Deglacial Tropical Atlantic subsurface warming links ocean circulation variability to the West African Monsoon

Matthew W Schmidt,Link Ji,Feng He,Ping Chang,Andrew O Parker

doi:10.1038/s41598-017-15637-6

Abstract

Multiple lines of evidence show that cold stadials in the North Atlantic were accompanied by both reductions in Atlantic Meridional Overturning Circulation (AMOC) and collapses of the West African Monsoon (WAM). Although records of terrestrial change identify abrupt WAM variability across the deglaciation, few studies show how ocean temperatures evolved across the deglaciation. To identify the mechanism linking AMOC to the WAM, we generated a new record of subsurface temperature variability over the last 21 kyr based on Mg/Ca ratios in a sub-thermocline dwelling planktonic foraminifera in an Eastern Equatorial Atlantic (EEA) sediment core from the Niger Delta. Our subsurface temperature record shows abrupt subsurface warming during both the Younger Dryas (YD) and Heinrich Event 1. We also conducted a new transient coupled ocean-atmosphere model simulation across the YD that better resolves the western boundary current dynamics and find a strong negative correlation between AMOC strength and EEA subsurface temperatures caused by changes in ocean circulation and rainfall responses that are consistent with the observed WAM change. Our combined proxy and modeling results provide the first evidence that an oceanic teleconnection between AMOC strength and subsurface temperature in the EEA impacted the intensity of the WAM on millennial time scales.

Highlights

Recent droughts across West Africa resulted in catastrophic socio-economic impacts on the developing nations in the region and highlight the need for a better understanding of the causes for these events[1]
Modeling results from ref.[13] showed that a similar reduction in Atlantic Meridional Overturning Circulation (AMOC) under Last Glacial Maximum (LGM) boundary conditions resulted in a subsurface warming across the tropical Atlantic, suggesting this oceanic mechanism operated in the past
Deep dwelling planktonic foraminifera can migrate vertically in the water column through time[19,20], a deeper depth habitat for G. crassaformis would still place it within the depth range of subsurface warming in the Eastern Equatorial Atlantic (EEA) associated with an abrupt decrease in AMOC strength based on the modeling results of our new transient Younger Dryas (YD) simulation

Summary

Introduction

Recent droughts across West Africa resulted in catastrophic socio-economic impacts on the developing nations in the region and highlight the need for a better understanding of the causes for these events[1]. Modern observational data and modeling studies show that a slowdown in the AMOC results in a subsurface warming in the tropical Atlantic, which, in turn, can impact the land-sea temperature contrast via upwelling and impact the strength of the WAM refs[7,8]. Modeling results from ref.[13] showed that a similar reduction in AMOC under Last Glacial Maximum (LGM) boundary conditions resulted in a subsurface warming across the tropical Atlantic, suggesting this oceanic mechanism operated in the past In these modeling experiments, the subsurface warming is propagated into the EEA where it impacts sea surface temperatures (SSTs) in the EEA cold tongue through equatorial and coastal upwelling. We use Mg/Ca ratios in the thermocline dwelling planktonic foraminifera Globorotalia crassaformis, (modern depth habitat between 200–270 m in the EEA ref.16), as a proxy for thermocline temperature variability

Methods

Results

Conclusion