Mesoscale eddies, also known as synoptic ocean eddies, refer to the eddies in the ocean with a diameter of 100-300km and a life span of 2-10 months. [1]
Mesoscale eddies are very similar to cyclones and anticyclones in the atmosphere, so they are also called synoptic ocean eddies. There are usually two kinds of vortex: one is "cyclonic vortex" (counterclockwise rotation in the Northern Hemisphere), whose center sea water moves from bottom to top, bringing the lower cold water to the warmer water in the upper layer, so that the water temperature inside the vortex is lower than the surrounding sea water (generally the center sea level is lower than the surrounding sea level), also known as cold vortex. The other is the "anticyclonic vortex" (clockwise rotation in the Northern Hemisphere). The central seawater moves from top to bottom, carrying the warm water from the upper layer into the cold water of the lower layer. The water temperature inside the vortex is higher than the surrounding water temperature (generally the central sea level is higher than the surrounding water), also known as the warm vortex.
Mesoscale eddies exist in all oceans of the world, most of which are concentrated in the North Atlantic, especially in the Bermuda Triangle. Since 1957, nearly 200 anticyclonic eddies have been found in the northwest of the Pacific Ocean.
The motion of mesoscale vortex can be divided into three types: rotation, translation and vertical.
The mesoscale vortex will change the original movement of the sea water flowing through the sea area, making the direction of the current varied, the velocity increased several times to dozens of times, and accompanied by strong vertical movement of the water body. The potential energy in the vortex center is the largest, and the farther away from the center, the smaller the potential energy.
The maximum value of vortex kinetic energy is not in the center, but in the region with the maximum linear velocity of water rotation. [2]
Impact on marine hydro physical properties
The first is the impact of mesoscale vortices on sea surface temperature (SST). Cyclone (anticyclone) type mesoscale vortices correspond to low (high) sea surface height (SSH). Under the effect of geostrophic, the sea water on the sea surface diverges (converges), thus causing the rise (fall) of the lower (upper) sea water as a supplement, thus making the sea surface present a low (high) SST. Therefore, mesoscale vortex can be divided into cold vortex and warm vortex, corresponding to cyclonic vortex and anticyclonic vortex respectively. The influence of mesoscale vortices on SST is not only caused by the rising or falling currents it causes, but also by the stirring effect of mesoscale vortices on the background SST. At the front of the ocean, the gradient of SST is very large. Under the strong agitation of the mesoscale vortex, cold water will be brought to warm water, and warm water will also be brought to cold water, so that the "cold wire" and "warm wire" of SST will be generated. After the mesoscale vortex affects SST, it will produce a series of chain effects, for example, it will change the air sea heat flux, which will not be discussed in detail here.
The second is the transport effect of mesoscale vortex on temperature, salt and particles, which is called eddy flux. For the average background field of temperature, salt, flow, etc., the existence of mesoscale vortices produces an irregular fluctuation on the background field. The combined effect of the generated fluctuating velocity and fluctuating temperature (salinity, particle density) produces the so-called heat (salt, particle) transport of vortices. Relevant studies show that the transport effect of this vortex can not be ignored compared with the background advection effect, and the transport effect of the vortex is more significant in the western boundary current and the extension body and the Antarctic circumpolar current sea area.
In addition to its own rotation, the mesoscale vortex is constantly "migrating". From the altimeter, it can be found that the mesoscale vortex in the ocean propagates westward at a speed similar to that of the long Rossby wave, which may indicate that the mesoscale vortex in the ocean is the local manifestation of the Rossby wave. In this sense, the energy in the ocean continuously propagates westward in the form of mesoscale vortex, thus adjusting the circulation of the entire ocean. In addition, the mesoscale vortex in the actual ocean is not strictly linear, that is, the streamline of the mesoscale vortex is not strictly closed (or the trajectory of the particles in the mesoscale vortex is not closed), which will make the mesoscale vortex continuously carry matter westward, but the relevant research in this area is still very scarce. [3]
Impact on marine chemical and biological environment
Mesoscale vortices also have an important influence on marine chemistry and biological environment. For example, the upwelling caused by the mesoscale vortex carries nutrients from the ocean bottom to the ocean euphotic layer, thus promoting the improvement of marine primary productivity. In addition, the agitation of the mesoscale vortex at the front and the vortex transport of particles by the mesoscale vortex will also affect the local marine chemical and biological environment. A recent report published on Science pointed out that a mesoscale vortex at the mid ridge of the Northeast Pacific Ocean is particularly barotropic, and its impact depth can reach the seafloor, thus helping the migration of biological communities at the seafloor hydrothermal outlet. It can be seen that the mesoscale vortex has a previously unknown impact on deep-sea marine ecology.
As the connection between large-scale processes and small-scale processes, mesoscale vortices play an important role in the energy product chain. First, due to baroclinic instability (or other reasons, such as barotropic instability), mesoscale vortices are generated from the background flow, and the energy absorbed from the background field is growing, thus transferring energy from large scale to mesoscale. The dissipation process of mesoscale vortex transfers energy from mesoscale to small-scale, and then finally converts into thermal energy dissipation. There is no conclusion on the generation and dissipation mechanism and process of mesoscale vortex, especially its dissipation mechanism and process, and the relevant reports are very scarce. Several reports have pointed out that the anticyclonic mesoscale vortex may be the "channel" for the downward propagation of near inertial energy, which is conducive to the mixing of deep seawater, thus promoting the transmission of energy to small scales; In addition, it is also reported that mesoscale eddies may produce Lee waves when passing through strong submarine topography, and the breaking of Lee waves causes the mixing of seawater.
