Quantification of modern moisture source variability to southwest US

Abstract:

There are a growing number of speleothem δ¹⁸O records emerging from the Southwest US that are interpreted to reflect, in part, past variability in moisture source. The present-day sources of moisture to the region, however, have yet to be quantified. We present results from a simulation of modern climate (1975-2013) using the Community Atmosphere Model (version 5)/Community Land Model (version 4) (1.9x2.5° grid resolution) that included six water tags to track water evaporated from over the North Atlantic, Gulf of Mexico, west and central North Pacific, far east Pacific and Gulf of California, the entire Southern Hemisphere, and land over the Northern Hemisphere. In the model, sea surface temperature and sea ice were prescribed, and the atmosphere and land were allowed to evolve freely. The results document a strong, NW-SE gradient in winter Pacific-derived precipitation, from 50% in southern California to 5% in east Texas. SE to NW gradients in annual precipitation derived from the Gulf of Mexico and Atlantic are similar, and range from 30% in east Texas to 5% in southern California. The proportions of winter precipitation from the Atlantic plus Gulf of Mexico negatively co-vary with that from the Pacific across the region. The contribution of water evaporated from land (i.e., terrestrial moisture) comprises the largest source water fraction (50%) during summer months over most of the region, contrary to the notion that Gulf of Mexico is the dominant moisture source during summer. Lastly, fractions of moisture source contributions are not correlated with precipitation amount, except for terrestrial moisture (positive correlation). The lack of correlation can account for the inferred decoupling between moisture source and amount suggested by the lack of co-variation in speleothem δ¹⁸O values and growth rate in recently published speleothem records from New Mexico and Texas. Future work will dynamically assess the climate processes driving variations in the proportion of source water contributions to the region.