The region off the Pacific southwest coast of Mexico has greater tropical cyclone activity per unit area than any other region of the world. This is in part due to high sea surface temperatures (Elsberry et al., 1987). From May to November frequent periods of intense convection occur over the ocean near Puerto Escondido. The disturbances take the form of thunderstorms associated with the ITCZ, mesoscale convective complexes and tropical depressions, storms, and hurricanes (Zehnder et al., 1999).

Water vapor samples were collected at Puerto Escondido around 0300, 1200 and 1900 UTC every day from 10 to 31 July 1998. They were analyzed for their hydrogen and oxygen isotope ratios. Over that time period there were large variations in isotope values. Trajectory analysis demonstrated that the large variations were directly related to the intensity of precipitation over the previous 48 hours upwind of the collection site. In addition a distinct contrast in the average isotope values existed before and after July 18. The isotope values were on average markedly lower during the first period. This shift was directly related to the shift in circulation regimes. In the first period air in Puerto Escondido originated to the south and west over the Pacific Ocean. In the second period air generally came from the east across the Mexican Isthmus or Guatemala from the Caribbean Sea.

The stable isotopic composition of leaf water in trees in the tropics in theory is primarily controlled by the isotopic composition of water vapor in the atmosphere. In theory this isotopic signal present in the leaf water is transferred to the tree cellulose as the tree grows. Therefore, the potential exists to document past circulation regimes along the southwest coast of Mexico by the isotopic analysis of tree cellulose.