate oscillations, including the Atlantic Multidecadal Oscillation, Pacific Decadal Oscillation, and El Niño Southern Oscillation. The Atlantic Multidecadal Oscillation (AMO) shows a strong negative correlation with tree ring ?18O values until ~1950s. The breakdown in the correlation with the AMO coincides with a major Pacific Decadal Oscillation-El Niño Southern Oscillation shift from warm to cool conditions (1947–1976 Cool Period II) that was followed by two of the strongest La Niña episodes in the last 50 years. Latewood treering oxygen isotopes from the decade of the 1950s strongly correlate with Niño 3.4 indices. Spectral analysis of the latewood tree-ring oxygen isotopes reveal significant periodicities of ~82.7, 33.7, 7.9, and 5.1 years. These periodicities may reflect solar activity such as the Gleissberg Period (82.7) and the Bruckner Cycle (33.7) and El Niño Southern Oscillation (7.9 and 5.1) influences on climate of the southeastern U.S. Five-tosix and seven-to-nine year periodicities have been related to the frequency of tropicalonly and baroclinically enhanced Atlantic hurricanes. Oxygen isotopes from tree-ring cellulose of sub-fossil longleaf pines recovered from Lake Louise, southern Georgia record climate conditions during a portion of the Little Ice Age (1580–1650) for the southeastern U.S. Oxygen isotope compositions for this time period are very similar to modern values (1895–1997) for this area. These results support previous studies that suggest the southeastern U.S. did not experience iv dramatic climate effects of the Little Ice Age. The slight overall enrichment of oxygen isotope ratios may primarily reflect changes in precipitation source and moisture stress. The results suggest that tropical cyclone activity was low to moderate for 1580-1640, but increased noticeably in the last decade of the study (1640s). v