Stormy and icy seas - Antarctica: Global Science from a Frozen Continent - page 141

of the East Wind Drift and katabatic winds generate offshore wind patterns that
are of great importance to sea ice formation.

Water mass formation

Temperature and the amounts of dissolved salts (salinity) characterise the
water masses in any ocean. Since they are not scattered randomly but occur in
well-de ned combinations this allows us to determine how each water mass gained

its speci ned by a pair of temperature and salinity

measurements and show up as points in a temperature/salinity plane, the so-called
T/S-diagram. Two water types mixing with each other are found on a line in the
T/S-diagram.
In addition to temperature and salinity, water masses can be distinguished by
their different content of dissolved substances, gases such as oxygen or carbon
dioxide, or nutrients, trace metals or anthropogenic substances such as CFCs.
Temperature and salinity of the oceans vary in both space and time
( Figures 5.4 and 5.5 ). They are determined by heat and fresh water exchanges at
the ocean surface. The ocean gains heat by solar radiation or heat transfer from
the warmer atmosphere and loses heat by radiation, contact with the colder
atmosphere or by evaporation. Freshwater is gained from precipitation, river in

c properties. Water types are de

ow
and melting ice and is lost by evaporation and sea ice formation. Since the ocean
gains heat in low latitudes and loses it in high latitudes, its temperature at the surface
normally decreases from low to high latitudes. Since there is an excess of evaporation
in the subtropics and of precipitation further north and south, there is generally a
decrease of sea surface salinity from the subtropics to the north and to the south.
A water mass originating in the subtropics would be warm and saline in comparison
to one from the mid-latitudes which would be colder and fresher. Ocean waters
of a given characteristic are transported by ocean currents out of the area where
they formed, initially forming tongues of warm or cold water but later mixing and
losing their individuality.
Temperature, salinity and pressure determine the density of seawater. Water
of higher temperature and/or lower salinity is less dense than water of lower
temperature and/or higher salinity so the warmest water is normally found at the
surface, resulting in a stable strati cation. If the water is cooled at the surface it gains

density, strati cation becomes unstable and the water starts sinking. If the salinity

increases by evaporation or by sea ice formation water increases its density as well
and sinking is enhanced. Therefore the most active sinking processes occur in high
latitudes where water increases density first by cooling and then by sea ice formation.

The vertical distribution of temperature and salinity in the Southern Ocean on a
transect from South Africa to the Antarctic continent is displayed in Figure 5.6 .