The presence of a permanent El Niņo-like Pliocene climate has been suggested by several authors (Molnar and Cane, 2002; Ravelo et al., 2004). During a present-day El Niņo, the collapse of the easterlies leads to an equatorial Kelvin wave, which is characterized by the eastward flow of warm waters from the WPWP, leading to increasing SSTs in the east Pacific. In consequence, the temperature gradient between the WPWP and the east Pacific diminishes and the east Pacific thermocline deepens (Wallace et al., 1998, and references therein). Upwelling intensity, however, is not changing, but the upwelled water now comes from the warmer waters above the thermocline instead of below the thermocline (Barber and Chavez, 1983; McPhaden and Picaut, 1990). These changes significantly influence extratropical climates, for example, they cause warmer and wetter conditions in North America, drier conditions in southeast Asia and Australia, and drier conditions over northeastern South America (Molnar and Cane, 2002, and references therein). Several Pliocene reconstructions show extratropical conditions similar to those of the present-day El Niņo (Zarate and Fasana, 1989; Dowsett and Poore, 1991; Wolfe, 1994; Archer et al., 1995). Evidence from the Pliocene tropical Pacific mainly comes from thermocline and SST gradient development. 
18O records of G. sacculifer and foraminiferal faunal assemblages for the east and west Pacific show that the present-day SST gradient and the slope of the thermocline depth between the east and the west Pacific did not fully develop until 1.5 Ma (Chaisson, 1995; Cannariato and Ravelo, 1997; Chaisson and Ravelo, 2000; Ravelo et al., 2006), although a first and significant step toward a shallow thermocline occurred between 5.3 and 4.0 Ma (Ravelo et al., 2006;
Steph et al., this volume).
The areas which experience the largest SST anomalies in the east Pacific during El Niņo events are the major upwellings areas along the coast of South America and along the equatorial divergence. Although the position of Site 1241 is, therefore, not the ideal place to record El Niņo-related changes, SST anomalies associated with El Niņo still show a warming of as much as 3°C during El Niņo events (www.cdc.noaa.gov). Including the backtrack location of Site 1241 closer to the equatorial divergence, our data provide indications on the existence of a permanent El Niņo-like climate during the Pliocene. The SSTMg/Ca record of G. sacculifer, however, does not show significantly higher temperatures for the early Pliocene in comparison with today, as would be expected. This result is also shown by Dowsett et al. (1996), who used foraminifer, diatom, and ostracode assemblages for a middle Pliocene time interval to reconstruct SSTs. However, they also show significant warming in mid and high latitudes, indicating that a globally warmer climate is not necessarily reflected by higher tropical Pacific SSTs, especially if the Pliocene atmospheric CO2 concentrations were not significantly higher than modern ones (Van der Burgh et al., 1993). Present-day mixed-layer SSTs in the tropical east Pacific vary between 24° and 27°C for the upper 40 m (Levitus and Boyer, 1994), below which the thermocline starts. Taking into account the paleolocation of Site 1241 (Pisias et al., 1995; Mix, Tiedemann, Blum et al., 2003), SSTs would be expected to be lower by ~1°C for the early Pliocene. SSTMg/Ca for the early Pliocene indicates variations between 24° and 26.5°C for the habitat depth of G. sacculifer. A study by Lea et al. (2000) from the equatorial east Pacific (Core TR163-19, with a comparable position to the paleoposition of Site 1241), showed similar SSTMg/Ca fluctuations between 23° and 28°C for the surface-dwelling Globigerinoides ruber for the last 350 k.y. Since present-day SSTs are indicative of interglacial, maximum temperatures, we conclude that the tropical thermal maxima during the early Pliocene were not significantly different than modern interglacial SSTs. Therefore, our east Pacific SSTMg/Ca results do not directly support the idea of an early Pliocene permanent El Niņo-like climate. An additional argument against a permanent El Niņo-like climate is provided by the significant shallowing of the thermocline between 5.3 and 4.0 Ma (Steph et al., this volume; Cannariato and Ravelo, 1997; Chaisson and Ravelo, 2000; Ravelo et al., 2006). On the other hand, a typical El Niņo-like scenario with a decreased or even reversed east-west Pacific SST gradient might also have been caused by lower SSTs in the west Pacific, as was suggested by Cannariato and Ravelo (1997). Their estimation of SST, however, was based on 18O analyses, assuming no influence of salinity on 18O. Andersson (1997) and Wang (1994) used transfer functions on total planktonic foraminiferal assemblages to reconstruct Pliocene SSTs for the west Pacific. Their SST reconstructions, using three different transfer functions, showed SSTs similar to present SSTs in the west Pacific. Andersson (1997) actually investigated the difference between two transfer functions for cold and warm seasons for Ontong Java Plateau Ocean Drilling Program Site 806. All estimates were close to present SSTs and significantly higher than our average SSTs for the east Pacific (Fig. F6). Andersson's (1997) lowest estimate (Modern Analog technique) for the cold season was 27.0°C and the warmest estimate for the warm season (Imbrie-Kipp Transfer Function method) was 29.6°C, whereas our average SSTMg/Ca for the east Pacific is 24.7°C (Fig. F6). Although the age model of Andersson (1997) is not tuned to our age model, the general trends reveal a strong similarity between the two records between 4.0 and 3.2 Ma with the west Pacific constantly being warmer than the east Pacific by ~3°C. Further comparison between both SST records suggests a strengthening of the tropical west-east Pacific SST gradient along with major intensification of NHG after 3.2 Ma.
Therefore, we conclude that a "normal" SST gradient rather than a very weak, El Niņo-like SST gradient between the west and the east Pacific seems more likely to have existed in the early Pliocene. Nevertheless, the closure of the Indonesian Gateway between 4.0 and 3.0 Ma (Cane and Molnar, 2001) and the decrease in east Pacific SST since 3.7 Ma might have played a role in further increasing the SST gradient toward present proportions.
