Some oceanography basics

This page contains some useful definitions and explainations for the various oceanographic/environmental data collected by Seagliders.

CTD data

Temperature

Seaglider CTD measures temperature, conductivity, and pressure and calculates salinity from the CTD data

temperature (in-situ): temperature measured at specific depth

potential temperature (\({\theta}\)): temperature a water parcel would be if brought adiabatically to the surface. Water heats up as it is compressed at depth, cools as it expands near surface. This is also referred to as theta (\({\theta}\))

conservative temperature: temperature per TEOS-10. For more info see teos-10.org: Conservative Temperature from in-situ temperature

Seaglider measures raw in-situ temperature, conducts some quality control (qc) assessment, and reports a final corrected temperature as well as a qc matrix on whether to trust that corrected temp or not. In .nc files these are temperature_raw, temperature_raw_qc, temperature, and temperature_qc respectively. It also reports potential temperature, based on corrected salinity, reported as theta and conservative temperature as conservative_temperature.

Conductivity

The CTD measures seawater electrical conductivity. Seaglider measures raw conductivity, conducts some qc, corrects for anomolies, and proivdes a qc matrix for corrected conductivity. In .nc files these are conductivity_raw, conductivity_raw_qc, conductivity, and conductivity_qc respectively.

Salinity

salinity (in-situ): salinity calculated from in-situ temperature, conductivity, and pressure (aka practical salinity). It is unitless (PSU = pratical salinity units)

absolute salinity: absolute salinity per TEOS-10. It is calculated with conductivity and some other factors and the units are g/kg. For more info see teos-10.org: Absolute Salinity from Practical Salinity

Salinity is calculated/provided in the Seaglider data as raw and corrected salinity. (1) From raw temperature and conductivity. In the .nc file, salinity_raw is from the temperature_raw and conductivity_raw. That has a corresponding qc matrix salinity_raw_qc. (2) Corrected for termal-inertia effects. In the .nc file this is salinity. It also includes a qc matrix salinity_qc. Absolute salinity is included as absolute_salinity.

Density

Seawater density can be calculated a variety of ways, many of which are calculated/reported by the Seaglider.

density (in-situ): density at depth/pressure, calculated from temperature, salinity, and pressure. This is reported as density_insitu in the Seaglider .nc file. The netCDF attribute states units are g/m3 but the values make more sense as either g/cm3 or kg/m3

potential density: density a parcel of water would be at the surface, calculated similar to density but using potential temperature and a reference pressure of 0. Reported by Seaglider in .nc file as density. The .nc attribute states its in units of g/m3 but the values make more sense as either g/cm3 or kg/m3

sigma (\({\sigma}\)): in-situ density calculated from in-situ temperature, salinity, and pressure. it is expressed in a shorthand way by subtracting 1000. E.g., if density is 1027.5 g/cm3 its 27.5 sigma units

\[ \sigma = \rho(S, T, P) - 1000 \, \frac{kg}{m^3} \]

Where:

  • \({\sigma}\) is the in-situ density anomaly.
  • \({\rho(S, T, P)}\) is the density calculated from in-situ salinity (\(S\)), in-situ temperature (\(T\)), and in-situ pressure (\(P\)).

sigma-t (\({\sigma_T}\)): density calculated using in-situ temperature and salinity but with effect of pressure removed (reference pressure 0)

\[ \sigma_T = \rho(S, T, 0) - 1000 \, \frac{kg}{m^3} \]

Where:

  • \({\sigma_t}\) is the sigma-t density anomaly.
  • \({\rho(S, T, 0)}\) is the density calculated from in-situ salinity (\(S\)) and in-situ temperature (\(T\)), but at surface pressure (0).
Warning

Seaglider uses sigma-t for ballasting!!

sigma-theta (\({\sigma_\theta}\)): potential density, calculated using potential temperature and in-situ salinity, also referenced to sea surface pressure 0. In the Seaglider .nc file this is sigma_theta

\[ \sigma_{\theta} = \rho(S, \theta, 0) - 1000 \, \frac{kg}{m^3} \]

Where:

  • \({\sigma_\theta}\) is the potential density anomaly.
  • \({\rho}\) is the density of the seawater.
  • \(S\) is the in-situ salinity.
  • \({\theta}\) is the potential temperature.
  • The pressure is referenced to the surface (0 decibars).

The Seaglider .nc file also reports sigma_theta (\({\sigma_\theta}\)) at a few reference pressures following the Gibbs-SeaWater (GSW) Oceanographic Toolbox: gsw_sigma0 with a reference pressure of 0 m, gsw_sigma3 with a reference pressure of 3000 m, and gsw_sigma4 with a reference pressure of 4000 m. gsw_sigma0 and sigma_theta should be the same (but I haven’t confirmed this!).

Sound speed

Seaglider .nc files provides speed of sound in sea water as the sound_velocity output (units m/s). This can be calculated from CTD data but there are several possible quations and it is not clear what equation is used to calculate this metric. I suspect it is the Gibbs Seawater Toolbox but need to ask IOP…

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