Pressure, temperature, salinity, and some thermohaline dynamics

Abstract

Pressure is the force exerted on a unit surface that is oriented perpendicular to the direction of that force. According to the international system for weights and measures (the SI system) the unit for pressure is Pascal, with symbol Pa, equivalent to N/m² (SUN 1985). The pressure in sea is one of the thermodynamic variables of seawater, determining together with temperature and salinity a range of seawater properties, e.g., density, specific heat, sound velocity, etc. (Fofonoff and Millard 1983). The net force, exerted by a pressure gradient on a water parcel, also is one of the main driving forces of the THC. The pressure used in oceanography is generally the sea pressure, P, which is the actual pressure minus one standard atmosphere (= 101 325 Pa). When the oceanic motions are restricted to low frequencies (no surface waves, internal waves, or convective motion), vertical accelerations can be ignored and the vertical momentum equation is to a very high accuracy approximated by the hydrostatic equilibrium:

$$ \frac{{\partial P}} {{\partial z}} = - \rho g.(3.1) $$

Here z is the vertical coordinate (upward positive), ρ is the density of seawater and g is the gravitational acceleration. With ρ ≈ 1000 kg/m³ and g ≈ 10 m/s² the pressure in sea increases with about 10,000 Pa for each depth increment of 1 m. In oceanography the use of the non-SI unit decibar (symbol dbar) is allowed. A pressure increase of 1 dbar (10⁴ Pa) corresponds numerically with a depth increase of about 1 m.