Mode water is defined as a particular type of

water mass An oceanographic water mass is an identifiable body of water with a common formation history which has physical properties distinct from surrounding water. Properties include temperature, salinity, chemical - isotopic ratios, and other physical ...

, which is nearly vertically homogeneous. Its vertical homogeneity is caused by the deep vertical

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

in winter. The first term to describe this phenomenon is ''18° water'', which was used by

Valentine Worthington Lawrence Valentine Worthington (March 6, 1920 – February 10, 1995), better known as Val Worthington was a British-American physical oceanographer. His most noted contributions are the discovery of mode water which he called ''18° water'', fir ...

to describe the

isothermal In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and a ...

layer in the northern

Sargasso Sea The Sargasso Sea () is a region of the Atlantic Ocean bounded by four currents forming an ocean gyre. Unlike all other regions called seas, it has no land boundaries. It is distinguished from other parts of the Atlantic Ocean by its charac ...

cool to a temperature of about 18 °C each winter. Then Masuzawa introduced the ''subtropical mode water'' concept to describe the thick layer of temperature 16–18 °C in the northwestern North Pacific subtropical gyre, on the southern side of the Kuroshio Extension. The terminology ''mode water'' was extended to the thick near-surface layer north of the Subantarctic Front by McCartney, who identified and mapped the properties of the

Subantarctic mode water Sub-Antarctic Mode Water (SAMW) is an important water mass in Earth's oceans. It is formed near the Sub-Antarctic Front on the northern flank of the Antarctic Circumpolar Current. The surface density of Sub-Antarctic Mode Water ranges between abou ...

(SAMW). After that, McCartney and Talley then applied the term ''subpolar mode water'' (SPMW) to the thick near-surface

mixed layer The oceanic or limnological mixed layer is a layer in which active turbulence has homogenized some range of depths. The surface mixed layer is a layer where this turbulence is generated by winds, surface heat fluxes, or processes such as evaporat ...

s in the North Atlantic’s subpolar gyre. Antarctic bottom water

Formation and erosion

Different mode waters have different formation and erosion mechanisms. The subtropical mode water (STMW) is formed mainly by

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

, the SPMW is formed mainly by other processes. SAMW is due to the combination of subduction and other processes. erosion mechanism of SPMW is a combination of

turbulent mixing In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

and air-sea flux. The erosion mechanism of STMW is likely air-sea flux. For SAMW erosion turbulent mixing may be the main factor.

Geographical distribution

Mode water formation areas are generally characterized by wintertime mixed layers that are relatively thick compared with other mixed layers in the same geographical region. North Atlantic, Southern eastern Indian Oceans and Ocean in the Pacific have the thickest mixed layers, so these thick layers are associated with the North Atlantic’s subpolar mode water and the Southern Ocean’s subantarctic mode water. Relatively thick mixed layers are also found in the subtropical mode water areas near the separated western boundary currents.

Temporal variability

One prominent feature of mode waters is they are stable in properties and locations, so that researchers can use data set from all decades to map the approximate core properties. The stability of properties is linked to the largest spatial-scale, longest time-scale wind and

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the p ...

forcing. It is not to say there is no variation in properties of the mode water. These variations in these near-surface water masses, in temperature,

salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

, density and thickness, are linked to surface forcing changes, although in some cases the connection is not yet obvious. For example, Suga and Hanawa show as the seasons progress, mode water moves away from the formation area and sometimes becomes permanently capped.

Detection

To detect the mode water we can use the minimum value in the vertical gradient of

potential density The potential density of a fluid parcel at pressure P is the density that the parcel would acquire if adiabatically brought to a reference pressure P_, often 1 bar (100 kPa). Whereas density changes with changing pressure, potential density of a f ...

, or equivalently in the

Brunt–Väisälä frequency In atmospheric dynamics, oceanography, asteroseismology and geophysics, the Brunt–Väisälä frequency, or buoyancy frequency, is a measure of the stability of a fluid to vertical displacements such as those caused by convection. More precisely ...

. Since temperature profiles are more abundant and both salinity and temperature are relatively homogeneous in mode water, vertical temperature gradients are sometimes used instead of

potential vorticity In fluid mechanics, potential vorticity (PV) is a quantity which is proportional to the dot product of vorticity and stratification. This quantity, following a parcel of air or water, can only be changed by diabatic or frictional processes. It i ...

or the vertical gradient of potential density to identify the core of the mode water. There is no specific values of those gradients to define the boundaries of a given mode water.

Importance

Mode waters have a big impact on nutrients distribution as they prevent deep-ocean nutrients from upwelling to the

euphotic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...

. Further more, they will control the

biological pump The biological pump (or ocean carbon biological pump or marine biological carbon pump) is the ocean's biologically driven sequestration of carbon from the atmosphere and land runoff to the ocean interior and seafloor sediments.Sigman DM & GH ...

, which plays an important role in

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

uptake. Dynamically, mode waters also control potential vorticity and

baroclinity In fluid dynamics, the baroclinity (often called baroclinicity) of a stratified fluid is a measure of how misaligned the gradient of pressure is from the gradient of density in a fluid. In meteorology a baroclinic flow is one in which the densi ...

in the subtropical North Atlantic.

References

{{Reflist Oceanography