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University of Southern California (2010)

Perspectives on drought and temperature variability for the southwestern United States from a new hydro-isotopic network

Berkelhammer, Max B

Titre : Perspectives on drought and temperature variability for the southwestern United States from a new hydro-isotopic network

Auteur : Berkelhammer, Max B

Université de soutenance : University of Southern California

Grade : Doctor of Philosophy (PhD) 2010

To directly test the relationship between atmospheric circulation and the isotopic composition of precipitation in the southwestern US, I develop a catalog of 120 individual storm events striking the west coast over a 5-year period. The cause of isotopic variability is assessed using an isotope-enabled GCM simulation that has been nudged to Reanalysis fields. The results from this analysis show that changes in meridional moisture flux from the low latitudes leave a tangible mark on precipitation in the region. This relationship can theoretically be quantified by a linear relationship between the modeled isotopic composition of precipitation and the relative percentage of low latitude tagged water that is delivered with the storm system. The controls on the isotopic variability of precipitation change substantially moving eastward into the North American Monsoon region where moisture is not delivered by large frontal storms but rather through localized convection. In these regions, variability of the moisture source is subdued and isotopic variability arises principally as a function of depth of convection, which leads to a close correlation between temperature and δ18O. Mechanistic constraints are placed on the cause of isotopic variability in the cellulose chronologies using a forward modeling approach where meteorological data and the isotopic composition of soil water and vapor from an ensemble of isotope-enabled GCM simulations are fed into a geochemical model that captures the isotopic fractionation associated with the biogeochemical processes in the tree prior to cellulose metabolism. At sites where precipitation is predominantly from winter precipitation the intra-ring isotopic cycles are driven largely by the relative humidity and temperature at the leaf boundary while the higher amplitude interannual variability arises from changes in the trees source water. Variations in the shape of the cycle reflect not only differences in growing season climate but also changes in the length of the growing season. At sites where moisture is predominantly from summer rains, the cycle directly tracks the isotopic composition of precipitation during the growing season. The isotope chronology from the Almagre Mountains in Colorado shares little covariance with the more western sites, because it relies predominately on monsoonal moisture whose isotopic composition tracks temperature. The record from this sites provides a 500-year reconstruction of growing season temperatures for this region. Growing season temperatures in the southern Rocky Mountains display a high degree of multi-decadal variability between the 18 th-20th century but only subdued variability prior (15th-18 th centuries). The temperature reconstruction presented here differs markedly from the tree-ring width based temperature reconstruction from the same site but agrees with regional temperature reconstructions based on instrumental temperature records and tree ring density. Because the relationship between temperature and the isotopic composition of precipitation is stable, I interpret the difference between the two reconstructions to be evidence that the ring-width based reconstruction is biased by non-stationary relationships between temperature and growth. Isotopic chronologies from Bristlecone Pines in the White Mountains show distinctive minima during each of the multi-year droughts of the 20 th century, suggesting that drought is sustained by a selective loss of low latitude moisture. This is dynamically consistent with an understanding that each of the major 20th century droughts was driven by cooler conditions in the eastern Tropical Pacific and a northward shift in the storm track. The exception to the isotope-aridity relationship is during the current drought, which has been associated with rising isotopic values and therefore appears dynamically distinct. Rising temperatures in the mid troposphere over the North Pacific during the current decade has diminished the pressure gradient between the subtropics and extratropics and has therefore reduced the convergence of the depleted northerly moisture source. Prior to the 20th century there is a dramatic change in the isotopic composition of precipitation at the White Mountain site. The cause of this is hypothesized to be an increase in low latitude moisture associated with cooler Northern Hemispheric conditions, which led to a more southerly storm track that consistently tapped into the enriched tropical moisture pool. Dramatic isotopic shifts in ice cores and lake sediments from the North Pacific confirm a major change in the midlatitude storm track at the terminus of the Little Ice Age

Mots clés : Droughts, Cellulose, Dendroclimatology, Stable isotopes

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