LAUSR

The mean state and variability of the tropical Pacific is influenced by the depth of the thermocline. During the Last Glacial Maximum (~21,000 years ago), the zonal sea surface temperature gradient across the equatorial Pacific was reduced and productivity was generally lower than modern. To understand the thermocline depth’s role in determining the Last Glacial Maximum tropical mean state, we reconstruct the upper ocean δO profile from multiple species of planktic foraminifera. We synthesize existing records of surface and subsurface dwelling foraminifera to reconstruct the vertical δO gradient throughout the eastern equatorial Pacific. We find the thermocline was deeper during the Last Glacial Maximum than the Holocene throughout the eastern equatorial Pacific region. The thermocline depth’s role in the dynamic forcing of the cold tongue contributed to the reduced zonal SST gradient across the equatorial Pacific, decreased productivity, and presumably impacted El Niño-Southern Oscillation variability relative to the Holocene. Plain Language Summary The eastern equatorial Pacific is cold and biologically rich because of a shallow thermocline. The thermocline is a subsurface ocean horizon where temperature rapidly decreases with depth. This shallow thermocline in the eastern equatorial Pacific is a key component of the ocean-atmospheric connection that characterizes the tropical Pacific and El Niño-Southern Oscillation. But how was the thermocline different in the past? Here we reconstructed the thermocline during the most recent ice age and found the thermocline was deeper than today. This deep thermocline contributed to the overall temperature pattern of the tropical Pacific, decreased the biological productivity and likely influenced the El Niño-Southern Oscillation.