Holocene evolution of the North Atlantic subsurface transport

Janne Repschläger,Dieter Garbe-Schönberg,Ralph Schneider,Mara Weinelt

doi:10.5194/cp-13-333-2017

Janne Repschläger, Dieter Garbe-Schönberg + Show 2 more

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https://doi.org/10.5194/cp-13-333-2017

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Abstract

Abstract. Previous studies suggested that short-term freshening events in the subpolar gyre can be counterbalanced by advection of saline waters from the subtropical gyre and thus stabilize the Atlantic Meridional Overturning Circulation (AMOC). However, little is known about the inter-gyre transport pathways. Here, we infer changes in surface and subsurface transport between the subtropical and polar North Atlantic during the last 11 000 years, by combining new temperature and salinity reconstructions obtained from combined δ18O and Mg ∕ Ca measurements on surface and subsurface dwelling foraminifera with published foraminiferal abundance data from the subtropical North Atlantic, and with salinity and temperature data from the tropical and subpolar North Atlantic. This compilation implies an overall stable subtropical warm surface water transport since 10 ka BP. In contrast, subsurface warm water transport started at about 8 ka but still with subsurface heat storage in the subtropical gyre. The full strength of intergyre exchange was probably reached only after the onset of northward transport of warm saline subsurface waters at about 7 ka BP, associated with the onset of the modern AMOC mode. A critical evaluation of different potential forcing mechanisms leads to the assumption that freshwater supply from the Laurentide Ice Sheet was the main control on subtropical to subpolar ocean transport at surface and subsurface levels.

Highlights

The Holocene, though often considered a generally stable warm climate mode, is characterized by distinct long-term climate trends, superimposed by strong oscillations on millennial to decadal timescales (e.g., Mayewski et al, 2004).The long-term evolution is formally divided into the early Holocene (11.6 to 8.2 ka BP), the mid-Holocene (8.2 to 4.3 ka BP), and the late Holocene (4.3–0 ka BP) (Walker et al, 2012)
In this study we show a stepwise evolution of the mixed layer temperature and salinity at the Azores Front (AF) during the Holocene that is closely linked to a northward migration of the AF and related transport of warm water within the North Atlantic Current (NAC) system
Subsurface transport into the North Atlantic evolved in three phases: an early Holocene meltwater phase (11–8 ka BP) with no subsurface transport, a mid-Holocene transitional phase (8–6 ka BP) with subsurface transport that reached the AF but not the subpolar NA and a late Holocene to modern phase (6–0 ka BP) with subsurface transport into the subpolar NA that coincides with the onset of Labrador Sea Water (LSW) formation

Summary

Introduction

The Holocene, though often considered a generally stable warm climate mode, is characterized by distinct long-term climate trends, superimposed by strong oscillations on millennial to decadal timescales (e.g., Mayewski et al, 2004).The long-term evolution is formally divided into the early Holocene (11.6 to 8.2 ka BP), the mid-Holocene (8.2 to 4.3 ka BP), and the late Holocene (4.3–0 ka BP) (Walker et al, 2012). Northern hemispheric insolation changes are inferred to control strength and position of the global wind systems and to have caused the early Holocene thermal maximum observed in the high northern latitudes (e.g., Leduc et al, 2010; Moros et al, 2006) This warming induced the final melting of glacially extended ice sheets and released a significant amount of meltwater from the Laurentide Ice Sheet into the North Atlantic (e.g., Jennings et al, 2015). With average duration of about 1500 years, are observed in many Holocene climate records, and are superimposed on the long-term trend The causes of these cycles are controversially discussed, with hypotheses ranging from meltwater pulses induced by variations in solar irradiance to internal ocean–ice–atmosphere feedback mechanisms or volcanism (e.g., Andrews and Giraudeau, 2003; Bond et al, 2001; Campbell et al, 1998; Schulz et al, 2004; Viau et al, 2006; Wanner, 2008). It is assumed that the stabilization is related to a strong Atlantic Meridional Overturning Circulation (AMOC), driven by deep-water formation in the Nordic seas and the Labrador Sea, and fueled by the northward transport of warm saline

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