Deep-ocean currents are the slow and steady movements of water in the depths of the ocean. They are driven by differences in density and temperature of seawater, and they play a vital role in regulating the climate, distributing nutrients, and transporting carbon dioxide around the world. But what are the factors that influence these hidden rivers of the sea? And how do they affect life on Earth? In this article, we will explore the causes, characteristics, and effects of deep-ocean currents.
Contents
What causes deep-ocean currents?
Deep-ocean currents are part of a global system of circulation known as the thermohaline conveyor belt. The term “thermohaline” comes from the Greek words for “heat” and “salt”, which are the two main factors that determine the density of seawater. Density is a measure of how much mass is packed into a given volume of a substance. The denser a substance is, the more it tends to sink.
The thermohaline conveyor belt starts at the surface of the ocean, where wind, waves, and solar radiation heat up and evaporate some of the water, leaving behind saltier and denser water. This process is more pronounced in regions where there is high evaporation and low precipitation, such as near the equator or in semi-arid areas. The saltier and denser water sinks to the bottom of the ocean, creating a downward pressure that drives the deep-ocean currents.
Another factor that contributes to the formation of deep-ocean currents is the freezing of seawater at high latitudes, such as near the poles. When seawater freezes, it forms ice crystals that exclude most of the salt, leaving behind colder and saltier water. This water also sinks to the bottom of the ocean, adding to the downward pressure.
The sinking water then flows along the ocean floor, following the contours of the seabed and the shape of the continents. It can travel thousands of kilometers before it reaches a region where it can rise back to the surface. This usually happens in areas where there is an upwelling of warmer and less salty water from below, such as near the equator or along coastal zones. The rising water then completes the cycle by flowing back to higher latitudes, where it can cool down and sink again.
The thermohaline conveyor belt takes about 1,000 years to complete one full circuit around the globe. It moves very slowly, at a rate of a few centimeters per second, compared to tens or hundreds of centimeters per second for surface currents. However, it carries a huge amount of water, more than 100 times the flow of all the rivers in the world combined.
What are the characteristics of deep-ocean currents?
Deep-ocean currents have different characteristics depending on their origin, direction, and depth. Some of the major deep-ocean currents are:
- The North Atlantic Deep Water (NADW), which forms in the Nordic Seas and near Greenland by cooling and sinking of surface water. It flows southward along the western edge of the Atlantic Ocean, reaching depths of up to 4,000 meters. It carries cold and oxygen-rich water that nourishes marine life in its path.
- The Antarctic Bottom Water (AABW), which forms near Antarctica by freezing and sinking of surface water. It flows northward along the bottom of all ocean basins, reaching depths of up to 6,000 meters. It carries very cold and dense water that fills most of the deep ocean.
- The Antarctic Circumpolar Current (ACC), which flows eastward around Antarctica at depths of 1,000 to 4,000 meters. It is driven by strong westerly winds that blow over the Southern Ocean. It connects all major ocean basins and transports heat and salt between them.
- The Pacific Deep Water (PDW), which forms in the North Pacific Ocean by cooling and sinking of surface water. It flows southward along the eastern edge of the Pacific Ocean, reaching depths of up to 3,000 meters. It carries relatively warm and nutrient-poor water that has low oxygen levels.
- The Indian Deep Water (IDW), which forms in the Indian Ocean by mixing of NADW and AABW. It flows westward along the southern edge of the Indian Ocean, reaching depths of up to 3,000 meters. It carries moderately cold and saline water that has intermediate oxygen levels.
What are the effects of deep-ocean currents?
Deep-ocean currents have significant effects on both marine ecosystems and global climate. Some of these effects are:
- They distribute heat and salt around the world, affecting regional temperatures and salinities.
- They transport carbon dioxide from the atmosphere to the deep ocean, where it can be stored for long periods of time. This helps to regulate the greenhouse effect and the global carbon cycle.
- They bring oxygen and nutrients to the deep ocean, supporting a diverse and productive marine life. They also remove waste and dead organic matter from the surface, preventing eutrophication and anoxia.
- They influence the formation and melting of sea ice, which affects the albedo and insulation of the Earth’s surface.
- They modulate the El Niño-Southern Oscillation (ENSO), which is a periodic fluctuation of sea surface temperatures and atmospheric pressures in the Pacific Ocean. ENSO affects weather patterns and precipitation across the world.
- They interact with surface currents and atmospheric circulation, creating feedback loops that can amplify or dampen climate changes.
Conclusion
Deep-ocean currents are the hidden rivers of the sea that flow beneath the surface of the ocean. They are driven by differences in density and temperature of seawater, and they form part of a global system of circulation known as the thermohaline conveyor belt. They have important effects on marine ecosystems and global climate, as they distribute heat, salt, carbon dioxide, oxygen, and nutrients around the world. They also influence sea ice formation, ENSO variability, and feedback loops that can affect climate changes. Understanding how deep-ocean currents work and how they respond to external forcings is essential for predicting and adapting to the future of our planet.
