A Deep Eastern Equatorial Pacific Thermocline During the Last Glacial Maximum

H L Ford,J E Hertzberg,J F Mcmanus,C L Mcchesney

doi:10.1029/2018gl079710

Abstract

AbstractThe 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 δ18O profile from multiple species of planktic foraminifera. We synthesize existing records of surface and subsurface dwelling foraminifera to reconstruct the vertical δ18O 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.

Highlights

The thermocline is a vital component of the coupled ocean-atmospheric system that determines the mean state and variability of the tropical Pacific (Fiedler & Talley, 2006)
We find the thermocline was deeper during the Last Glacial Maximum than the Holocene throughout the eastern equatorial Pacific region
The multispecies reconstruction at Site 849 indicates the thermocline strength was weaker and the thermocline depth was deeper during the Last Glacial Maximum (LGM) relative to the Holocene (Figure 2)

Summary

Introduction

The thermocline is a vital component of the coupled ocean-atmospheric system that determines the mean state and variability of the tropical Pacific (Fiedler & Talley, 2006). The eastern equatorial Pacific (EEP; Figure 1) is characterized by a cold tongue and an Eastern Pacific Warm Pool (EPWP). The thermocline is shallow in the east; in the cold tongue, upwelling favorable winds bring cold, nutrient rich water to the surface, whereas in the EPWP the surface is warm and well stratified. The western equatorial Pacific has warm sea surface temperatures (SSTs) with a deep thermocline (Rebert et al, 1985). The cold eastern and the warm western equatorial Pacific create a strong zonal SST gradient that generates strong atmospheric circulation, which reinforces the zonal SST gradient and thermocline tilt (deep in the west, shallow in the east). Perturbations in the thermocline depth, zonal SST gradient, and atmospheric circulation generate El NiñoSouthern Oscillation (ENSO) events (Guilyardi et al, 2009)

Methods

Results

Conclusion