Reversed North Atlantic gyre dynamics in present and glacial climates

Marisa Montoya,Anders Levermann,Andreas Born

doi:10.1007/s00382-009-0729-y

Abstract

The dynamics of the North Atlantic subpolar gyre (SPG) are assessed under present and glacial boundary conditions by investigating the SPG sensitivity to surface wind-stress changes in a coupled climate model. To this end, the gyre transport is decomposed in Ekman, thermohaline, and bottom transports. Surface wind-stress variations are found to play an important indirect role in SPG dynamics through their effect on water-mass densities. Our results suggest the existence of two dynamically distinct regimes of the SPG, depending on the absence or presence of deep water formation (DWF) in the Nordic Seas and a vigorous Greenland–Scotland ridge (GSR) overflow. In the first regime, the GSR overflow is weak and the SPG strength increases with wind-stress as a result of enhanced outcropping of isopycnals in the centre of the SPG. As soon as a vigorous GSR overflow is established, its associated positive density anomalies on the southern GSR slope reduce the SPG strength. This has implications for past glacial abrupt climate changes, insofar as these can be explained through latitudinal shifts in North Atlantic DWF sites and strengthening of the North Atlantic current. Regardless of the ultimate trigger, an abrupt shift of DWF into the Nordic Seas could result both in a drastic reduction of the SPG strength and a sudden reversal in its sensitivity to wind-stress variations. Our results could provide insight into changes in the horizontal ocean circulation during abrupt glacial climate changes, which have been largely neglected up to now in model studies.

Highlights

During the past 800,000 years, climate has been relatively cold with ice sheets covering large parts of the Northern Hemisphere continents most of the time
Numerical ice sheet modeling could improve the understanding of this period but most studies focus on the primary inception regions over North America and western Siberia (Marshall and Clarke, 1999; Zweck and Huybrechts, 2005; Calov et al, 2005a; Kubatzki et al, 2006; Charbit et al, 2007; Peyaud et al, 2007)
Ice caps grow outside Scandinavia, most notably on the islands in the Barents and Kara Seas and in western and far-eastern continental Siberia. These results show that sea surface temperatures have strong impact on the mass balance in Scandinavia

Summary

Introduction

During the past 800,000 years, climate has been relatively cold with ice sheets covering large parts of the Northern Hemisphere continents most of the time. Numerical ice sheet modeling could improve the understanding of this period but most studies focus on the primary inception regions over North America and western Siberia (Marshall and Clarke, 1999; Zweck and Huybrechts, 2005; Calov et al, 2005a; Kubatzki et al, 2006; Charbit et al, 2007; Peyaud et al, 2007) This is probably due to the lack of constraints on timing and extent of ice from data, and because the steep Scandinavian topography and variable climate are a challenge for the realistic representation of glaciers. Despite low insolation levels at 115 ka and ice growth at similar latitudes over North America and Siberia, climate is too warm over Scandinavia to support ice growth (Fig. 1) This result is robust and no ice grows even in years with cold summers. Probably through its impact on the sea ice edge, plays an important role for surface air temperature and land ice growth over Scandinavia (Fig. 4). Cooling the sea surface by 4 K allows additional ice growth over northern

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Conclusions