Southward Displacement of the North Atlantic Subtropical Gyre Circulation System During North Atlantic Cold Spells

Abstract

During times of deglacial Atlantic Meridional Overturning Circulation (AMOC) perturbations, the tropical Atlantic experienced considerable warming at subsurface levels. Coupled ocean‐atmosphere simulations corroborate the tight teleconnection between the tropical Atlantic and climate change at high northern latitudes but still underestimate the relevance of the subsurface North Atlantic subtropical gyre (STG) for heat and salt storage and its sensitivity to rapid climatic change. We here reconstruct vertical and lateral temperature and salinity gradients in the tropical west Atlantic and the Caribbean over the last 30 kyr, based on planktic deep and shallow dwelling foraminiferal Mg/Ca and δ18O records. The rapid and large‐amplitude subsurface changes illustrate a dynamic STG associated with abrupt shifts of North Atlantic hydrographic and atmospheric regimes. During full glacial conditions, the STG has been shifted southward while intensified Ekman downwelling associated to strengthened trade winds fostered the formation of warm and saline salinity maximum water (SMW). The southward propagation of SMW was facilitated by the glacially eastward deflected North Brazil Current. During periods of significant AMOC perturbations (Heinrich Stadials 1 and the Younger Dryas), extreme subsurface warming by ~6 °C led to diminished lateral subsurface temperature gradients. Coevally, a deep thermocline suggests that SMW fully occupied the subsurface tropical west Atlantic and that the STG reached its southernmost position. During the Holocene, modern‐like conditions gradually developed with the northward retreat of SMW and the development of a strong thermocline ridge between the Subtropical Gyre and the tropical west Atlantic.

Single Tests of Thermocline Dwelling Foraminifera Globorotalia inflata as Recorder of Upper Water Column Structure off Mauritania (NW Africa): Methodology and Paleoceanographic Use

“…A southward shift of the southern edge of the STG, similar to that recorded from the Caribbean (Reißig et al, 2019), would increase exposure of the site to the northern waters of the gyre. The 2°to the north water temperatures are 1.5°C warmer at 150 m, so such a shift could explain the 1.6°C warmer than Holocene temperatures measured for the LGM.…”

“…This is due to the east-west expansion of the STG in the winter months (Stramma & Siedler, 1988) meaning that the site is less peripheral to the gyre in winter. On glacial-interglacial time scales, periods of Northern Hemisphere cooling are associated with still larger shifts in the position of the STG, due to southward displacement of the Intertropical Convergence Zone and major wind belts in the Atlantic, as well as the collapse of the North Brazil Current (Arbuszewski et al, 2013;Peterson et al, 2000;Reißig et al, 2019).…”

“…During the LGM and in times of reduced AMOC, the North Brazil Current, which usually flows as a western boundary current, weakens. This allows warm saline water to fill the tropical Atlantic subsurface (Reißig et al, 2019;Schmidt et al, 2012). Salinity at site GeoB7926 is likely increased by high salinity water entering the gyre circulation in the west Atlantic, as well as higher salinity in the STG, where surface water is entrained to the subtropical cell (Bingham et al, 2014;Snowden & Molinari, 2003).…”

“…Subsurface hydrography is less well explored, although observational studies suggest that the water depths between 300 and 600 m are a key region for AMOC sensitivity (Zhang, 2007). Available subsurface temperature and salinity data tend to be focused on the western side of the Atlantic, as this is where warm and highly saline water forms, which enters the Atlantic subsurface (Reißig et al, 2019;Rühlemann et al, 2004;Schmidt et al, 2012). There are few subsurface records from the tropical East Atlantic (Elderfield & Ganssen, 2000;Huang et al, 2012).…”

Plio‐Pleistocene Variability of the East Pacific Thermocline and Atmospheric Systems

“…For sample treatment, we followed the cleaning procedure of Schmidt and Lynch‐Stieglitz (), supplemented by a final reductive cleaning step described in Reißig et al (). All samples were measured at the GEOMAR, using a Varian 720‐ES ICP OES.…”