Glacial-Interglacial Variability of Eastern Pacific Hydroclimate

Abstract

Unprecedented drought conditions in southwest North America (SWNA, defined as the region from southern California to southern Mexico, west of the continental divide) has raised alarms regarding water resource scarcity in a region with dense urban populations and vital agricultural industries. Modern hydroclimate in SWNA is complex as rainfall seasonality and moisture source ranges from the Mediterranean climate of California, bimodal desert southwest, monsoonal northwest Mexico, and Central American tropics, and there is a high degree of interannual rainfall variability due to the importance of short-lived, high-intensity events to regional moisture budgets. A deeper understanding of the spatio-temporal patterns of mean annual and seasonal rainfall variability and their response to global climate perturbations is thus critical for future planning and water resource management in the region. The eastern Pacific is a key modulator of rainfall variability in SWNA. Pacific ocean-atmosphere interactions affect North American monsoon intensity, the seasonal migration of the intertropical convergence zone (ITCZ), the strength, location, and waviness of the mid-latitude storm tracks, and the occurrence of extreme precipitation events associated with atmospheric rivers, tropical cyclones, and the El Niño/Southern Oscillation. Paleoclimate records from the Pacific can provide crucial constraints on how these processes responded to past global climate perturbations. A unified view of Pacific-North American interactions is currently limited by sparse proxy data from the Eastern Pacific Warm Pool (EPWP) and the Gulf of California, specifically of reconstructions that extend beyond the Last Glacial Maximum (LGM; 19--23 ka). The work presented in this dissertation sheds new light onto the late Pleistocene climatic history of SWNA based on novel organic geochemical proxy reconstructions of eastern Pacific sea surface temperatures and North American monsoon rainfall variability. The proxy data indicate that the northern EPWP (along the northwest Mexican Margin near the mouth of the Gulf of California) did not experience glacial cooling. Rather, elevated sea surface temperatures regularly occur during intervals of low greenhouse gases and increased global ice volume across the last glacial cycle. Meanwhile, monsoonal rainfall was elevated during warm interglacials and suppressed during glacial periods. I additionally compare the proxy data to general circulation model simulations of the LGM and Last Interglacial (LIG; $\sim$117--130 ka) periods to investigate the effects of orbital forcing (insolation), greenhouse gases, and ice sheets on moisture source, precipitation seasonality and geographic patterns of rainfall variability across SWNA. This research fills a critical data gap and ultimately provides a mechanistic understanding of the patterns and drivers of hydroclimate variability in SWNA that indicates a complexity in the response of SWNA temperature and precipitation to global climate forcings.