Properties of Water and Seawater

Water is a unique substance, with unusual attributes because of its structure. The molecule consists of a central oxygen atom with two attached hydrogen atoms forming a bond angle of about 105°. As oxygen is more 

While the concept will be considered in detail below, the term salinity (S%o) is introduced here as a measure of the salt content of seawater, a 

From an oceanographic perspective, the fundamental properties of seawater are temperature, salinity and pressure i.e., depth dependent). Together, these parameters control the density of the water, which in turn determines the buoyancy of the water and pressure gradients. Small density differences integrated over oceanic scales cause considerable pressure gradients and result in currents. 

Surface water temperatures are extremely variable, obviously influenced by location and season. The minimum temperature found in polar latitudes approaches the freezing point of nearly —2 °C. Equatorial oceanic waters can reach 30 °C. Temperature variations with depth are far from consistent. In a region where mixing prevails, as observed especially in the surface waters, a layer forms with a relatively uniform temperature. The zone immediately beneath normally exhibits a sharp change in temperature, known as the thermocline. The thermocline in the ocean extends down to about 1000 m within equatorial and temperate latitudes. It acts as an important boundary in the ocean, separating the surface and deep layers and limiting mixing between these two reservoirs. 

Below the thermocline, the temperature changes only little with depth. The temperature is a non-conservative property of seawater because adiabatic compression causes a slight increase in the in situ temperature measured at depth. For instance in the Mindanao Trench in the Pacific Ocean, the temperature at 8500 and 10,000 m is 2.23 and 2.48 °C, respectively. The term potential temperature is defined to be the temperature that the water parcel would have if raised adiabatically to the ocean surface. For the examples above, the potential temperatures are 1.22 and 1.16 °C, respectively. Potential temperature of seawater is a conservative index. 

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Wang D.-P. and MiUero E. J. (1973) Precise representation of the P-V-T properties of water and seawater determined from sound speeds. J. Geophys. Res. 78, 7122—7128. 

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