Steingrímur Jónsson

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.

Seasonal and interannual variability of wind stress curl over the Nordic Seas

Over 32 years of wind data, which are based on surface pressure maps, from the Hindcast data base that has been developed by the Norwegian Meteorological Institute have been used to study the wind stress curl field, over the Nordic Seas. The mean wind stress curl pattern is characterized by very large values over most of the area, especially over the Greenland Sea. Within the framework of Sverdrup dynamics this gives rise to a cyclonic circulation in the area, with a maximum transport in the western boundary current of about 35 Sv. Only one gyre is present, and its center is situated between Jan Mayen and Greenland. The seasonal variation of the wind stress curl is very large, with almost negligible values during summer, from May through August. During September the wind stress curl starts to build up and has reached its full winter strength by November. This maximum is maintained until April, when it begins to decline to typical summer values. In order to study the spatial and temporal scales of the wind stress curl, an empirical orthogonal function analysis was performed on the wind stress curl after filtering it with a half-power period of 50 days and then resampling the wind stress curl every 30 days. A Monte Carlo method is used to estimate the statistical significance of the various modes. The first 11 modes are found to be significant, and they represent 83.6% of the total variance of the data set. The first mode clearly shows the seasonal fluctuations of the mean wind stress curl pattern, representing the seasonal variation in the intensity of the Icelandic low. The next two modes show mostly interannual variability, with a decadal time scale. Most of these variations are happening in the northern part of the Greenland Sea. The other significant modes are mainly describing variability that is limited either in space or in time. A large part of this anomalous variability occurs in the 1980s.

The structure and atmospheric forcing of the mesoscale velocity field in Fram Strait

We consider the connection between the wind field and the mesoscale circulation in the northern Greenland Sea, using over 50 record years of current observations to define the current structure in space and time. We find that east of the East Greenland Polar Front the motion is highly coherent in the vertical and that it scales in the horizontal as the internal Rossby radius. The structure depends very little on frequency. The East Greenland Current is dominated by smaller-scale phenomena in both space and time, and the kinetic energy is low. The available energy in the wind field may be the largest in the world, especially over the Greenland and Iceland seas. The wind power shows a marked seasonal signal that east of the East Greenland Polar Front is paralleled by a similar signal in the fluctuating kinetic energy of the current, and the former leads the current fluctuations by 1–2 months. The mesoscale motions are probably dominated by nonlinear interactions, and the observed vertical and horizontal structure is in general agreement with predictions of the theory of quasi-geostrophic turbulence forced by atmospheric fluctuations. We conclude that the majority of the mesoscale fluctuations in Fram Strait east of the East Greenland Polar Front are generated by the wind field. However, with few exceptions, the current field is not coherent with the curl of the wind stress, probably primarily owing to the isolating effect of the rough topography. The extent to which the wind field also generates mesoscale motions in the East Greenland Current is uncertain, although the eddy kinetic energy east of the East Greenland Polar Front is probably sufficient to account for conditions in the East Greenland Current by westward advection of eddies.

Freshwater Fluxes East of Greenland

The northern North Atlantic features areas of strong surface cooling which sets up a southward flow of sinking cold, dense waters in the deep ocean. A northward flow of warm surface waters replaces the sinking southbound waters; a loop often termed the meridional overturning circulation (MOC). The associated northward heat transport is an important moderator of the high latitude climate along the flow. It is a major concern that excessive amounts of freshwater added to the northern North Atlantic could alter the dense water formation and associated ocean density contrasts driving this part of the MOC (Hakkinen 1999; Haak et al. 2003). The region has been undergoing a remarkable freshening since the mid-1960s (Curry and Mauritzen 2005; Curry et al. 2003; Dickson et al. 2002). Sources of freshwater input are runoff from Greenland, net precipitation, and export of freshwater from the Arctic in the form of sea ice and melt water through Fram Strait and the Canadian Archipelago. In the late 1960s a major freshening event contributed with as much as half of the extra freshwater required to account for the observed 1965– 1995 freshening (Curry and Mauritzen 2005). The event was labeled the Great Salinity Anomaly (GSA) (Dickson et al. 1988), and appeared as extraordinarily fresh water circulating in the Subpolar gyre during the 1970s. The freshwater release has been attributed to an anomalous export of sea ice through Fram Strait during the late 1960s (Hakkinen 1993; Karcher et al. 2005). Pulses of excess freshwater and sea ice appear to have been emitted from the Arctic also after the GSA;

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.

4. Recent developments in oceanographic research in icelandic waters

The ocean around Iceland has much to offer in terms of interesting physical oceanographic phenomena. Among these phenomena are great contrasts in water mass properties, strong atmospheric forcing in the area, both dynamic and thermodynamic, and the fact that a large part of the thermohaline circulation of the world ocean has its origin there or passes over the ridges on either side of the country. Icelandic waters also show large interannual and decadal variability. In recent years, research has been more oriented towards quantitative studies. In this paper we will illustrate some of those properties using examples from recent research projects carried out in the area. Oceanographic research in the area around Iceland has a long history. In the 19th and the beginning of the 20th century, the research was mostly conducted by other nations. Now the oceanographic research is done mostly by Icelandic institutions, often in close co-operation with foreign colleagues.

Measurement and analysis of currents along the Danish west coast

Current meter records from summer and late fall at three positions have been analyzed and related to sea level and wind data. Spectral analysis shows that the most energetic fluctuations are due to tides with an amplitude corresponding to 0.2 m/s. The variability is mainly in the alongshore direction for tidal and subtidal frequencies. The adjusted sea level, which is decreasing northwards, has the strongest response for winds from land in the wintertime. In the summer a balance between bottom friction and alongshore wind stress is found with a resistance coefficient of 0.1 cm/s, while a more complicated balance exists in the winter. The mean flow during summer is about 2 cm/s. For an estimated length scale of 170 km this corresponds to an alongshore transport of 0.15·106m3/s (=0.15 Sv).

On the Nature of the Mesoscale Variability in Denmark Strait

AbstractTime series data from a mooring in the center of Denmark Strait and a collection of shipboard hydrographic sections occupied across the sill are used to elucidate the mesoscale variability of the dense overflow water in the strait. Two dominant, reoccurring features are identified that are referred to as a bolus and a pulse. A bolus is a large, weakly stratified lens of overflow water associated with cyclonic rotation and a modest increase in along-stream speed of 0.1 m s−1. When a bolus passes through the strait the interface height of the overflow layer increases by 60 m, and the bottom temperature decreases by 0.4°C. By contrast, a pulse is characterized by anticyclonic rotation, a strong increase in along-stream speed of >0.25 m s−1, a decrease in interface height of 90 m, and no significant bottom temperature signal. It is estimated that, on average, boluses (pulses) pass through the strait every 3.4 (5.4) days with no seasonal signal to their frequency. Both features have the strongest along...

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.

A comparison between wind stresses, based on geostrophically derived and observed winds, at Weather Ship M in the Norwegian sea

Abstract A comparison is made of wind stresses based on geostrophically derived winds from the Hindcast database, that covers all the Nordic Seas, and observed winds at the Weather Ship M at 66°N, 2°E in the Norwegian Sea. In the time domain the two data sets agree very well. A vector EOF-analysis shows a first mode that accounts for about 90% of the total variance of the two data sets, and it is almost equally distributed among the two series. The spatial eigenvector of the first mode also shows a very similar structure for the two series. In the frequency domain the geostrophically derived wind stresses are not representative for the real (observed) wind stresses for periods less than about two days. For longer periods, however, there is a high inner coherence with phase close to zero between the two data sets. The database would therefore be an excellent tool for studying wind driven low frequency motion in the ocean.