The ocean is a complex fluid environment in constant, dynamic motion. This movement spans an immense range of scales, from massive, slow-moving currents to rapid, localized swirls. The chaotic, irregular motion of seawater known as turbulence is the physical process that governs how energy and matter are distributed throughout the world’s oceans.
Defining Chaotic Ocean Motion
Oceanic turbulence is the irregular, chaotic movement of water, which stands in contrast to smooth, predictable currents, known as laminar flow. This motion is characterized by fluctuations in velocity and pressure that adhere to specific physical principles. The defining characteristic of turbulence is the presence of eddies, which are swirling masses of water that vary dramatically in size.
These eddies occur throughout the entire water column, ranging in diameter from kilometers down to mere centimeters. Large eddies, often generated by major currents or winds, transfer their energy through a process called the energy cascade. These large structures break down into progressively smaller swirls. This breakdown continues until the smallest scales are reached, typically at the centimeter level, where the energy is converted into heat by the water’s internal viscosity. These microscale eddies are particularly effective at mixing water properties across density layers.
External Forces Driving Mixing
The energy required to initiate and maintain this turbulence is supplied by several large-scale external forces that inject kinetic energy into the ocean.
Wind Stress
Wind stress on the sea surface is a primary driver, creating friction that generates waves and surface eddies. These surface instabilities promote mixing in the upper layers of the ocean, especially during strong storms.
Tidal Interactions
Another significant energy source is the tide, particularly where tidal currents interact with underwater topography, such as continental slopes and seamounts. This interaction generates large internal waves that propagate along density interfaces deep beneath the surface. When these internal waves, which can be hundreds of meters high, break, they release tremendous energy that generates intense turbulence.
Density Gradients
Density gradients, driven by variations in temperature and salinity, also contribute to mixing by creating buoyancy forces. When warm, less dense water sits atop colder, saltier water, the resulting stratification can become unstable. This instability triggers turbulent mixing that works to homogenize the layers.
Turbulence and Global Climate Regulation
Small-scale turbulence has a profound, large-scale influence on the climate system by regulating the ocean’s capacity to store heat and gases. The ocean is naturally layered by density, and stratification acts as a barrier preventing surface water from interacting with deep water. Turbulence weakens this barrier through vertical mixing, facilitating the movement of heat and carbon across these density layers.
This vertical exchange is fundamental to the global overturning circulation, known as the Meridional Overturning Circulation (MOC). The MOC acts as a planetary conveyor belt, transporting warm surface waters from the tropics toward the poles. There, the water cools, becomes dense, and sinks to the deep ocean. Turbulence is necessary in the deep ocean, allowing that dense, cold water to eventually mix with warmer, lighter water and slowly return to the surface, maintaining the continuous flow of the circulation.
Turbulence also facilitates the ocean’s sequestration of atmospheric carbon dioxide (\(text{CO}_2\)). The ocean absorbs approximately 30 percent of human-produced \(text{CO}_2\) from the atmosphere. Turbulence allows the carbon-rich surface water to be moved downward and sequestered in the deep ocean, effectively removing it from contact with the atmosphere for centuries.
Impact on Marine Life and Nutrient Cycling
Beyond its geophysical effects, turbulence is the physical mechanism that sustains marine life by driving nutrient cycling. The sunlit surface layer, or photic zone, is where photosynthesis occurs, but it is typically depleted of essential nutrients. These nutrients, such as nitrogen and phosphorus, accumulate in the deep, dark ocean from the decay of sinking organic matter.
Turbulence acts as a natural elevator system, driving the upwelling and vertical mixing that transport these nutrient salts from the deep water to the surface. This continuous supply of nutrients fuels the growth of phytoplankton, the microscopic algae that form the base of the entire marine food web. Without this mixing, the surface ocean would become an unproductive biological desert.
Turbulence also directly influences the behavior and feeding success of microscopic organisms. The movement of small swirls and eddies affects how marine bacteria absorb dissolved organic material, a key step in recycling matter back into the food web. Moderate levels of microscale turbulence can enhance the contact rate between predators, such as copepods and larval fish, and their planktonic prey, while high levels of turbulence can disrupt feeding.