Potential density of seawater

Density is what stratification, mixing and buoyancy actually depend on, and temperature alone is a poor stand-in for it wherever salinity varies. The Gulf of Maine is one of those places: Scotian Shelf inflow arrives cold and fresh, and the two pull the density in opposite directions, so a cold anomaly can be either denser or lighter than the water it displaces depending on how fresh it is.

Usage

potential_density(
  env_dat,
  temperature = "SST",
  salinity = "SSS",
  sigma = TRUE,
  name = NULL
)

Arguments

env_dat

an sf POINT object with one row per location and time step, as datamatch's access functions return

temperature

name of the temperature column, in degrees C. Should be a potential temperature

salinity

name of the salinity column, in PSU

sigma

return sigma-theta, that is density minus 1000. FALSE returns absolute density

name

name for the new column

Value

env_dat with a density column, in kg/m^3

Details

Uses the UNESCO (1983) equation of state at one atmosphere. Copernicus thetao is already a potential temperature, so applying it here gives potential density directly, conventionally reported as sigma-theta: density in kg/m^3 minus 1000.

"One atmosphere" describes the pressure the density is referenced to, not the depth the temperature and salinity came from. Potential density is exactly the quantity you want for water sampled at depth — it is what that water would weigh if brought to the surface, which is what makes two levels comparable. datamatch::accessCopernicus() takes a depth argument, so this applies to any level, and buoyancy_frequency() uses two of them.

What it is not

This is the density a parcel would have if brought to the surface. It is the right quantity for comparing water masses and for deciding what floats over what, and it deliberately ignores pressure, so it is not in-situ density and should not be used where the compressibility of deep water matters.

Applying it to a temperature that is not a potential temperature gives in-situ surface density instead, which is the same number at the surface and increasingly wrong with depth.

Range

The polynomial is fitted over roughly -2 to 40 degrees C and 0 to 42 PSU. Outside that it still returns a number, and that number is an extrapolation of a fit rather than a density, so values beyond the range are warned about. Fresh water from a river mouth and ice-melt surface layers are the usual culprits.

References

UNESCO (1983). Algorithms for computation of fundamental properties of seawater. UNESCO Technical Papers in Marine Science 44.

See also

water_mass_fraction(), vertical_gradient(), cell_anomaly()

Examples

if (FALSE) { # \dontrun{
env <- potential_density(env)

# Density anomalies say more about inflow than temperature anomalies do,
# because they combine the cold and the fresh into one number.
env <- cell_anomaly(env, "sigma_theta")
} # }