USING STABLE ISOTOPE RATIOS TO ASSESS MOISTURE SOURCES IN THE EARLY OCTOBER, 2015 STORM IMPACTING THE CAROLINAS

Rainwater from tropical cyclones and highly convective storms has oxygen and hydrogen isotope ratios (δ¹⁸O and δD) that are more negative than normal meteoric precipitation events from similar latitudes; the so-called “amount effect.” This study uses δ¹⁸O and δD values of rainwater to assess whether the storm in early October 2015 that hit the Carolinas drew moisture from Hurricane Joaquin, the Gulf of Mexico, or both. 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. We propose that the initial stage of the storm drew moisture from Hurricane Joaquin over the Atlantic to the east, and the latter stage abruptly shifted to drawing moisture from the Gulf of Mexico to the south. Future work will compare these results to HYSPLIT analysis, local river water data, and subsequent rain events.