Wind forcing of salinity anomalies in the Denmark Strait overflow

S. Hall,K. J. Heywood,S. R. Dye,M. R. Wadley

doi:10.5194/os-7-821-2011

S. Hall, K. J. Heywood + Show 2 more

Open Access

https://doi.org/10.5194/os-7-821-2011

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Abstract

Abstract. The overflow of dense water from the Nordic Seas to the North Atlantic through Denmark Strait is an important part of the global thermohaline circulation. The salinity of the overflow plume has been measured by an array of current meters across the continental slope off the coast of Angmagssalik, southeast Greenland since September 1998. During 2004 the salinity of the overflow plume changed dramatically; the entire width of the array (70 km) freshened between January 2004 and July 2004, with a significant negative salinity anomaly of about 0.06 in May. The event in May represents a fresh anomaly of over 3 standard deviations from the mean since recording began in 1998. The OCCAM 1/12° Ocean General Circulation Model not only reproduces the 2004 freshening event (r=0.96, p<0.01), but also correlates well with salinity observations over a previous 6 year period (r=0.54, p<0.01), despite the inevitable limitations of a z-coordinate model in representing the mixing processes at and downstream of the Denmark Strait sill. Consequently the physical processes causing the 2004 anomaly and prior variability in salinity are investigated using the model output. Our results reject the hypotheses that the anomaly is caused by processes occurring between the overflow sill and the moorings, or by an increase in upstream net freshwater input. Instead, we show that the 2004 salinity anomaly is caused by an increase in volume flux of low salinity water, with a potential density greater than 27.60 kg m−3, flowing towards the Denmark Strait sill in the East Greenland Current. This is caused by an increase in southward wind stress upstream of the sill at around 75° N 20° W four and a half months earlier, and an associated strengthening of the East Greenland Current.

Highlights

The overflow of cold, dense water from the Nordic Seas to the Atlantic Ocean across the Greenland-Scotland Ridge is an important component of the global thermohaline circulation
This system of overflow and entrainment is the main contributor to North Atlantic Deep Water (NADW), the deep limb of the global thermohaline circulation (Dickson and Brown, 1994)
They speculated that stronger southward winds 27.8 isopycnal separates Iceland Scotland Overflow Water (ISOW) from the overlying Labrador could result in a greater volume of low salinity water flowing Sea Water (LSW), formed by wintertime convection in the from the north towards Denmark Strait in the East Greenland Current (EGC), with a Labrador Sea (Yashayaev, 2007) and Irminger Sea

Summary

Introduction

The overflow of cold, dense water from the Nordic Seas to the Atlantic Ocean across the Greenland-Scotland Ridge is an important component of the global thermohaline circulation. We use the DSOW definition of Dickson et al (2008) as water with a potential density (σθ ) greater than 27.85 (values of σθ will be quoted without units here) This system of overflow and entrainment is the main contributor to North Atlantic Deep Water (NADW), the deep limb of the global thermohaline circulation (Dickson and Brown, 1994). In addition to DSOW, the surface waters of the East Greenland Current (EGC) flow south through Denmark Strait, and Pickart et al (2005) identified a separate southward current formed from dense water cascading over the East Greenland shelf edge south of the sill, known as the East Greenland Spill Jet. The purpose of this paper is to establish the cause of observed salinity anomalies in DSOW.

Salinity time series at the Angmagssalik moorings

10 August 2000 to 13 June 2002

Findings

Summary