Underwater rivers: a case of wet on wet
References to “underwater rivers” pop up in the news and literature. But what are we to make of a river defined by water within water? Recent documentation of massive water flows on the Australian continental shelf were described as a network of underwater rivers. But perhaps it is best to think of these flows as currents instead of rivers. Not currents formed by wind, but currents powered by density.
Scientists refer to the water flows on the Australian shelf as Dense Shelf Water Cascades (DSWC). The term describes water flowing across the sea bottom on the continental shelf and cascading over its edge into the ocean abyss. This seaward flow of water occurs because gravity turns small differences in seawater density into large scale motion.
The two factors responsible for creating dense water are temperature and salinity. When seawater cools, it becomes denser. Also, as water turns saltier its density increases. Both of these controlling factors play a role in the development of Australia’s DSWC.
Dense Shelf Water Cascades
Along a 10,000 kilometer stretch of Australia’s continental shelf, density differences in the seawater generate currents that flow towards the open ocean. Shelf density flows on this scale are not seen anywhere else in the world.
The development of higher density water starts near the coast under the heat of the summer sun. The solar radiation beats down on areas with little freshwater input, and the heat causes seawater evaporation. Because evaporation only removes freshwater and leaves the salt, the salinity and density of shallow coastal water rise in tandem with increasing evaporation. When cooler temperatures prevail during late autumn and winter, the water density further increases due to cooling.
These processes set the stage for gravity to do its work and let the dense water sink to the ocean bottom, and then flow down-slope towards the edge of the continental shelf. This water eventually cascades off the edge of the shelf, carrying a range of particles and nutrients into the deep-ocean ecosystem. Australia’s coastal evaporation and seasonal cooling rates are the highest in the world, making this unique hydrologic system possible.
Despite the uniqueness of Australia’s shallow-water density flows, they are but one facet of the density currents that power our oceans. Somewhere in the North Atlantic, cold salty water is sinking into the abyss. Perhaps this action occurs in the Labrador Sea, or maybe the descent takes place in the waters between Greenland and Scotland. But the water is disappearing from the surface, sinking to the seafloor and feeding a deep-ocean current known as the “global conveyor belt.”
This process starts in the Arctic when polar sea ice forms, removing freshwater from the Arctic sea and increasing water salinity. When this cold, salty water enters the Atlantic Ocean, it is denser than the surrounding seawater and sinks, leaving the more buoyant warmer water at the ocean surface. When the denser water reaches the seafloor, it continues moving downslope under the influence of gravity, starting the global conveyor belt.
The volume of water moving along the conveyor belt is immense, measuring more than 100 times the flow of the Amazon River. But life on the bottom of the ocean moves at a leisurely pace with the current flowing at just a few centimeters per second.
This water sinking to the bottom of the North Atlantic starts a journey that lasts up to 1600 years before the water surfaces again in the North-Central Pacific Ocean. This journey takes water from the Arctic to the Antarctic, and then around the globe in the Southern Ocean. When the water reaches the Pacific Ocean, it spins off into a current and travels northward, traversing the ocean’s length. This conveyor belt is another underwater river, but it travels the globe along the deep ocean bottoms.
These large-scale, density-driven systems are also referred to as overturning circulation or thermohaline circulation. “Thermo” since cold water is denser than warm water, and “haline” because saltwater is denser than freshwater. This type of circulation transfers water from the surface to the ocean bottom and back again.
Thermohaline circulation affects weather and climatic conditions around the globe. Also, the overturn of surface water to the ocean bottom keeps the deep seas oxygenated. Without this circulation, abyssal zones lose contact with the oxygenated surface waters and risk anoxia (loss of all oxygen). Life would largely disappear from this deep realm if anoxic conditions developed.
Density-currents power our oceans and hence support massive marine ecosystems extending from the surface of the sea to abyssal depths. These underwater rivers carry oxygen and nutrients with them, making life possible. Without the assistance of thermohaline circulation, the oceans would look very different than what we experience today.