Stable Isotope Ratios and the Extratropical Transition of Tropical Cyclones

As hurricanes move poleward they weaken and often tranform into extratropical cyclones, yet continue producing enormous quantities of rain. Rain and water vapor samples were collected in the paths of Hurricanes Opal (1995), Bertha (1996), Fran (1996) and Floyd (1999) and analyzed for their 18O and deuterium contents. Stable isotope ratios reflected the structure and evolution of the storms. On average the isotope ratios remained anomalously low, as is characteristic of hurricane rains, but increased toward the north as maximum cloud tops lowered and the storms weakened. In addition, the transition of the storms to extratropical status was accompanied by growing asymmetry of isotope ratios, with highest values generally in the warm sector.