Warm Pliocene mid-latitude upwelling sites, with implications to future southwestern North America aridity under climate change

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The early Pliocene (5.3–3 million years ago) was the most recent naturally-occurring warm period. It had an estimated carbon dioxide concentration of 400 ppm, similar to the level reached at present due to anthropogenic emissions. The Pliocene climate is often held up as a past analog for 21st-century climate. Today, cold deep water rises to the surface in the California coastal ocean upwelling system. Climate reconstructions suggest that the surface water in that region during the Pliocene was dramatically warmer than at present—by some 9 degrees C. Other evidence suggests that southwestern North America was very wet at that time, also in contrast to today. Those Pliocene reconstructions differ significantly from Intergovernmental Panel on Climate Change (IPCC) projections of near future climate. The IPCC projections suggest that parts of the southwestern USA will get drier, not wetter. The IPCC projections of the California upwelling system are very uncertain. The proposed research will examine a possible explanation of the Pliocene observations. It will lead to a better understanding of possible future changes to mid-latitude coastal upwelling systems. Such systems are critical to the fishing industry. The study will also lead to a better understanding of our ability to predict the aridity of southwest North America in a warmer climate. The project Broader Impacts include training of a graduate student, undergraduate students doing term and summer research projects, and high school student summer interns. The Broader Impacts activities will also include outreach to STEM college students via an open course on the science of climate change.The planned work will examine a proposed feedback that may explain these past climate observations. In this feedback, weaker upwelling leads to a warmer upwelling-region sea-surface temperature (SST), which enhances precipitation over the adjacent land and creates wetlands there, and then the existence of wetlands weakens the upwelling-favorable coastal winds and, therefore, further weakens the upwelling itself. The significance of the proposed study arises from the exploration of a novel feedback with important socioeconomic consequences. This work will study some related fundamental aspects of ocean upwelling systems and use a hierarchy of ocean and atmospheric models that will provide the opportunity to test the proposed feedback mechanism and study its consequences.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.