Mesoscale eddies are the ocean’s equivalent of atmospheric weather systems: powerful, swirling masses of water spanning tens to hundreds of kilometers. Found throughout the global ocean, these three-dimensional structures extend deep below the surface, influencing climate patterns and the location of marine life. Containing more than half of the kinetic energy in the ocean, eddies are a component of ocean circulation. Their constant motion and ability to transport water over vast distances make them a subject of scientific interest.
Understanding Mesoscale Eddy Genesis and Properties
Mesoscale eddies form through various dynamic processes. A primary mechanism is the instability of strong ocean currents, like the Gulf Stream, which can shed large, rotating rings of water as they meander. Wind stress on the ocean surface and the interaction of currents with underwater mountains also contribute to their formation. These eddies can persist for weeks to months, and sometimes for years.
These oceanic features are classified into two main types based on their rotation. Cyclonic eddies rotate counter-clockwise in the Northern Hemisphere and clockwise in the Southern, and are characterized by a cold, dense water core that causes an upward doming of the ocean’s layers. In contrast, anticyclonic eddies rotate in the opposite direction and feature a core of warmer, less dense water that depresses the ocean layers downward. The temperature and salinity within an eddy are often distinctly different from the surrounding waters, creating mobile anomalies that drift across ocean basins.
Eddies’ Role in Ocean Dynamics
The rotational movement of mesoscale eddies makes them effective at transporting large volumes of water, along with the heat, salt, and dissolved gases it contains. Eddies are a dominant mechanism for moving heat from the equator towards the poles, a process that helps regulate Earth’s climate. In the Southern Ocean, for example, eddies are responsible for most of the poleward heat transport across the Antarctic Circumpolar Current.
By carrying water with different properties, eddies are a major contributor to ocean mixing. An eddy from a warm current moving into a colder region transfers heat and alters local water properties. This stirring prevents the ocean from becoming overly stratified and affects the distribution of chemical substances like oxygen and nutrients, influencing the makeup of entire ocean regions.
The influence of eddies extends to large-scale ocean circulation, as they can affect the pathways of major currents and the ocean’s energy balance. They also play a part in the ocean’s capacity to absorb atmospheric carbon dioxide. By transporting carbon-rich surface waters into the ocean’s interior, eddies contribute to the ocean’s role as a carbon sink.
Impacts on Marine Life and Ecosystems
The physical dynamics of mesoscale eddies create unique biological effects, turning them into mobile hotspots or deserts for marine life. The direction of rotation is a determining factor in an eddy’s productivity. Cyclonic eddies feature an upwelling of deep, cold water, which brings an abundance of nutrients to the surface. This injection of nutrients often triggers extensive phytoplankton blooms, forming the base of the marine food web.
These blooms create feeding grounds that attract a wide range of organisms. Zooplankton graze on the phytoplankton and are consumed by small fish and squid. These aggregations, in turn, attract larger animals like tuna, sharks, and marine mammals, which follow the eddies to exploit these temporary oases of food. The eddies function as traveling ecosystems, concentrating life within their boundaries.
Conversely, anticyclonic eddies have lower surface productivity because the downwelling of water pushes nutrients away from the sunlit surface, limiting phytoplankton growth. However, they can trap organisms that were present when the eddy formed. Eddies are also a mechanism for transporting the larvae of fish, corals, and other invertebrates over long distances. This can connect geographically separate populations and influence genetic diversity.
Observing and Tracking Ocean Eddies
Scientists employ a range of technologies to study mesoscale eddies. Satellite oceanography provides a global view, with altimeters measuring sea surface height to map their locations and movements. Cyclonic eddies have a lower sea surface height, while anticyclonic eddies have a higher one. Other satellites measure sea surface temperature and ocean color, revealing the eddies’ cores and any phytoplankton blooms.
To understand their three-dimensional structure, researchers use in-situ instruments. Autonomous profiling floats drift with currents and move through the water column, collecting data on temperature and salinity. Ocean gliders, which are underwater vehicles, can be programmed to survey eddies for extended periods, providing detailed cross-sections of their internal structure.
Numerical models are also used to simulate the ocean and predict eddy behavior. These computer programs can forecast the paths of eddies and help scientists understand their long-term effects on climate and marine ecosystems. By combining data from satellites, in-situ instruments, and models, researchers build a comprehensive picture of how these oceanic features function.