Reconstructing Hydrologic Variability in Southwestern North America Using Speleothem Proxies and Precipitation Isotopes from California
Precipitation in southwestern North America has exhibited significant natural variability over the past few thousand years. This variability has been attributed to sea surface temperature regimes in the Pacific and Atlantic oceans, and to the attendant shifts in atmospheric circulation patterns. In particular, decadal variability in the North Pacific has influenced precipitation in this region during the twentieth century, but links to earlier droughts and pluvials are unclear. Here I assess these links using δ18O measurements from a speleothem from southern California that spans AD 854- 2007. I show that variations in the oxygen isotopes of the speleothem correlate to sea surface temperatures in the Kuroshio Extension region of the North Pacific, which affect the atmospheric trajectory and isotopic composition of moisture reaching the study site. Interpreting our speleothem data as a record of sea surface temperatures in the Kuroshio Extension, I find a strong 22-year periodicity, suggesting a persistent solar influence on North Pacific decadal variability. A comparison with tree-ring records of precipitation during the past millennium shows that some droughts occurred during periods of warmth in the Kuroshio Extension, similar to the instrumental record. However, other droughts did not and instead were likely influenced by other factors. The carbon isotope record indicates drier conditions are associated with higher δ13C values and may be a suitable proxy for reconstructing past drought variability. More research is needed to determine the controls on trace element concentrations. Finally, I find a significant increase in sea surface temperature variability over the past 150 years, which may reflect an influence of greenhouse gas concentrations on variability in the North Pacific. While drought is a common feature of climate in this region, most climate models also project extreme precipitation events to increase in frequency and severity because the climate changes largely due to increased water vapor content in a warmer atmosphere. I also utilize precipitation data and isotopic analysis from precipitation samples collected weekly from near the cave site at Giant Forest, Sequoia National Park, California, from 2001 to 2011, to analyze climate mode patterns during extreme precipitation events and to construct an isotopic data base of precipitation samples. Composite maps indicate extreme precipitation weeks consist of a weaker Aleutian Low, coupled with a deep low pressure cell located northwest of California and enhanced subtropical moisture. I find extreme precipitation weeks occur more often during the La Niña phase and less during the positive Eastern Pacific (EP) phase or during the Central Pacific (CP) neutral phase at our site. Analyses of climate mode patterns and precipitation amounts indicate that when the negative Arctic Oscillation (AO), negative and neutral Pacific North American pattern (PNA), and positive Southern Oscillation Index (SOI) (La Niña) are in sync, the maximum amount of precipitation anomalies are distributed along the Western US. Additionally, the central or eastern Pacific location of El Nino Southern Oscillation sea surface temperature anomalies can further enhance predictive capabilities of the landfall location of extreme precipitation.