Gibbs Seawater (GSW)

3.6.21 verified Sat May 09 auth: no python

GSW is a Python implementation of the Thermodynamic Equation of Seawater (TEOS-10) for oceanographic computations. Current version 3.6.21, requires Python >=3.11. It provides over 200 functions for oceanographic variables, including density, salinity, sound speed, potential temperature, etc. Release cadence is sporadic, following updates to the TEOS-10 algorithms.

pip install gsw

Common errors

error ImportError: No module named gsw ↓

cause gsw not installed or Python environment not activated.

fix

Run 'pip install gsw' in the correct environment.

error TypeError: ufunc '...' not supported for the input types ↓

cause Inputs are not numpy arrays or have incompatible dtypes (e.g., strings).

fix

Ensure all inputs are numeric numpy arrays: e.g., SP = np.array([34, 35], dtype=float).

error ValueError: Invalid input: pressure must be non-negative ↓

cause Negative pressure values are not physically meaningful for ocean depths.

fix

Check data; pressure must be >=0 dbar. For surface pressure use 0.

Warnings

gotcha GSW functions expect numpy arrays; passing plain Python lists may raise errors in some operations. ↓

fix Convert inputs to numpy arrays (e.g., np.array([...])).

breaking gsw 3.0+ dropped support for Python <3.11. If you are on an older Python version, pin to gsw<3.0. ↓

fix Upgrade Python to 3.11+ or install gsw==2.x with pip install 'gsw<3'.

deprecated Some older function names (e.g., gsw.rho, gsw.alpha) have been renamed to use TEOS-10 standard names (e.g., gsw.rho_t_exact, gsw.alpha_wrt_CT_exact). ↓

fix Check the documentation for the exact function name; the old names are still available but may be removed in future.

gotcha Pressure must be in dbar (decibars), not in Pa. Many newcomers mistakenly pass pressure in Pa. ↓

fix Convert Pa to dbar by dividing by 10000.

Imports

gsw
```
import gsw
```
There is no common wrong import; just import gsw and use gsw.<function>()

Quickstart

Compute absolute salinity and conservative temperature from practical salinity and pressure.

import numpy as np
import gsw

# Example: compute absolute salinity from practical salinity
SP = np.array([35.0, 35.5, 36.0])
p = np.array([0, 10, 100])  # pressure in dbar
lon = np.array([-150, -150, -150])
lat = np.array([30, 30, 30])

SA = gsw.SA_from_SP(SP, p, lon, lat)
print(SA)

# Example: conservative temperature from potential temperature (0 dbar reference)
CT = gsw.CT_from_pt(SA, 15.0)
print(CT)