Wait, What? Dense water can cross an underwater mountain pass, accelerate downhill along the seafloor and become a much larger current by swallowing surrounding Atlantic water on the way.
Denmark Strait Overflow Water (DSOW) is one of the major dense-water overflow branches connecting the Nordic Seas to the deep North Atlantic. Cold, dense water crosses the Greenland–Iceland ridge through Denmark Strait, descends the continental slope, entrains surrounding Irminger Sea and Atlantic water, and contributes to the lower part of North Atlantic Deep Water.
Scientific Job Claimed by This Manual
This manual owns one Ocean World process: dense Nordic Seas source water → sill crossing through Denmark Strait → bottom-intensified downslope overflow → shear-driven entrainment of surrounding water → transformed DSOW plume → contribution to the deep western North Atlantic. The North Atlantic Deep Water Learning Manual owns the composite NADW system. The Labrador Sea Water Learning Manual owns the Labrador convection branch. This manual owns the Denmark Strait sill-overflow transformation.
Primary: What Is an Underwater Overflow?
Imagine a bowl of dense water separated from a deeper basin by a low underwater ridge. Once the dense water reaches the ridge crest, it can spill over and descend the slope on the other side.
Denmark Strait acts like such an underwater gateway between the Nordic Seas and the North Atlantic.
Why the Water Stays Near the Bottom
The overflow water is denser than much of the water above and around it. Gravity therefore keeps it pressed toward the slope as it descends.
The current is not a free-falling waterfall in empty space. It is a dense gravity current moving within another fluid.
Secondary: Why the Overflow Speeds Up After the Sill
Once dense water crosses the sill and reaches the steeper Atlantic side, gravitational potential energy is converted into kinetic energy. The bottom-intensified current accelerates downslope.
As it speeds up, strong velocity shear develops between the dense plume and surrounding water. That shear makes the interface turbulent.
Entrainment: Why the Current Gets Bigger While Losing Its Original Signature
Turbulence mixes surrounding Atlantic water into the overflow. This process is called entrainment.
The total volume transport grows downstream, while the original overflow becomes warmer, saltier or fresher depending on the properties of the entrained water. The final deep current is therefore a transformed mixture, not pristine Nordic water.
JC: Hydraulic Control at the Sill
Dense overflows through narrow straits can behave like hydraulically controlled gravity currents. The sill geometry, density contrast and upstream reservoir help determine how much dense water can pass through the gateway.
Downstream of the sill, the flow becomes strongly supercritical in places, then adjusts through mixing, bottom drag and interaction with topography.
Why Topography Routes the Overflow
The plume follows the continental slope around southern Greenland rather than spreading uniformly across the basin. Coriolis forces, bottom slope and density structure help keep dense water organised along the boundary.
This is one pathway into the deep western boundary current system.
Connection to North Atlantic Deep Water
The North Atlantic Deep Water Learning Manual owns NADW as a composite deep-water mass. Denmark Strait Overflow Water contributes to its denser lower portion after it has crossed the sill and been modified by entrainment.
Connection to Labrador Sea Water
The Labrador Sea Water Learning Manual owns deep convection in the Labrador Sea. LSW and DSOW can occupy overlapping regions of the subpolar North Atlantic but carry different formation histories and tracer signatures.
Connection to the Bottom Boundary Layer
The Ocean Bottom Boundary Layer Learning Manual owns the generic frictional layer above the seabed. DSOW is an energetic real-world case in which bottom stress, shear and rough topography strongly influence a dense gravity current.
How Do We Know?
Scientists use moored current meters, CTDs, lowered acoustic Doppler current profilers, tracers and repeated hydrographic sections across Denmark Strait and downstream of Greenland. These observations show dense water crossing the sill and increasing its transport as it entrains surrounding water.
NOAA AOML identifies Denmark Strait as one of the two major northern Nordic Seas overflows feeding the Atlantic deep-water system, together with overflow through the Iceland–Scotland passages.
Observation Versus Water-Mass Attribution
An instrument directly measures current speed, temperature, salinity and density. Calling the water DSOW requires a source and pathway interpretation: the dense layer must be consistent with Nordic Seas overflow through Denmark Strait and with downstream transformation.
Tracer evidence helps separate DSOW from Labrador Sea Water and other neighbouring deep-water components.
Can You Predict It?
- A larger density contrast across the sill: stronger gravity-driven overflow becomes possible.
- More turbulent entrainment downstream: total transport grows while the original water-mass signature becomes diluted.
- Rougher topography and stronger shear: mixing can intensify.
- A fresher Nordic source becomes less dense: the overflow’s downstream depth and transport can change.
Transfer Test
A dense overflow crosses a sill with transport of 3 units. Farther downstream, measurements show 6 units of transport but weaker source-water salinity and temperature signatures. What process best explains both observations?
Entrainment. Ambient water has been mixed into the plume, increasing its volume while diluting its original properties.
Model Boundary
DSOW varies over time and contains filaments, eddies and multiple source contributions. “Dense water crosses the sill and flows south” is a first-order model. Real plume thickness, entrainment and pathways depend on tides, source-water properties, topography and background circulation.
Useful Misconceptions to Correct
- DSOW is not a waterfall through air.
- It does not remain chemically or physically unchanged after crossing the sill.
- Greater downstream transport does not mean more source water was created; entrainment can increase transport.
- DSOW is one component of NADW, not the whole system.
- Labrador Sea Water and DSOW have different formation pathways.
Canonical External Sources
Teaching Method
Begin with the contradiction: “How can an underwater current become twice as large while becoming less like the water it started as?” Make students use entrainment to solve the puzzle before introducing the DSOW name.
For Primary learners, use dense water spilling over an underwater ridge. For Secondary learners, add downslope acceleration and mixing. For JC learners, add sill control, shear and tracer evidence, then give unfamiliar overflow data and require students to infer whether transport growth came from stronger source flow or entrainment.