Exercise 4: Water masses.

The similarity of the TS-diagram between widely separate stations indicates the presence of water masses. Water masses are formed through interaction with the atmosphere which - through rainfall and evaporation - influences the salinity and - through heating and cooling - influences the sea surface temperature. A water mass is a body of water with a common formation history. Water masses formed in high rainfall areas are characterised by relatively low salinity, water masses formed in polar regions by very low temperatures etc The convergence of all TS-curves towards one TS-point at low temperatures indicates the presence of Antarctic Bottom Water (AABW). This water mass is formed at the Antarctic continent, where cool and saline surface water sinks down the continental slope to the ocean floor. AABW spreads northward from the Southern Ocean and fills all oceans below about 4000 m depth.

Water mass formation regions

The minima and maxima in the TS-diagram indicate the presence of two other water masses. Antarctic Intermediate Water (AAIW) is formed in the region of the southern hemisphere westerlies, where rainfall is large, and therefore has low salinity. It sinks to about 1000 m depth and moves northward in all oceans. In a TS-diagram it is seen as a salinity minimum. The diagram above indicates AAIW as a dark circle. As this water moves northward it mixes with the more saline water below and above. This increases its salinity and shifts the salinity minimum towards higher salinity values, along the dark line shown in the diagram.

The salinity maximum indicates the presence of North Atlantic Deep Water (NADW). This water mass is formed in the Greenland and Labrador Seas where it sinks to about 3000 m depth and moves southward through the Atlantic Ocean, eventually reaching the Southern Ocean and from there entering all other oceans. The diagram above shows it as a green circle. Like AAIW, the NADW mixes with the water below and above, which reduces its salinity as it progresses southward and produces a southward decrease of the associated salinity maximum along the green line shown in the diagram.

The water mass of the upper 1000 m, where temperature and salinity change rapidly with depth along a well defined line in the TS-diagram, is known as Central Water (CW). Each ocean has its own varieties of Central Water in both hemisperes, which we are not going to follow up here.

The next few pages study the distribution and pathways of some key water massses in data animations.