The present-day location of Site 1241 (5°50´N, 86°26´W) is within the eastward-flowing North Equatorial Countercurrent (NECC). A tectonic backtrack of the Cocos plate, on which Site 1241 is positioned, locates Site 1241 at a more southwesterly position around 3°N, 87.7°W between 4 and 5 Ma (Mix, Tiedemann, Blum, et al., 2003; Pisias et al., 1995) (Fig. F1). The closer proximity to the equatorial divergence and the South Equatorial Current (SEC) suggests that expected SST and SSS at Site 1241 for the early Pliocene were ~0.5°–1°C cooler and 0.35 units less saline than present, if other physical properties remained unchanged (Mix, Tiedemann, Blum, et al., 2003). Surface hydrography in the tropical Pacific is strongly controlled by seasonal variations in wind strength, causing seasonal changes in thermocline depth. In general, the NECC is closely linked to the Intertropical Convergence Zone (ITCZ), the zone of maximum heating and convergence of the northeast and southeast trade winds (Donguy and Meyers, 1996; Huber, 2002). North of the ITCZ, the northeast trade winds drive the North Equatorial Current toward the west and the southeast trade winds drive the SEC also to the west, but south of the ITCZ. Below the SEC and the equator, the Equatorial Undercurrent (EUC) flows toward the east from a depth of ~300 m in the WPWP to a depth of ~30 m in the east Pacific, parallel to the shallowing thermocline. The EUC is driven by the west-east pressure gradient and confined by the Coriolis force to the equator between 2°N and 2°S (Fig. F1). From August to December, the ITCZ is in its northernmost position and the southeast trade winds are at maximum strength, resulting in a strong SEC and NECC. The SEC carries cool waters from the southeastern upwelling areas to the west, and the NECC carries warm, low-salinity waters to the east out of the WPWP, depressing the thermocline (Donguy and Meyers, 1996). During this interval, the EUC is very weak. From March to May the ITCZ is at its southernmost position and the southeast trade winds are weak. As a result, the SEC and NECC are also weak. The EUC, in contrast, strengthens and causes the thermocline to shoal (Halpern and Weisberg, 1989; Ravelo and Shackleton, 1995). During El Niño events, easterly winds in the tropical Pacific become very weak, warm waters from the WPWP flow to the east, the thermocline deepens, the EUC is very weak, and SST increases, resulting in a decreased west-east SST gradient (Wallace et al., 1998).
Between the Atlantic/Caribbean and the east Pacific, a salinity gradient of 1–1.5 units exists, which is mainly determined by the net transport of water vapor from the Caribbean Sea over Central America into the east Pacific by the trade winds (Weyl, 1968; Zaucker et al., 1994). During the early Pliocene, free exchange of surface watermasses between the east Pacific and the Caribbean was still possible because of the open Panamanian Gateway. A salinity gradient did not develop because the inflow of lower salinity waters from the Pacific into the Caribbean prevented a significant salinity increase in the Caribbean. Haug et al. (2001) and Keigwin (1982) showed that 
18OG.sacculifer records of the east Pacific and the Caribbean started to diverge between 4.7 and 4.2 Ma. This was interpreted as an indication that the Isthmus of Panama had shoaled to a depth of <100 m, thereby restricting the inflow of low-salinity Pacific surface water masses into the Caribbean.
Notably, the present-day east Pacific is characterized by relatively low temperatures. The southeast trade winds cause strong upwelling along the Peruvian coast and along the equator, thus forming the Pacific cold tongue, which extends toward the west along the equatorial divergence (Mitchell and Wallace, 1992). Accordingly, the thermocline in these areas is very shallow (<50 m). In contrast, the Caribbean/western Atlantic warm pool is marked by a distinctly deeper thermocline (100–150 m). At a water depth of 50 m, the temperature contrast between the Caribbean and the east Pacific is 5°–6°C, whereas at the surface, SST is ~27°C, both at Site 1241 and in the Caribbean (Levitus and Boyer, 1994). North of the equator, however, SSTs rise and form the eastern Pacific warm pool (EPWP), which is characterized by seasonally maximum SSTs over 28.5°C and extends between 7°N and 27°N and as far west as 110°W (Wang and Enfield, 2001; Xie et al., 2005). An important feature of the EPWP is upwelling in the Costa Rica Dome (centered at 9°N, 90°W), a permanent cyclonic eddy possibly caused by strong "gap winds" blowing through the Papagayo gap in the Central American cordillera (Umatani and Yamagata, 1991; Xie et al., 2005).
Molnar and Cane (2002) described the early Pliocene as a period of permanent El Niño-like conditions. During a present-day El Niño, the collapse of the easterlies leads to an equatorial Kelvin wave, which is characterized by the eastward flow of warm waters from the WPWP toward the east Pacific. This causes the cessation of upwelling, a decreasing temperature gradient between the WPWP and the east Pacific, and deepening of the thermocline in the east Pacific (Wallace et al., 1998). A permanent El Niño-like climate state during the Pliocene should thus be characterized by significantly higher SSTMg/Ca in the mixed layer than is found in present-day SSTs in the east Pacific, or by lower SSTs in the WPWP. The presence of a deeper thermocline can be confirmed by using the 18O and Mg/Ca records of planktonic foraminifers living at different water depths and calculating their gradients (Steph et al., this volume).
