Variability of the Denmark Strait overflow: A numerical study (original) (raw)
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Structure and variability of the Denmark Strait Overflow: Model and observations
Journal of Geophysical Research, 2003
We report on a combined modeling and observational effort to understand the Denmark Strait Overflow (DSO). Four cruises over the course of 3 years mapped hydrographic properties and velocity fields with high spatial resolution. The observations reveal the mean path of the dense water, as well as the presence of strong barotropic flows, energetic variability, and strong bottom friction and
Mesoscale variability in Denmark strait: The PV outflow hypothesis*
1998
The outflow through Denmark Strait shows remarkable mesoscale variability characterized by the continuous formation of intense mesoscale cyclones just south of the sill. These cyclones have a diameter of about 30 km and clear signatures at the sea surface and in currents measured near the bottom. They have a remnant of Arctic Intermediate Water (AIW) in their core.
Journal of Physical Oceanography, 2017
Initial results are presented from a yearlong, high-resolution (~2 km) numerical simulation covering the east Greenland shelf and the Iceland and Irminger Seas. The model hydrography and circulation in the vicinity of Denmark Strait show good agreement with available observational datasets. This study focuses on the variability of the Denmark Strait overflow (DSO) by detecting and characterizing boluses and pulses, which are the two dominant mesoscale features in the strait. The authors estimate that the yearly mean southward volume flux of the DSO is about 30% greater in the presence of boluses and pulses. On average, boluses (pulses) are 57.1 (27.5) h long, occur every 3.2 (5.5) days, and are more frequent during the summer (winter). Boluses (pulses) increase (decrease) the overflow cross-sectional area, and temperatures around the overflow interface are colder (warmer) by about 2.6°C (1.8°C). The lateral extent of the boluses is much greater than that of the pulses. In both cases...
Evolution of Denmark Strait Overflow Cyclones and Their Relationship to Overflow Surges
Geophysical Research Letters, 2020
Mesoscale features present at the Denmark Strait sill regularly enhance the volume transport of the Denmark Strait overflow (DSO). They are important for the Atlantic Meridional Overturning Circulation and ultimately, for the global climate system. Using a realistic numerical model, we find new evidence of the causal relationship between overflow surges (i.e., mesoscale features associated with high-transport events) and DSO cyclones observed downstream. Most of the cyclones form at the Denmark Strait sill during overflow surges and, because of potential vorticity conservation and stretching of the water column, grow as they move equatorward. A fraction of the cyclones form downstream of the sill, when anticyclonic vortices formed during high-transport events start collapsing. Regardless of their formation mechanism, DSO cyclones weaken starting roughly 150 km downstream of the sill, and potential vorticity is only materially conserved during the growth phase. Plain Language Summary Ocean currents affecting the global climate are sustained by cold and dense water that sinks in the North Atlantic Ocean. A large portion of this water passes through Denmark Strait, the ocean channel located between Greenland and Iceland. The amount of water entering the strait varies from day to day and is controlled by ocean vortices. Knowing the mechanisms associated with these vortices is of key importance for understanding and predicting Earth's climate. Using a realistic numerical model, we find that the vortices are generated in the strait during dense water surges. In one scenario, the ocean vortices cross Denmark Strait rotating in the same direction as the Earth (counterclockwise). These vortices strengthen as they move toward the south. If the vortices initially rotate clockwise, they move slowly and quickly collapse. The water converging south of them triggers the formation of new vortices rotating counterclockwise. All of these energetic vortices move toward the equator. First they quickly grow, then they lose energy starting roughly 150 km south of Denmark Strait.
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.
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...
Mesoscale mixing of the Denmark Strait Overflow in the Irminger Basin
Ocean Modelling, 2017
The Denmark Strait Overflow (DSO) is a major export route for dense waters from the Nordic Seas forming the lower limb of the Atlantic Meridional Overturning Circulation, an important element of the climate system. Mixing processes along the DSO pathway influence its volume transport and properties contributing to the variability of the deep overturning circulation. They are poorly sampled by observations however which hinders development of a proper DSO representation in global circulation models. We employ a high resolution regional ocean model of the Irminger Basin to quantify impact of the mesoscale flows on DSO mixing focusing on geographical localization and local time-modulation of water property changes. The model reproduces the observed bulk warming of the DSO plume 100-200 km downstream of the Denmark Strait sill. It also reveals that mesoscale variability of the overflow ('DSO-eddies', of 20-30 km extent and a time scale of 2-5 day) modulates water property changes and turbulent mixing, diagnosed with the vertical shear of horizontal velocity and the eddy heat flux divergence. The spacetime localization of the DSO mixing and warming and the role of coherent mesoscale structures should be explored by turbulence measurements and factored into the coarse circulation models.
Mesoscale Eddies Observed at the Denmark Strait Sill
Journal Of Geophysical Research: Oceans, 2019
The Denmark Strait overflow is the major export route of dense water from the Arctic Mediterranean into the North Atlantic. At the Strait's shallow sill, the overflow is a bottom-intensified cold and dense plume, bound to the east by a thermal front formed with the warmer, northward flowing North Icelandic Irminger Current. More than two decades of observations at the sill show strong fluctuations of volume flux on daily time scales. To better understand the source of this variability, a five-mooring array was installed at the sill, capturing nearly 1 year of velocity and bottom temperature measurements at a high temporal and spatial resolution. Bottom temperature fluctuations that exceed 4 • C indicate a meandering of the front between the plume and the North Icelandic Irminger Current. Current vector rotation shows trains of alternating cyclones and anticyclones at the sill. An eddy crosses the sill every 3 to 6 days with a mean velocity of 0.4 m/s and a typical diameter of 30 to 40 km. The results suggest that anticyclones, with centers passing through the deepest part of the sill, may be responsible for periods of increased volume flux-also referred to as boluses and pulses in previous studies. Although the relationship between eddies, pulses, and boluses is still unclear, the results show that eddies are directly linked to fluctuations in the strength, thickness, and position of the overflow plume. Plain Language Summary The southward flow of dense water from the Arctic ocean plays a crucial role in global ocean circulation but is almost immediately constrained on its way south by a submarine ridge that connects Greenland, Iceland, the Faroe Islands, and Scotland. The southward flow is therefore forced to pass through several straits and up and over relatively shallow sills. Most of the flow passes over a sill in the Denmark Strait, located between Greenland and Iceland. In this study, we present observations from an array of instruments, which measure the southward flow as it passes over the Denmark Strait sill. The flow is characterised by trains of eddies (vortices), with an alternating sense of rotation; meaning a counterclockwise eddy is usually followed by a clockwise eddy. The eddies are 30 to 40 km wide and need about 1 day to pass over the sill. These eddies help to explain pronounced changes in the flow across the sill, as they can help either to speed up the flow or slow it down. The results of this study contribute to understanding mesoscale fluctuations, which influence local mixing processes and water mass transports.
Revised circulation scheme north of the Denmark Strait
Deep Sea Research Part I: Oceanographic Research Papers, 2013
The circulation and water mass transports north of the Denmark Strait are investigated using recently collected and historical in situ data along with an idealized numerical model and atmospheric reanalysis fields. Emphasis is placed on the pathways of dense water feeding the Denmark Strait Overflow Water plume as well as the upper-layer circulation of freshwater. It is found that the East Greenland Current (EGC) bifurcates at the northern end of the Blosseville Basin, some 450 km upstream of the Denmark Strait, advecting overflow water and surface freshwater away from the boundary. This "separated EGC" flows southward adjacent to the previously identified North Icelandic Jet, indicating that approximately 70% of the Denmark Strait Overflow Water approaches the sill along the Iceland continental slope. Roughly a quarter of the freshwater transport of the EGC is diverted offshore via the bifurcation. Two hypotheses are examined to explain the existence of the separated EGC. The atmospheric fields demonstrate that flow distortion due to the orography of Greenland imparts significant vorticity into the ocean in this region. The negative wind stress curl, together with the closed bathymetric contours of the Blosseville Basin, is conducive for spinning up an anti-cyclonic gyre whose offshore branch could represent the separated EGC. An idealized numerical simulation suggests instead that the current is primarily eddy-forced. In particular, baroclinic instability of the model EGC spawns large anti-cyclones that migrate offshore and coalesce upon reaching the Iceland continental slope, resulting in the separated EGC. Regardless of the formation mechanism, the recently obtained shipboard data and historical hydrography both indicate that the separated EGC is a permanent feature of the circulation north of the Denmark Strait.