Dynamical and observational constraints on tropical Pacific sea surface temperatures at the last glacial maximum

Asynchronously coupled atmosphere and ocean general circulation model simulations are used to examine the consequences of changes in the west/east sea-surface t mperature (SST) gradient across the equatorial Pacific at the last glacial maximum (LGM). Simulations forced by the CLIMAP SST for the LGM, where the west/east SST gradient across the Pacific is reduced compared to present, produce a reduction in the strength of the trade winds and a decrease in the west/east slope of the equatorial thermocline that is incompatible with thermocline depths newly inferred from foraminiferal assemblages. Strongerthan-present trade winds, and a more realistic simulation of the thermocline slope, are produced when eastern Pacific SSTs are 2øC cooler than western Pacific SSTs. Our study highlights the importance of spatial heterogeneity in tropical SSTs in determining key features of the glacial climate. The first synthesis of sea-surface t mperatures (SSTs) at the last glacial maximum (LGM: ca 21,000 calendar yr B.P.), made by the CLIMAP Project (CLIMAP, 1981) indicated that the average cooling of the tropics was ca 2øC. Climate simulations in which the SSTs were prescribed from the CLIMAP data set were unable to reproduce the magnitude of the cooling inferred from the depression of snowlines on tropical mountains (Rind and Peteet, 1985). Several lines of evidence now suggest hat the tropical SST cooling might have been larger than shown by CLIMAP. Studies based on noble gas palaeothermometry and Sr/Ca ratios in corals have shown cooling of--5-6øC in several regions (e.g. Guilderson et al., 1994; Stute M. et al., 1995). However, new syntheses of both terrestrial evidence for tropical lowland cooling (Farrera et al., 1999) and alkenone-estimates of SST changes in the tropical Indian Ocean (Bard et al., 1997) limit the plausible magnitude of cooling in some tropical regions to less than 3øC. One key aspect of the CLIMAP synthesis that has been overlooked in the current debate about tropical cooling is the strong geographical patterning of SST anomalies. In the tropical Pacific, the SST anomaly shows a distinct pattern with a reduced Copyright 2000 by the American Geophysical Union. Paper number 1999GL002321. 0094-8276/00/1999GL002321505.00 west/east SST gradient relative to present. The SST cooling is at the order of 2øC in the western Pacific warm pool, but smaller in the eastern tropical Pacific (Fig. l). The CLIMAP SSTs in the western Pacific are consistent with terrestrial evidence from New Guinea and Indonesia for relatively little change (<2øC) in temperature at low elevations (Farrea et al., 1999). In contrast, the CLIMAP SST in the eastern tropical Pacific, which shows little cooling (or even warming in the southern eastern tropical Pacific), seems to be inconsistent with other observations. Recent analysis of radiolarian assemblages (Pisias and Mix, 1997) indicates that the SSTs in the eastern equatorial Pacific were 3 to