gsw_gibbs function

Gibbs Energy of Seawater, and its Derivatives

gsw_gibbs(ns, nt, np, SA, t, p = 0)

Arguments

• ns: An integer, the order of the SA derivative. Must be 0, 1, or 2.
• nt: An integer, the order of the t derivative. Must be 0, 1, or 2.
• np: An integer, the order of the p derivative. Must be 0, 1, or 2.
• SA: Absolute Salinity [ g/kg ]. The valid range for most gsw functions is 0 to 42 g/kg.
• t: in-situ temperature (ITS-90) [ degC ]
• p: sea pressure [dbar], i.e. absolute pressure [dbar] minus 10.1325 dbar

Returns

Gibbs energy [ J/kg ] if ns=nt=np=0. Derivative of energy with respect to SA [ J/kg/(g/kg)^ns ] if ns is nonzero and nt=np=0, etc. Note that derivatives with respect to pressure are in units with Pa, not dbar.

Implementation Note

This R function uses a wrapper to a C function contained within the GSW-C system as updated 2022-10-11 at https://github.com/TEOS-10/GSW-C with git commit 657216dd4f5ea079b5f0e021a4163e2d26893371.

The C function uses data from the library/gsw_data_v3_0.mat

file provided in the GSW-Matlab source code, version 3.06-11. Unfortunately, this version of the mat file is no longer displayed on the TEOS-10.org website. Therefore, in the interests of making GSW-R be self-contained, a copy was downloaded from http://www.teos-10.org/software/gsw_matlab_v3_06_11.zip on 2022-05-25, the .mat file was stored in the developer/create_data directory of https://github.com/TEOS-10/GSW-R, and then the dataset used in GSW-R was created based on that .mat file.

Please consult http://www.teos-10.org to learn more about the various TEOS-10 software systems.

Caution

The TEOS-10 webpage for gsw_gibbs does not provide test values, so the present R version should be considered untested.

Examples

library(gsw)
p <- seq(0, 100, 1)
SA <- rep(35, length(p))
t <- rep(-5, length(p))
## Check the derivative wrt pressure. Note the unit change
E <- gsw_gibbs(0, 0, 0, SA, t, p)
# Estimate derivative from linear fit (try plotting: it is very linear)
m <- lm(E ~ p)
print(summary(m))
plot(p, E)
abline(m)
dEdp1 <- coef(m)[2]
# Calculate derivative ... note we multiply by 1e4 to get from 1/Pa to 1/dbar
dEdp2 <- 1e4 * gsw_gibbs(0, 0, 1, SA[1], t[1], p[1])
## Ratio
dEdp1 / dEdp2

References

http://www.teos-10.org/pubs/gsw/html/gsw_gibbs.html