1 Introduction

Broad evidence suggests that the mean state of the tropical Pacific Ocean has changed throughout the twentieth century in response to rising greenhouse gas concentrations (Capotondi et al., 2015; Collins et al., 2010; DiNezio et al., 2010; Karnauskas et al., 2009; Yeh et al., 2009). Warming is clear over most of the basin but attempts to relate warming to particular mechanisms have yielded competing conceptual models. The “weaker Walker” model holds that atmospheric warming produces greater increases in water vapor relative to precipitation, causing reduced vertical vapor transport (Held & Soden, 2006). Via the Bjerknes feedback, this would slow the easterly trade winds, weakening oceanic circulation and upwelling and leading to warming across the tropical Pacific (Cai et al., 2015; Collins et al., 2010; Vecchi & Soden, 2007). Alternatively, the ocean dynamical thermostat (ODT) model suggests that the eastern and western Pacific should experience differential warming: increased upwelling in the east should strengthen the zonal sea surface temperature (SST) gradient and the Walker circulation, amplifying the trades and damping warming in the eastern and central Pacific (Clement et al., 1996). A third model posits that evaporative heat loss should be more efficient in areas of low humidity or strong winds, thus predicting greater equatorial warming relative to the subtropics and in the western relative to the eastern Pacific (DiNezio et al., 2009; Vecchi et al., 2008; Xie et al., 2010).

Evaluating these hypotheses is difficult because observations remain ambiguous regarding the nature and direction of SST trends in the eastern tropical Pacific (ETP) (Abram et al., 2016; An & Im, 2013; DiNezio et al., 2009; Sayani et al., 2011). In situ (e.g., Wolff, 2010), instrumental (Deser, Alexander, et al., 2010), and proxy records (Tierney et al., 2015) do not agree on the climatic history of the region, and models reproduce SST patterns poorly (Vecchi et al., 2008).

Paleoclimate data can help overcome the region's paucity of instrumental climate records. Previous work has leveraged data from the Galápagos archipelago, where geochemical records from corals extend the observational baseline provided by in situ data beginning in the mid‐1960s (Wolff, 2010). However, highly variable SST associated with El Niño challenges coral survival. In particular, the extreme 1982–83 El Niño event caused coral reef mortality of up to 97% (Glynn et al., 2015). Thus, despite a long history of coral paleoclimatology in the Galápagos, existing records stop in 1982 (e.g., Dunbar et al., 1994; Guilderson & Schrag, 1998; Shen et al., 1992) and do not show modern trends (Cole & Tudhope, 2016).

As a result, SST trends in the Galápagos are still debated. While some studies suggest neutral or cooling SSTs during the late twentieth century (Karnauskas et al., 2015; Wolff, 2010), others show a stepwise SST increase following the 1976 Pacific climate shift (Guilderson & Schrag, 1998) or even suggest that warming began before the twentieth century (Conroy et al., 2009).

Here we present a new SST reconstruction that spans 1940–2010 from two Wolf Island corals, in the northern Galápagos archipelago (Figure 1). Our record is the first from Galápagos to bridge the 1982–1983 event. Together with other coral, instrumental, and satellite‐based gridded data sets, we use this record to investigate twentieth century SST trends throughout the ETP. The spatial patterns of recent trends allow us to assess the mechanisms controlling the trajectory of the tropical Pacific's response to climate change.