Impact of abrupt deglacial climate change on tropical Atlantic subsurface temperatures

Abstract

Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface experiences an even larger warming because of rapid reorganizations of ocean circulation patterns at intermediate water depths. Here, we reconstruct high-resolution temperature records using oxygen isotope values and Mg/Ca ratios in both surface- and subthermocline-dwelling planktonic foraminifera from a sediment core located in the TNA over the last 22 ky. Our results show significant changes in the vertical thermal gradient of the upper water column, with the warmest subsurface temperatures of the last deglacial transition corresponding to the onset of the Younger Dryas. Furthermore, we present new analyses of a climate model simulation forced with freshwater discharge into the North Atlantic under Last Glacial Maximum forcings and boundary conditions that reveal a maximum subsurface warming in the vicinity of the core site and a vertical thermal gradient change at the onset of AMOC weakening, consistent with the reconstructed record. Together, our proxy reconstructions and modeling results provide convincing evidence for a subsurface oceanic teleconnection linking high-latitude North Atlantic climate to the tropical Atlantic during periods of reduced AMOC across the last deglacial transition.

Millennial-scale tropical atmospheric and Atlantic Ocean circulation change from the Last Glacial Maximum and Marine Isotope Stage 3

“…1). The Florida Current overlaying the core site is comprised of wind-driven recirculated gyre water (∼17 Sv) and cross-equatorial flow from the South Atlantic (∼13 Sv) (Schmitz and McCartney, 1993) forming the northward flowing surface branch of the AMOC. The waters in the Florida Current are sourced from the Caribbean, the tropical Atlantic, and western tropical South Atlantic, and travel through the Yucatan Straights and the Gulf of Mexico before reaching the Florida Straits (Murphy et al, 1999;Schmitz and Richardson, 1991).…”

Tropical North Atlantic subsurface warming events as a fingerprint for AMOC variability during Marine Isotope Stage 3

“…Modeling results in Schmidt et al . [] showed that a major reduction in AMOC under Last Glacial Maximum (LGM) forcings and boundary conditions can produce a strong subsurface warming in the western TNA and equatorial Atlantic, providing evidence this mechanism could have operated under glacial conditions. Schmidt et al .…”

“…Schmidt et al . [] also analyzed Mg/Ca ratios in the subthermocline dwelling planktonic foraminifera Globorotalia crassaformis in core VM12‐107 from the Southern Caribbean to reconstruct a record of deglacial subsurface temperature change (see Figure for core location). Their results showed significant subsurface warming at the start of the Younger Dryas and Heinrich event 1 that covaried with changes in AMOC variability across the deglacial, demonstrating a link between AMOC weakening and subsurface warming in the TNA, likely through the first step of the proposed mechanism.…”

The role of North Brazil Current transport in the paleoclimate of the Brazilian Nordeste margin and paleoceanography of the western tropical Atlantic during the late Quaternary

“…Most climate models simulating slowing of AMOC transport (whether addressing global warming or past NACE events) produce a distinctive North Atlantic-South Atlantic SST anomaly dipole with much less change of the SST in the equatorial Atlantic region (e.g., Vellinga and Wood, 2002, Zhang and Delworth, 2005, Liu et al, 2009, Zhang et al, 2011, Menary et al, 2012and Schmidt et al, 2012). …”