Changing Trends and Variance in Eastern Equatorial Pacific Sea Surface Temperatures over the Twentieth Century

Global climate models and instrumental datasets often disagree regarding sea surface temperature (SST) trends in the tropical Pacific. Coral Sr/Ca records with sub-seasonal resolution provide SST proxies that complement and extend limited instrumental records. We present a new partially replicated Sr/Ca-SST record from two Galápagos corals (Isla Wolf, at 1°24'N, 91°48'W), that spans 1937-2010. The record shows high SST variance in the region, which increases nearly twofold after the late 1970s. Similarly, there is little trend in the mean until 1976, after which SSTs warm during all seasons. Both the increase in variance and the trend are strongest during the warm season, leading to progressively more anomalous conditions during El Niño events. To investigate recent changes in the eastern equatorial Pacific since the 1976/1977 climate transition, we compare the Galápagos record to a published coral Sr/Ca-SST record from Clipperton Atoll (10°18'N, 109°13'W, spanning 1874-1993; Wu et al., 2014, Palaeogeogr. Palaeoclimatol. Palaeoecol.). As in the Galápagos, Clipperton corals show increasing SSTs in all seasons after 1976. The trend at Clipperton is greater than in the Galápagos, though the variance is smaller and does not change significantly throughout the record. Finally, the north-south temperature gradient between Clipperton and Galápagos has increased slightly over the interval in which the two records overlap (1937-1993). Gridded instrumental SST data generally agree with the coral Sr/Ca-SST results, though the gridded data suggest lower variance at both sites. In sum, we show that an increase in the mean and variance of SSTs in the eastern equatorial Pacific is associated with an enhanced meridional SST gradient over the twentieth century, and especially since 1976. These results contrast with recent suggestions that a weakened meridional SST gradient in the equatorial Pacific may be leading to stronger El Niño events. Our results support the consensus that the 1976/1977 climate transition was a key turning point in the state of the tropical Pacific.