Andreas Macrander

Interannual changes in the overflow from the Nordic Seas into the Atlantic Ocean through Denmark Strait

The global thermohaline circulation is an important part of Earths climate system. Cold, dense water formed in the Nordic Seas enters the Atlantic Ocean as overflows across the sills of the Greenland-Scotland Ridge. The Denmark Strait Overflow (DSO) is one of the main sources of North Atlantic Deep Water. Until now the DSO has been believed to be stable on interannual timescales. Here, for the first time, evidence is presented from a 4-year program of observations showing that overflow transports in 1999/2000 were approximately 30% higher than previous estimates. Later, transports decreased remarkably during the observation period, coincident with a temporary temperature increase of about 0.5°C.

Validation of GRACE Gravity Fields by In-Situ Data of Ocean Bottom Pressure

GRACE observes the temporally changing gravity field of the Earth with unprecedented accuracy. Compared to the gravity signals of the continental hydrological cycle, local ocean mass variability reflecting ocean current, density and sea level changes are a challenge for the GRACE mission. Hence, validation of GRACE with in-situ observations of ocean bottom pressure is critical to evaluate the capability of GRACE to observe oceanic mass redistribution. Here, GRACE data is compared with in-situ ocean bottom pressure at a hundred sites located in all of the world’s oceans. The advances made by recent GRACE product releases are shown, and gravity fields provided from different data centres are compared. In some regions, particularly at high-latitude sites with comparatively strong ocean bottom pressure variability and dense satellite coverage, GRACE captures oceanic variability quite well. This is a robust feature for all gravity field solutions. In contrast, some other regions show remarkably large differences between different GRACE solutions, suggesting that discrepancies in de-aliasing models play a major role in defining the skill of GRACE to realistically observe local oceanic mass variability.

Modelling the Overflows Across the Greenland–Scotland Ridge

The Atlantic Meridional Overturning Circulation (AMOC) is part of a global redistribution system in the ocean that carries vast amounts of mass, heat, and freshwater. Within the AMOC, water mass transformations in the Nordic Seas (NS) and the overflows across the Greenland-Scotland Ridge (GSR) contribute significantly to the overturning mass transport. The deep NS are separated by the GSR from direct exchange with the subpolar North Atlantic. Two deeper passages, Denmark Strait (DS, sill depth 630 m) and Faroe Bank Channel (FBC, sill depth 840 m), constrain the deep outflow. The outflow transports are assumed to be governed by hydraulic control (Whitehead 1989, 1998). According to the circulation scheme by Dickson and Brown (1994), there is an overflow of 2.9 Sv (1 Sv = 1 Sverdrup = 106 m3 s–1) through DS, 1.7 Sv through FBC and another 1 Sv from flow across the Iceland%Faroe Ridge (IFR). To the south of the GSR, the overflows sink to depth and then spread along the topography, eventually merging to form a deep boundary current in the western Irminger Sea. During the descent, the dense bottom water flow doubles its volume by entrainment of ambient waters (e.g. Price and Baringer 1994) so that there is a deep water transport of 13.3 Sv once the boundary current reaches Cape Farvel (Dickson and Brown 1994). Thus the overflows and the overflow-related part of the AMOC account for more than 70% of the maximum total overturning, which is estimated from observations to be about 18 Sv (e.g. Macdonald 1998)

Improved transport estimate of the East Icelandic Current 2002–2012

The East Icelandic Current (EIC) is one of the major export pathways from the Iceland Sea north of Iceland, carrying mostly cold and fresh waters of Arctic origin. In this study, volume and freshwater transports are estimated using current profiles and salinity time series from a mooring deployed from 2011 to 2012 over the insular slope northeast of Iceland. These data are extended by hydrographic sections spanning the entire EIC four times per year. In combination with altimetry, geostrophic current profiles of the whole section are obtained for the period 2002–2012. The data are analyzed with respect to volume and freshwater transport variability and their relation to atmospheric forcing. The observations show a mean transport of 0.75 ± 0.08 Sv, and a mean freshwater transport of 3.4 ± 0.3 mSv in the upper 170 m. There is large interannual variability which appears to depend more on local conditions rather than large-scale atmospheric forcing. The freshwater transport is small compared to the export in the East Greenland Current.

Liquid freshwater transport estimates from the East Greenland Current based on continuous measurements north of Denmark Strait

Liquid freshwater transports of the shelfbreak East Greenland Current (EGC) and the separated EGC are determined from mooring records from the Kogur section north of Denmark Strait between August 2011 and July 2012. The 11-month mean freshwater transport (FWT), relative to a salinity of 34.8, was 65 ± 11 mSv to the south. Approximately 70% of this was associated with the shelfbreak EGC and the remaining 30% with the separated EGC. Very large southward FWT ranging from 160 mSv to 120 mSv was observed from September to mid-October 2011 and was foremost due to anomalously low upper-layer salinities. The FWT may, however, be underestimated by approximately 5 mSv due to sampling biases in the upper ocean. The FWT on the Greenland shelf was estimated using additional inshore moorings deployed from 2012-14. While the annual mean ranged from nearly zero during the first year to 18 mSv to the south during the second year, synoptically the FWT on the shelf can be significant. Furthermore, an anomalous event in autumn 2011 caused the shelfbreak EGC to reverse, leading to a large reduction in FWT. This reversed circulation was due to the passage of a large, 100 km wide anticyclone originating upstream from the shelfbreak. The late summer FWT of -131 mSv is 150% larger than earlier estimates based on sections in the late-1990s and early-2000s. This increase is likely the result of enhanced freshwater flux from the Arctic Ocean to the Nordic Seas during the early 2010s. This article is protected by copyright. All rights reserved.