TRACING MOISTURE SOURCES OF THE EXTREME STORM EVENT IMPACTING THE CAROLINAS IN OCTOBER, 2015 USING O, H, AND SR ISOTOPE RATIOS

Oxygen, hydrogen, and strontium isotope ratios of rainwater were analyzed to trace moisture sources of the extreme storm that impacted the Carolinas in early October, 2015. Rain samples were collected in Durham, North Carolina every two hours during the height of the storm to every four to six hours as the storm subsided. In the first 21 hours of the storm, δ¹⁸O and δD values were very negative, ranging from −13.1 to −10.1‰ and −94.1 to −72.0‰, respectively. Within two hours, rainwater samples abruptly shifted to higher values ranging from −6.8 to −0.8‰ and −9.3 to −0.4‰, respectively. These data form two distinct populations falling on the Global Meteoric Water Line. Our preliminary ⁸⁷Sr/⁸⁶Sr ratios increase from the beginning to the end of the storm, trending toward a marine value (0.708689 to 0.709200). HYSPLIT backward trajectories indicate that low-level (1000 m AGL) air masses from the initial stages of the storm originated near Newfoundland, Canada, and then switched to a source southeast of the study area over the Atlantic Ocean. The switch to a southeastern source occurred with the abrupt shift to higher stable isotope ratios. The very negative δ¹⁸O and δD values early in the storm likely resulted from colder condensation temperatures at high latitudes and the “orographic effect” (δ¹⁸O and δD values of rainwater become more negative as moist air moves farther from its source area). The abrupt shift to higher stable isotope ratios resulted from moisture drawn from the outer bands (i.e., not near the eye) of Hurricane Joaquin reflecting near-surface convergence of warm, moist air over the Atlantic Ocean southeast of the sampling site. Strontium isotope ratios are consistent with our interpretation with less radiogenic (i.e., more terrestrial) values at the start of the storm and a more marine influence toward the end of the storm.