This is analogous to
swSigma0(), but referenced to 4000db pressure.
swSigma4( salinity, temperature = NULL, pressure = NULL, longitude = NULL, latitude = NULL, eos = getOption("oceEOS", default = "gsw") )
either practical salinity (in which case
pressure must be provided) or an
oce object, in
temperature (in the ITS-90 scale; see
next item), etc. are inferred from the object, ignoring the
other parameters, if they are supplied.
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,
longitude of observation (only used if
latitude of observation (only used if
equation of state, either
"unesco" (references 1 and 2)
"gsw" (references 3 and 4).
Potential density anomaly (kg/m\(^3\)).
See citations provided in the
Other functions that calculate seawater properties: