Revised circulation scheme north of the Denmark Strait (original) (raw)

On the hydrography of Denmark Strait

Journal of Geophysical Research: Oceans

Using 111 shipboard hydrographic sections across Denmark Strait occupied between 1990 and 2012, we characterize the mean conditions at the sill, quantify the water mass constituents, and describe the dominant features of the Denmark Strait Overflow Water (DSOW). The mean vertical sections of temperature, salinity, and density reveal the presence of circulation components found upstream of the sill, in particular the shelfbreak East Greenland Current (EGC) and the separated EGC. These correspond to hydrographic fronts consistent with surface-intensified southward flow. Deeper in the water column the isopycnals slope oppositely, indicative of bottom-intensified flow of DSOW. An end-member analysis indicates that the deepest part of Denmark Strait is dominated by Arctic-Origin Water with only small amounts of Atlantic-Origin Water. On the western side of the strait, the overflow water is a mixture of both constituents, with a contribution from Polar Surface Water. Weakly stratified ''boluses'' of dense water are present in 41% of the occupations, revealing that this is a common configuration of DSOW. The bolus water is primarily Arctic-Origin Water and constitutes the densest portion of the overflow. The boluses have become warmer and saltier over the 22 year record, which can be explained by changes in end-member properties and their relative contributions to bolus composition.

Fates and Travel Times of Denmark Strait Overflow Water in the Irminger Basin

Journal of Physical Oceanography, 2013

The Denmark Strait Overflow (DSO) supplies about one third of the North Atlantic Deep 6 Water and is critical to the global thermohaline circulation. Knowledge of the pathways of 7 DSO through the Irminger Basin and its transformation there is still incomplete however. We 8 deploy over 10,000 Lagrangian particles at Denmark Strait in a high resolution ocean model 9 to study these issues. The particle trajectories show that: First, the mean-position and 10 potential density of dense waters cascading over the Denmark Strait sill evolve consistently 11 with hydrographic observations. These sill particles transit the Irminger basin to the Spill Jet 12 section (65.25 o N) in 5-7 days and to the Angmagssalik section (63.5 o N) in two-three weeks. 13 Second, the dense water pathways on the continental shelf are consistent with observations 14 and particles released on the shelf in the Strait constitute a significant fraction of the dense 15 water particles recorded at the Angmagssalik section within 60 days (∼ 25%). Some particles 16 circulate on the shelf for several weeks before they spill off the shelf break and join the 17 overflow from the sill. Third, there are two places where the water density following particle 18 trajectories decreases rapidly due to intense mixing: southwest of the sill and southwest of 19 the Kangerdlugssuaq Trough on the continental slope. After transformation in these places, 20 the overflow particles exhibit a wide range of densities. 21 1 23 waters formed in the Arctic Ocean and the Nordic Seas. The dense waters pass through the 24 Irminger Basin toward the North Atlantic where they supply about one third of the North 25 Atlantic Deep Water, a major component of the global thermohaline circulation (Dickson 26 and Brown 1994). The DSO transmits the climate signals from its source regions, modified 27 en route by mixing and entrainment, and affects the properties throughout the water column 28 in the North Atlantic (Dickson et al. 2008; Yashayaev and Dickson 2008). 29 The 620-m deep Denmark Strait (DS) sill is the main gateway for dense waters exiting 30 the Greenland Sea to the Irminger Basin and is a key location for observing the DSO at 31 the start of its transit to the North Atlantic (Dickson et al. 2008). Measurements show that 32 the dense overflow through the sill is fast (speeds frequently exceed 1 m/s) and occurs as 33 pulses of dense water (referred to as boluses) cascading to the deep water south of the sill 34 at intervals of 2-5 days. On longer timescales, DSO appears as a steadier and hydraulically 35 controlled flow with a mean transport of approximately 3 Sv (1 Sv = 10 6 m 3 s −1 ; Käse and 36 Oschlies 2000; Macrander et al. 2007; Jochumsen et al. 2012). DSO temperature and salinity 37 vary on a timescale of a few-days, owing to mesoscale activity and intense mixing processes 38 near the sill (Rudels et al. 1999; Tanhua et al. 2005). The seasonal signals in the DSO 39 transport and properties are weak (Dickson and Brown 1994; Jochumsen et al. 2012). The 40 overflow composition exhibits interannual-to-decadal variations, however, most likely linked 41 to changes in the upstream source waters or pathways (Rudels et al. 2002a). These changes 42 in turn are possibly linked to atmospheric forcing and in particular to variations in the North 43 Atlantic Oscillation (Yashayaev and Dickson 2008; Serra et al. 2010).

Variability of the Denmark Strait overflow: A numerical study

Journal of Geophysical Research, 2001

Mesoscale variability in the Denmark Strait overflow is investigated using a version of the Miami Isopycnic Coordinate Ocean Model that includes a recently developed intermittent and vigorous, turbulent and diffusive diapycnal mixing scheme. Earlier idealized modeling work on the subject is further extended in that a realistic replica of the shelf-slope topography and irregular coastline geometry of the Denmark Strait area is also employed. Compared with earlier studies, our experiments reveal that (1) the introduction of the new diapycnal scheme and the true shelf-slope topography do not affect the formation of the intense cyclonic eddies south of the sill and along the continental slope and (2) the new diapycnal scheme has a significant effect on the bottom plume, in that the distribution, volume transport and properties of the bottom plume appear to be more realistic and more in line with what is observed. The first conclusion adds support to the vortex-stretching mechanism suggested in earlier studies as the cause for the formation of the cyclonic eddies. This mechanism is therefore a robust feature and provides an explanation for the observed variability in the Denmark Strait overflow. The second conclusion underscores the importance of including realistic parameterizations of diapycnal mixing in isopycnal coordinate ocean models. Specifically, the results show that the water that descends along the bottom south of the sill becomes lighter and is more voluminous compared to experiments without the new mixing scheme. Even when including the mixing scheme, the bottom plume separates into distinct boluses of enhanced thickness. Moreover, these boluses are also associated with water of anomalously high density, as well as with the cyclonic eddies in the upper water colunto. 22,277 22,278 SHI ET AL: VARIABILITY OF DENMARK STRAIT OVERFLOW K•ise, image available at http://www. ifm.unikiel.de/to/sfb460/a I/res 1 .html)

Frontogenesis and variability in Denmark Strait and its influence on overflow water

Journal of Physical Oceanography

A high-resolution numerical model, together with in situ and satellite observations, is used to explore the nature and dynamics of the dominant high-frequency (from one day to one week) variability in Denmark Strait. Mooring measurements in the center of the strait reveal that warm water “flooding events” occur, whereby the North Icelandic Irminger Current (NIIC) propagates offshore and advects subtropical-origin water northward through the deepest part of the sill. Two other types of mesoscale processes in Denmark Strait have been described previously in the literature, known as “boluses” and “pulses,” associated with a raising and lowering of the overflow water interface. Our measurements reveal that flooding events occur in conjunction with especially pronounced pulses. The model indicates that the NIIC hydrographic front is maintained by a balance between frontogenesis by the large-scale flow and frontolysis by baroclinic instability. Specifically, the temperature and salinity t...

On the origin and propagation of Denmark Strait overflow water anomalies in the Irminger Basin

Journal of Geophysical Research: Oceans, 2015

Denmark Strait Overflow Water (DSOW) supplies the densest contribution to North Atlantic Deep Water and is monitored at several locations in the subpolar North Atlantic. Hydrographic (temperature and salinity) and velocity time series from three multiple-mooring arrays at the Denmark Strait sill, at 180 km downstream (south of Dohrn Bank) and at a further 320 km downstream on the east Greenland continental slope near Tasiilaq (formerly Angmagssalik), were analyzed to quantify the variability and track anomalies in DSOW in the period 2007-2012. No long-term trends were detected in the time series, while variability on time scales from interannual to weekly was present at all moorings. The hydrographic time series from different moorings within each mooring array showed coherent signals, while the velocity fluctuations were only weakly correlated. Lagged correlations of anomalies between the arrays revealed a propagation from the sill of Denmark Strait to the Angmagssalik array in potential temperature with an average propagation time of 13 days, while the correlations in salinity were low. Entrainment of warm and saline Atlantic Water and fresher water from the East Greenland Current (via the East Greenland Spill Jet) can explain the whole range of hydrographic changes in the DSOW measured downstream of the sill. Changes in the entrained water masses and in the mixing ratio can thus strongly influence the salinity variability of DSOW. Fresh anomalies found in downstream measurements of DSOW within the Deep Western Boundary Current can therefore not be attributed to Arctic climate variability in a straightforward way. Downstream of Denmark Strait, entrainment warms the overflow and hence decreases its density. Nevertheless, DSOW remains the densest water mass in the Irminger Sea with temperatures below 2 C, descending to more than 2000 m depth. Here it is overlain by Iceland Scotland Overflow Water (ISOW), which also influences the less dense portion of the plume by isopycnal mixing. The average overflow water transport at the sill of Denmark Strait is around 3.4 Sv [Jochumsen et al., 2012], which increases further downstream to 10.7 Sv near Angmagssalik due to entrainment and the combination with ISOW [Dickson et al., 2008].

Significant role of the North Icelandic Jet in the formation of Denmark Strait overflow water

Nature Geoscience, 2011

The Denmark Strait overflow water is the largest dense water plume from the Nordic seas to feed the lower limb of the Atlantic Meridional Overturning Circulation. Its primary source is commonly thought to be the East Greenland Current. However, the recent discovery of the North Icelandic Jet-a deep-reaching current that flows along the continental slope of Iceland-has called this view into question. Here we present high-resolution measurements of hydrography and velocity north of Iceland, taken during two shipboard surveys in October 2008 and August 2009. We find that the North Icelandic Jet advects overflow water into the Denmark Strait and constitutes a pathway that is distinct from the East Greenland Current. We estimate that the jet supplies about half of the total overflow transport, and infer that it is the primary source of the densest overflow water. Simulations with an ocean general circulation model suggest that the import of warm, salty water from the North Icelandic Irminger Current and water-mass transformation in the interior Iceland Sea are critical to the formation of the jet. We surmise that the timescale for the renewal of the deepest water in the meridional overturning cell, and its sensitivity to changes in climate, could be different than presently envisaged.

Synoptic sections of the Denmark Strait Overflow

Geophysical Research Letters, 2001

We report on a rapid high-resolution survey of the Denmark Strait overflow (DSO) as it crosses the sill, the first such program to incorporate full-water-column velocity profiles in addition to conventional hydrographic measurements. Seven transects with expendable profilers over the course of one week are used to estimate volume transport as a function of density. Our observations reveal the presence of a strongly barotropic flow associated with the nearly-vertical front dividing the Arctic and Atlantic waters. The sevensection mean transport of water denser than er0 = 27.8 is 2.7 + 0.6 Sv, while the mean transport of water colder than 2.0øC is 3.8 + 0.8 Sv. Although this is larger than the 2.9 Sv of 0 • 2øC water measured by a 1973 current meter array, we find that a sampling of our sections equivalent to the extent of that array also measures 2.9Sv of cold water. Both the structure and magnitude of the measured flow are reproduced well by a high-resolution numerical model of buoyancy-driven exchange with realistic topography.