Exercise 4: Water masses.

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• The figure below consists of four diagrams. On the right is a map of the Atlantic Ocean showing the station positions where the data were taken. Africa and Europe are seen on the right, South America on the left. The equator is shown as a thick horizontal line, the 30°S, 30°N and 60°N latitudes by thinner horizontal lines. The two vertical lines give the 30°W and 60°W meridians.

• On the bottom left is a TS-diagram. It gives temperature (TEMP) on the vertical axis from -10°C to 30°C and salinity (SALT) on the horizontal axis from 33 to 38 and shows all data points, from the surface to the ocean floor, obtained at all stations. We shall come back to the TS-diagram in a moment.

• On the top left is a salinity section. The vertical scale gives pressure (PRES) in decibars, which is roughly equivalent to depth in meters. There is no horizontal scale, but the distance can be appreciated by comparing the section with the ship's track on the right and by noting some topographic features (You may find that you can put to good use what you learned in Exercise 2): Antarctica is to the left of the figure. The section begins at 53°S, which is the latitude of the southern tip of South America. It then runs along the centre of the western Atlantic, crossing the Rio Grande Rise (the first rise of the ocean floor in the left part of the salinity section). It crosses the Mid-Atlantic Ridge (the large double-peaked rise in the centre of the section) at the equator and continues through the eastern basins until it reaches Iceland on the right.

• The diagram in the bottom centre shows the location of the data points along the section. As you can see, the data density in the upper ocean is so dense that the individual points blend into a red area, but at greater depth the sampling is much less dense. This diagram is not of much concern to us in this context.

The salinity in the salinity section is indicated by colour. The same colour scaling is used in the TS-diagram: Salinities less than 34.7 are shown in blue and aquamarine, salinities between 35.2 and 35.5 in white, and salinities above 35.5 in various shades of orange. A simple way of remembering the colour code is blue means fresh, orange means salty.

With this rule in mind we can interpret the salinity section as follows: A layer of relatively fresh (blue) water extends from the surface near Antarctica towards the equator, spreading out at about 800 - 1000 m depth. This is the Antarctic Intermediate Water (AAIW).

The diagrams are linked by a marker spot which identifies particular data points. In the diagrams below, the marker spot is placed at the southernmost station. In the figure on the right it is therefore seen at the southern end of the ship's track, while in the salinity section it is found at the extreme left near the surface. In the TS-diagram the marker spot can be seen near the lowest salinities at around 1 2 °C .

The marker spot confirms our earlier finding that Antarctic Intermediate Water is seen in a TS-diagram as a salinity minimum. At the southernmost station, which is located in the formation area of AAIW, the marker spot is found at the lowest observed salinity. As AAIW spreads northward, it mixes with the water above and below, and its salinity increases. We can verify this by moving along the section, always at the level of the salinity minimum. This will show us the northward movement of AAIW and the change of its properties along its path. The animation shows this with a moving marker.

Things to watch for in the animation:

• The path of AAIW is followed from its formation region in the south to about 15°N; check this in the Atlantic Ocean map.

• AAIW moves northward at about 1000 m depth; check this in the salinity section.

• As it moves northward its salinity, which is very low initially, increases through mixing with the water above and below. Check this in the TS-diagram. The section shows this, too; the marker spot moves from blue into aquamarine and eventually white.

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