Multiple lines of evidence suggest that southwestern North America (SWNA), like many subtropical continents, was much wetter during the Pliocene epoch, a climate interval featuring reduced ice volume and CO 2 concentrations above preindustrial levels (Figure 1). Sedimentological data document widespread perennial and ephemeral lakes in southern California and Arizona (Pound et al., 2014;Ibarra et al., 2018) (Figure 1), and palynological and macrobotanical evidence from southern California suggests expanded tree cover and the presence of species that today only grow in regions with mesic conditions and summer rainfall (Ballog & Malloy, 1981;Remeika et al., 1988). Faunal remains from Baja California contain Crocodylus spp. fossils, which require freshwater habitats, further suggesting increased water resources in regions that are arid at present (Miller, 1980;Salzmann et al., 2009). At face value, this evidence for a wet Pliocene is at odds with the theoretical and model-derived prediction that regions like SWNA, and subtropical continents more broadly, will continue to dry in coming centuries as a result of elevated greenhouse gases (Byrne & O' Gorman, 2015;Seager et al., 2010).

The early to middle Pliocene (5.3-3 Ma) was the most recent epoch of prolonged global warmth relative to modern day. Proxy records point to several notable differences between Pliocene and modern-day climates. The Pliocene was characterized by strongly polar-amplified warming (Dowsett et al., 1994;Thompson & Fleming, 1996), with slightly smaller Antarctic ice sheets and minimal Northern Hemisphere ice coverage (Ravelo et al., 2004). The mean east-west sea surface temperature (SST) gradient in the tropical equatorial Pacific may have been weaker (

One particularly intriguing feature of proxy reconstructions of Pliocene sea surface temperatures (SSTs) is the dramatically higher temperatures inferred at several midlatitude coastal upwelling sites. Midlatitude coastal upwelling today occurs along the eastern boundaries of the Atlantic and Pacific basins and results in narrow strips of cold SST extending 10-50 km off the coasts (