Congratulations to Xiaojing Du, who defended her dissertation on Friday, March 27, 2020

Advisor: Ingrid Hendy

Abstract
Southern California has a Mediterranean climate characterized by wet winters and dry summers. With both significant seasonal precipitation variability and unusually large interannual variance relative to the rest of the US, Southern California presents a huge challenge to water resource management as state water demands continue to grow. In this dissertation, I use multiple lines of evidence, including precipitation reconstructions, climate model outputs, and reconstructed vegetation, to explore the causes of hydroclimate change in this region. This understanding is critical for regional climate projections, water resource management, and forest ecosystems sustainability.

A robust 9000-year high-resolution Bayesian age model was generated using 89 accelerator mass spectrometric 14C dates for laminated marine sediments from central Santa Barbara Basin (SBB), California. Multiple flood (extreme precipitation events with an average return interval of ~100 years) and turbidite (earthquake induced slope failures with an average return interval of ~500 years) layers were identified. A master stratigraphy for the Holocene was created using these layers and correlated multiple marine sediment cores from SBB to place all published SBB proxy paleoclimate records into the new chronology. A sub-annually resolved Southern California precipitation record was reconstructed using ITRAX scanning X-ray fluorescence titanium counts from the same laminated sediment sequence in SBB. Instrumental precipitation data from the 20th and 21st centuries was analyzed to demonstrate that Southern California precipitation is significantly correlated with El Niño Southern Oscillation (ENSO) on interannual timescales via an ENSO teleconnection between the tropical Pacific and North America. The Ti-based precipitation reconstruction over the Common Era demonstrates the ENSO teleconnection is modulated by both tropical forcing and mid-latitude atmospheric pressure systems (the Aleutian Low). Strong interannual precipitation variability occurred when greater ENSO variance was observed in the tropical Pacific and when the Aleutian Low deepens in the northeast Pacific (e.g. 1370-1540 CE). Interannual precipitation was weak when either ENSO variance is reduced (700-900 CE) or the ENSO teleconnection was muted by a weakening of the AL (1540-1680 CE).

The sub-annual SBB precipitation reconstruction was extended back through the Holocene (9,000 years) to demonstrate that interannual precipitation variability increased significantly during late Holocene (after 4,500 years ago) when the Intertropical Convergence Zone (ITCZ) migrated southward and the Aleutian Low strengthened. The influence of ITCZ and Aleutian Low position was examined using model simulation outputs: a southward shifted ITCZ weakens across-equatorial trade wind, potentially enhancing tropical Pacific ENSO variability and strengthening the Aleutian Low in the North Pacific. Such atmospheric circulation changes lead to a stronger ENSO teleconnection with Southern California and thus greater interannual precipitation variability in the region. A new multidecadal-resolution pollen record in SBB was compared to regional precipitation reconstructions to explore the vegetation response to different aspects of hydroclimate change including interannual precipitation variability. The drought-adapted endemic vegetation appears insensitive to both interannual precipitation variability and extreme precipitation events, displaying only a subtle long-term response to orbital-forced seasonal insolation variance over the Holocene. However, prolonged or severe droughts may have a minor impact on the distribution of the coastal mosaic oak woodland/ chaparral/ sage scrub communities in this region on multidecadal to centennial timescales.