A stable Faroe Bank Channel overflow 1995–2015

Bogi Hansen,Karin Margretha Húsgarð Larsen,Hjálmar Hátún,Svein Østerhus

doi:10.5194/os-12-1205-2016

Bogi Hansen, Karin Margretha Húsgarð Larsen + Show 2 more

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https://doi.org/10.5194/os-12-1205-2016

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Journal: Ocean Science	Publication Date: Nov 17, 2016
Citations: 56	License type: CC BY 3.0

Affiliation: Havstovan, Uni Research (Norway)

Abstract

Abstract. The Faroe Bank Channel (FBC) is the deepest passage across the Greenland–Scotland Ridge (GSR) and there is a continuous deep flow of cold and dense water passing through it from the Arctic Mediterranean into the North Atlantic and further to the rest of the world ocean. This FBC overflow is part of the Atlantic Meridional Overturning Circulation (AMOC), which has recently been suggested to have weakened. From November 1995 to May 2015, the FBC overflow has been monitored by a continuous ADCP (acoustic Doppler current profiler) mooring, which has been deployed in the middle of this narrow channel. Combined with regular hydrography cruises and several short-term mooring experiments, this allowed us to construct time series of volume transport and to follow changes in the hydrographic properties and density of the FBC overflow. The mean kinematic overflow, derived solely from the velocity field, was found to be (2.2 ± 0.2) Sv (1 Sv = 106 m3 s−1) with a slight, but not statistically significant, positive trend. The coldest part, and probably the bulk, of the FBC overflow warmed by a bit more than 0.1 °C, especially after 2002, increasing the transport of heat into the deep ocean. This warming was, however, accompanied by increasing salinities, which seem to have compensated for the temperature-induced density decrease. Thus, the FBC overflow has remained stable in volume transport as well as density during the 2 decades from 1995 to 2015. After crossing the GSR, the overflow is modified by mixing and entrainment, but the associated change in volume (and heat) transport is still not well known. Whatever effect this has on the AMOC and the global energy balance, our observed stability of the FBC overflow is consistent with reported observations from the other main overflow branch, the Denmark Strait overflow, and the three Atlantic inflow branches to the Arctic Mediterranean that feed the overflows. If the AMOC has weakened during the last 2 decades, it is not likely to have been due to its northernmost extension – the exchanges across the Greenland–Scotland Ridge.

Highlights

Overflow is the term generally used to describe the bottomintensified flow of cold and dense water from the Nordic seas through the deep passages across the Greenland–Scotland Ridge (GSR) to the North Atlantic (Saunders, 2001)
After crossing the GSR, the overflow is modified by mixing and entrainment, but the associated change in volume transport is still not well known. Whatever effect this has on the Atlantic Meridional Overturning Circulation (AMOC) and the global energy balance, our observed stability of the Faroe Bank Channel (FBC) overflow is consistent with reported observations from the other main overflow branch, the Denmark Strait overflow, and the three Atlantic inflow branches to the Arctic Mediterranean that feed the overflows
The third overflow branch east of Iceland is the deep flow through the Faroe Bank Channel (FBC), FBC overflow, which is the main focus of this study

Summary

Introduction

Overflow is the term generally used to describe the bottomintensified flow of cold and dense water from the Nordic seas through the deep passages across the Greenland–Scotland Ridge (GSR) to the North Atlantic (Saunders, 2001). The strongest overflow branch in terms of volume transport passes through the Denmark Strait. The Denmark Strait overflow (DS overflow) is estimated to transport around 3.5 Sv (1 Sv = 106 m3 s−1) of dense (σθ > 27.8 kg m−3) water (Jochumsen et al, 2012; Harden et al, 2016). A similar amount of overflow is generally considered to pass east of Iceland in three branches where the overflow across the Wyville Thomson Ridge is weak (< 0.3 Sv; Østerhus et al, 2008). With an average volume transport close to 2 Sv, the FBC overflow is generally estimated to contribute ca. one-third of the total overflow, and is second only to the DS overflow (Østerhus et al, 2008)

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