Exploring Australia's Unique Underwater Rivers and Currents
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Chapter 1: Understanding Underwater Rivers
References to "underwater rivers" often emerge in various discussions and literature. However, what precisely do we mean when we describe a river as being surrounded by water? Recent observations of substantial water flows on the Australian continental shelf have been characterized as a system of underwater rivers. Yet, it may be more accurate to conceptualize these flows as currents rather than traditional rivers, specifically currents driven by differences in water density.
Scientists have termed these water flows on Australia's shelf as Dense Shelf Water Cascades (DSWC). This designation refers to water that moves along the seabed of the continental shelf and spills over its edge into the depths of the ocean. This outward movement occurs because gravity transforms minor variations in seawater density into significant currents.
The two primary factors responsible for generating dense water are temperature and salinity. As seawater cools, it becomes denser, and an increase in salinity further raises its density. Both of these elements are crucial in forming Australia's DSWC.
Along a vast 10,000-kilometer stretch of Australia's continental shelf, variations in seawater density create currents flowing toward the ocean. Such large-scale shelf density flows are unparalleled globally.
The process of forming denser water begins near the coast, particularly under the summer sun's heat. Solar radiation warms areas with minimal freshwater influx, causing seawater to evaporate. Since evaporation removes freshwater but leaves salt behind, the salinity and density of the shallow coastal waters rise with increasing evaporation. Cooler temperatures in late autumn and winter further enhance water density due to cooling.
These mechanisms set the stage for gravity to act, allowing the denser water to descend to the ocean floor and flow down-slope toward the continental shelf's edge. Eventually, this water cascades off the shelf, transporting various particles and nutrients into the deep-sea ecosystem. Australia’s coastal evaporation and seasonal cooling rates are the highest globally, facilitating this unique hydrological phenomenon.
The first video, titled "Found Embarrassing Amount of Stuff Underwater in River (Scuba Diving)," showcases the surprising discoveries made while exploring underwater environments, highlighting the hidden treasures beneath the surface.
Section 1.1: The Nature of Density Currents
Despite the uniqueness of Australia's shallow-water density flows, they represent just one aspect of the density currents that drive our oceans. Somewhere in the North Atlantic, cold, salty water is sinking into the abyss. This process may occur in the Labrador Sea or in the waters between Greenland and Scotland. In this case, water disappears from the surface, descends to the seafloor, and contributes to a deep-ocean current known as the "global conveyor belt."
This phenomenon begins in the Arctic, where the formation of polar sea ice removes freshwater from the sea, increasing salinity. When this cold, saline water enters the Atlantic, it becomes denser than the surrounding seawater and sinks, allowing the warmer water to remain buoyant at the surface. Upon reaching the seafloor, the denser water continues to move downslope, driven by gravity, initiating the global conveyor belt.
The volume of water circulating in this conveyor belt is immense, exceeding 100 times the flow of the Amazon River, although the pace of movement at the ocean bottom is leisurely—only a few centimeters per second.
This water that sinks to the North Atlantic embarks on a journey lasting up to 1,600 years before resurfacing in the North-Central Pacific Ocean. This path takes the water from the Arctic to the Antarctic, and then it travels globally through the Southern Ocean. Upon reaching the Pacific, it diverges into a current and heads northward, spanning the ocean's length. This conveyor belt represents another form of an underwater river, navigating the deep ocean floor.
Subsection 1.1.1: Thermohaline Circulation Explained
These extensive, density-driven systems are often referred to as overturning circulation or thermohaline circulation. "Thermo" denotes the fact that cold water is denser than warm water, while "haline" indicates that saltwater is denser than freshwater. This circulation type facilitates the transfer of water from the surface to the ocean depths and back again.
Thermohaline circulation influences weather patterns and climate conditions globally. Additionally, the turnover of surface water to the ocean floor is essential for oxygenating the deep seas. Without this circulation, the abyssal zones could lose contact with oxygen-rich surface waters, leading to anoxic conditions (a state devoid of oxygen). Such conditions could threaten life in these deep environments.
Density currents are vital for sustaining our oceans and supporting extensive marine ecosystems, extending from the surface to the abyssal depths. These underwater rivers transport essential oxygen and nutrients, making marine life feasible. Without thermohaline circulation, our oceans would be drastically different than what we observe today.