Evidence for El Niño–like conditions during the Pliocene

The modern tropical Pacific Ocean is characterized by strong east-west asymmetry in sea surface temperature and subsurface thermocline depth coupled to easterly trade winds and zonal atmospheric, or Walker, circulation. Walker circulation and the “normal” east-west asymmetry of sea surface temperature and thermocline depth break down temporarily during El Niño events. Since these temporary deviations from the “normal” tropical climate state are known to have global impacts, it is important to consider whether permanent shifts in the mean tropical Pacific climate state are an integral part of global climate change on longer time scales. To understand the link between tropical conditions and global warmth, we focus our study on the early Pliocene, the most recent period in Earth’s history of sustained global warmth relative to today. A data synthesis of tropical paleoceanographic data, including a new alkenone unsaturation index ( U 37 )–based sea surface temperature record from the eastern equatorial Pacific, indicates that, in the early Pliocene, the east-west asymmetry in sea surface temperature and thermocline depth was reduced compared to today and the tropical Pacific was in a permanent El Niño–like state. Thus, the “normal” mean state of the modern tropical Pacific is not a persistent feature of Earth’s climate over long time scales. INTRODUCTION Studies of the el niño Southern Oscillation (enSO) phenomenon indicate that, through atmospheric teleconnections, small changes in the pattern of tropical Pacific sea surface temperature (SSt) have a global impact on interannual time scales (Alexander et al., 2002). Although the mechanisms responsible for the enSO do not directly apply to studies of climate changes on longer time scales, enSO events provide a clear example of how changes in the distribution of SSt across the Pacific Ocean can have far-field climate effects such as higher than average rainfall in the southwestern United States and higher than average temperature in temperate regions of north America. the potential global effect of small, long-term changes in the tropical SSt pattern is substantiated by modeling studies (Yin and Battisti, 2001; Barreiro et al., 2005). However, while the impact of changes in the mean SSt pattern of the tropical Pacific on global climate is recognized, the circumstances under which they could occur are difficult to predict. For example, in simulations of future climate change forced with enhanced greenhouse gases, climate models do not give consistent results in the tropical Pacific: some predict no long-term changes; some predict el niño–like mean conditions; still others predict la niña–like mean conditions (cane, 2005; collins, 2005). Because the instrumental record is too short to examine multidecadal and longer-term climate changes, paleoceanographic studies are needed to establish whether modern mean tropical SSt patterns across tropical basins are stable over long time periods. these data-based studies can then be used to test and improve theoretical and computer models of long-term climate change, including those that are used to predict future climate change. While much can be learned from studying the extreme globally cool climate of the last Glacial Maximum (lGM), it is also important to focus on past periods of global warmth prior to the ice ages of the past few million years. Paleoceanographic studies generally indicate that the mean SSt of the Pacific tropical ocean was stable within a few degrees over millions of years, yet these studies rarely include enough data to characterize the east-west SSt difference across the Pacific. For example, the Pliocene warm period (ca. 4.5–3.0 Ma) (Fig. 1) has been the focus of much interest among paleoclimatologists because of the need to understand climate processes in past times of global warmth. landmark studies, such as those by the Pliocene research, interpretation, and Synoptic Mapping (PriSM) group, including compilations of oceanic and terrestrial data (Dowsett et al., 1996, 2005; thompson and Fleming, 1996) and modeling studies (Haywood et al., 2000; Sloan et al., 1996), indicate that the Pliocene was significantly warmer than today, especially in extratropical regions. However, the PriSM reconstructions include very little data from the tropical Pacific Ocean and therefore do not provide insight into changes in tropical SSt patterns. crowley (1996) pointed out the urgent need for more tropical data in order to further constrain the mechanisms that explain global climate conditions in the Pliocene, and in the last decade, several studies were conducted that focus on the tropical Pacific utilizing Pliocene-age material obtained by the Ocean Drilling Program.