Compute \(\sigma_\theta\), the potential density of seawater (minus 1000 kg/m\(^3\)), referenced to 4000db pressures. This is defined as \(\sigma_1=\sigma_\theta=\rho(S,\theta(S,t,p),4000\)
1000 kg/m\(^3\).
swSigma4(
salinity,
temperature = NULL,
pressure = NULL,
longitude = NULL,
latitude = NULL,
eos = getOption("oceEOS", default = "gsw")
)either practical salinity (in which case temperature
and pressure must be provided) or an oce object, in
which case salinity, temperature (in the ITS-90 scale; see
next item), etc. are inferred from the object.
in-situ temperature (\(^\circ\)C), defined
on the ITS-90 scale. This scale is used by GSW-style calculation (as
requested by setting eos="gsw"), and is the value contained within
ctd objects (and probably most other objects created with data
acquired in the past decade or two). Since the UNESCO-style calculation is
based on IPTS-68, the temperature is converted within the present function,
using T68fromT90().
pressure (dbar)
longitude of observation (only used if eos="gsw";
see ‘Details’).
latitude of observation (only used if eos="gsw"; see
‘Details’).
equation of state, either "unesco" (references 1 and 2)
or "gsw" (references 3 and 4).
Potential density anomaly (kg/m\(^3\)).
See citations provided in the swRho() documentation.
Other functions that calculate seawater properties:
T68fromT90(),
T90fromT48(),
T90fromT68(),
swAbsoluteSalinity(),
swAlphaOverBeta(),
swAlpha(),
swBeta(),
swCSTp(),
swConservativeTemperature(),
swDepth(),
swDynamicHeight(),
swLapseRate(),
swN2(),
swPressure(),
swRho(),
swRrho(),
swSCTp(),
swSTrho(),
swSigma0(),
swSigma1(),
swSigma2(),
swSigma3(),
swSigmaTheta(),
swSigmaT(),
swSigma(),
swSoundAbsorption(),
swSoundSpeed(),
swSpecificHeat(),
swSpice(),
swTFreeze(),
swTSrho(),
swThermalConductivity(),
swTheta(),
swViscosity(),
swZ()