On the origin and propagation of Denmark Strait overflow water anomalies in the Irminger Basin (original) (raw)
Related papers
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).
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...
Upstream sources of the Denmark Strait Overflow: Observations from a high-resolution mooring array
Deep Sea Research Part I: Oceanographic Research Papers, 2016
We present the first results from a densely instrumented mooring array upstream of the Denmark Strait sill, extending from the Iceland shelfbreak to the Greenland shelf. The array was deployed from September 2011 to July 2012, and captured the vast majority of overflow water denser than 27.8 kg m −3 approaching the sill. The mean transport of overflow water over the length of the deployment was 3.54 ± 0.16 Sv. Of this, 0.58 Sv originated from below sill depth, revealing that aspiration takes place in Denmark Strait. We confirm the presence of two main sources of overflow water: one approaching the sill in the East Greenland Current and the other via the North Icelandic Jet. Using an objective technique based on the hydrographic properties of the water, the transports of these two sources are found to be 2.54 ± 0.17 Sv and 1.00 ± 0.17 Sv, respectively. We further partition the East Greenland Current source into that carried by the shelfbreak jet (1.50 ± 0.16 Sv) versus that transported by a separated branch of the current on the Iceland slope (1.04 ± 0.15 Sv). Over the course of the year the total overflow transport
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.
Causes of Changes in the Denmark Strait Overflow
Journal of Physical Oceanography, 2007
The warming Nordic seas potentially tend to decrease the overflow across the Greenland-Iceland-Scotland Ridge (GISR) system. Recent observations by Macrander et al. document a significant drop in the intensity of outflowing Denmark Strait Overflow Water of more than 20% over 3 yr and a simultaneous increase in the temperature of the bottom layers of more than 0.4°C. A simulation of the exchange across the GISR with a regional ocean circulation model is used here to identify possible mechanisms that control changes in the Denmark Strait overflow and its relations to changed forcing condition. On seasonal and longer time scales, the authors establish links of the overflow anomalies to a decreasing capacity of the dense water reservoir caused by a change of circulation pattern north of the sill. On annual and shorter time scales, the wind stress curl around Iceland determines the barotropic circulation around the island and thus the barotropic flow through Denmark Strait. For the overlapping time scales, the barotropic and overflow component interactively determine transport variations. Last, a relation between sea surface height and reservoir height changes upstream of the sill is used to predict the overflow variability from altimeter data. Estimated changes are in agreement with other recent transport estimates based on current-meter arrays.
Denmark Strait overflow water (DSOW) is one of the main components of the thermohaline circulation. There has in the past been no consensus on where it is formed and by which way it is brought to the Denmark Strait sill. It was argued by Jonsson and Valdimarsson (2004) that a major part of DSOW is transported by a subsurface current along the slope northwest of Iceland. This supports theories suggesting that the Iceland Sea is a major source for the DSOW and this has consequences for the way in which climate change affects the thermohaline circulation. This was based on measurements from the years 2001 and 2002 with a vessel mounted ADCP as well as CTD data. Here we extend these data to include the year 2003 as well as establishing a connection to hydrographic data further east in the Iceland Sea. The water is traced to the northernmost station on the Siglunes section off central northern Iceland using water mass properties. This station is over 1000 m deep and lies just west of the...