Tropical cyclogenesis and stable isotope ratios of water

During the period from 10 to 31 July 1998 three tropical disturbances brushed the western coast of Mexico near Puerto Escondido (14 N, 97 W). The first dissipated but the next two developed into Tropical Storm Celia and Hurricane Estelle respectively. Vapor and rain samples collected during the three- week period and analyzed for their stable isotope ratios had anomalously low values only during the time and for about a day after each of the disturbances passed by. The low isotope ratios develop in mesoscale convective systems with high cloud tops and preceded tropical cyclogenesis by at least a day. During periods of isolated convection or no precipitation, isotope ratios remained high. Low isotope ratios therefore appear to serve as a necessary but not sufficient indicator of tropical cyclogenesis.