Thermal response of the western tropical Atlantic to slowdown of the Atlantic Meridional Overturning Circulation

Author(s)

Crivellari, Stefano ^{[1, 2]} ; Chiessi, Cristiano Mazur ^[1] ; Kuhnert, Henning ^[3] ; Haeggi, Christoph ^{[3, 4]} ; Mollenhauer, Gesine ^[5] ; Hefter, Jens ^[5] ; Portilho-Ramos, Rodrigo ^[3] ; Schefuss, Enno ^[3] ; Mulitza, Stefan ^[3]

Affiliation

^[1] Univ Sao Paulo, Sch Arts Sci & Humanities, Sao Paulo - Brazil

^[2] Univ Sao Paulo, Inst Geosci, Sao Paulo - Brazil

^[3] Univ Bremen, MARUM Ctr Marine Environm Sci, Bremen - Germany

^[4] Univ Southern Calif, Dept Earth Sci, Los Angeles, CA - USA

^[5] Alfred Wegener Inst Polar & Marine Res, Bremerhaven - Germany

• Document type: Journal article
• Source: Earth and Planetary Science Letters; v. 519, p. 120-129, AUG 1 2019.
• Web of Science Citations: 0

Abstract

The western tropical Atlantic plays an important role in the interhemispheric redistribution of heat during millennial-scale changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC). The proper evaluation of this role depends on a clear understanding of sea surface temperature (SST) variations during AMOC slowdown periods like Heinrich Stadials (HS) in the western tropical Atlantic. However, published SST records from the western tropical Atlantic between ca. 4 degrees S and 7 degrees N show inconsistencies that are apparently related to the employed temperature proxy (i.e., Mg/Ca versus alkenone unsaturation index U-37(k')). In general, while Mg/Ca values indicate warming during Heinrich Stadials, U-37(k') values show cooling. To assess this issue, we sampled core GeoB16224-1 retrieved off French Guiana (i.e., 6 degrees 39.38'N) and reconstructed water temperatures at high resolution using Mg/Ca on the foraminifera species Globigerinoides ruber, U-37(k'), TEX86 and modern analogue technique (MAT) transfer functions using planktonic foraminifera assemblages calibrated for 50 m water depth. Our results show that Mg/Ca and TEX86 values recorded an increase in SST related to AMOC slowdown. Conversely, U-37(k') and MAT values registered a decrease in temperatures during HS3 and HS1. Our U-37(k') and Mg/Ca results thus confirm the previously reported inconsistency for the period between 48-13 cal ka BP. We suggest that several non-thermal physiological effects probably imparted a negative temperature bias on the U-37(k') temperatures during Heinrich Stadials. However, MAT-based temperatures show similar variability with U-37(k')-based temperatures. Hence, we also suggest that during severe slowdown periods of the AMOC, a steeper meridional temperature gradient together with a southward shift of the Intertropical Convergent Zone produced not only an increase in SST but also a stronger upper water column stratification and a shoaling of the thermocline, decreasing subsurface temperatures. Our new high resolution temperature records allow a better characterization of the thermal response of the upper water column in the tropical western Atlantic to slowdown events of the AMOC, reconciling previously discrepant records. (C) 2019 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 13/22521-2 - Effects of abrupt changes in the Atlantic meridional overturning circulation over deep-water masses: an isotopic approach
• Grantee: Stefano Crivellari
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)
• FAPESP's process: 12/17517-3 - Response of the Western Atlantic Ocean to changes in the Atlantic meridional overturning circulation: from millennial to seasonal variability
• Grantee: Cristiano Mazur Chiessi
• Support Opportunities: Research Program on Global Climate Change - Young Investigators