Beneath the grey swells separating Greenland from Iceland lies a phenomenon more voluminous than any river on land. The Denmark Strait overflow, known in Danish and Norwegian as Greenlandspumpen or Grønlandspumpa (“the Greenland pump”), transports quantities of water on a scale hard to imagine. Each second, roughly 3.2 million cubic metres rush through, eclipsing even the Amazon River’s famous discharge.
Stretching for about 1,000 kilometres under the Denmark Strait, this column of water is around 200 metres wide and just as thick. Its journey begins where the colder, denser waters of the Nordic Seas meet the relatively warmer Irminger Sea to the south. At the Denmark Strait’s high point, the Greenland–Iceland Rise, the chillier northern water spills over and plunges along the ocean floor to a depth near 3,000 metres.
The hidden architect of Atlantic deep waters
The Denmark Strait overflow is not classified as a waterfall, but its scale and drop are unmatched by anything on land. If brought above sea level, its cascade, descending more than 3.5 kilometres, would dwarf even Angel Falls, the world’s tallest vertical waterfall on land, by a factor of three.
This oceanic torrent is created by differences in water density on either side of the Denmark Strait. Because the cold water from the north is denser, it continuously flows southward underneath the lighter, warmer water, pulled onward by gravity along the sloping seafloor. The Coriolis effect, a result of Earth’s rotation, bends the plume, pushing the overflow towards the west. This creates a remarkable disparity across the channel, where the height of the descending water on the Greenland side is about a kilometre higher than on the Iceland side.
Known scientifically as Denmark Strait Overflow Water (DSOW), this mass originates as water from the Greenland Sea, the Iceland Sea, and the eastern reaches of the Nordic Sea. It is delivered to the brink of the Denmark Strait mainly by the North Icelandic Jet and two arms of the powerful East Greenland Current.
A vital link in the ocean’s global engine
The Denmark Strait overflow supplies nearly half the dense overflows feeding North Atlantic Deep Water, a foundational current in the global system scientists call thermohaline circulation. This movement of dense, cold water shapes not just the depths of the Atlantic but also influences long-term climate rhythms, including the Atlantic Multidecadal Oscillation.
