Moisture transport pathways into the American Southwest from multiple oceanic sources as deduced from hydrogen isotopes.

There has been a long-standing controversy regarding the oceanic sources of atmospheric moisture over the southwestern US, especially during the summer monsoon circulation. Past arguments have been made for moisture sources in the Gulf of Mexico, Gulf of California, Eastern Pacific, or some combination thereof. To help resolve this problem, we are using hydrogen isotopes as a tracer to reconstruct the history of moisture being advected into central New Mexico. The hydrogen isotope composition (δD) of water vapor is dependent on many factors, including the temperature of the source ocean, rainout history, contributions from evapotranspiration over the continent, and mixing between air masses. Since April 1, 2005 we have been measuring δD of atmospheric water vapor (δDwv) at ground level 1 to 3 times per day. In addition, we periodically collect water vapor in ~300m vertical increments to ~3km above the surface using a light aircraft. We see significant temporal variations in δDwv on several different scales in the ground-level δDwv measurements. Lower δDwv values in the winter are consistent with moisture transport from the Pacific Ocean, while larger δDwv values in the summer imply moisture from a warmer body of water. During the fall, winter, and spring, large variations in δDwv (up to 80‰) can occur over the period of a few days to a few hours whereas during the summer monsoon season the δDwv values are much less variable. The relationship between δDwv and dew point also varies throughout the year; good correlations exist through portions of spring, fall, and winter, while at other times no correlation is apparent. During the summer monsoon season, a distinctive anticorrelation exists between dew point and δDwv. Using trajectory analyses, we have been able to demonstrate that many of the variations in δDwv can be explained by changes in moisture transport pathways. It appears that the source of the moisture being advected into central New Mexico can switch from the Gulf of Mexico to the Gulf of California in as little as 12 hours. Variations of δDwv are also observed within vertical profiles, where multiple layers of water vapor with distinctive δDwv values are usually noted. Trajectory analyses terminated at different altitudes allow us to correlate these variations of δDwv with different source regions. It appears that within a single column of air, water vapor from multiple source regions may be present. We also conclude that water vapor contributions from evapotranspiration in this semi-arid area are too small to significantly affect δDwv values.