The Origin of Moisture Sources for the North American Monsoon Using a Numerical Model and Precipitation Stable Isotopes

Abstract:

This work evaluates the moisture sources that contribute to North American Monsoon precipitation over a 34-year period. The modified analytical dynamic recycling model (DRM) is used to evaluate the contributions from both oceanic and terrestrial regions. This computationally-efficient modeling framework reveals previously overlooked moisture source regions such as Central America and the Caribbean Sea in addition to the well-known Gulf of California and Gulf of Mexico source regions. Our results show that terrestrial evapotranspiration is as important as oceanic evaporation for NAM precipitation, and terrestrial sources contribute to approximately 40% of monsoonal moisture. There is a northward progression of moisture sources, beginning with Central America during the early season and transitioning north into the NAM region itself during the peak of the monsoon season. The most intense precipitation occurs towards the end of the season and tends to originate in the Gulf of California and tropical Pacific associated with tropical cyclones and gulf surges.

Heavy stable isotopes of hydrogen and oxygen in precipitation (δD and δ¹⁸O) collected for every precipitation event measured in Tucson, AZ for the period 1981-2013 complement our numerical results. Our analysis shows that precipitation events linked to sources from the Gulf of Mexico and Caribbean Sea have a more positive isotopic composition than sources from the Gulf of California and Tropical Pacific. We also see that terrestrial regions that derive their precipitation from the Gulf of Mexico have more positive isotopic composition than those that derive their moisture from the Pacific.