This exercise looks at the water masses of the deep ocean. It is the first in this series of exercises which is by neccessity entirely based on oceanographic observations. Most pages of the exercise are based on data collected along the meridional section in the Atlantic Ocean shown on the right.

This diagram shows how temperature varies with depth at three locations: in the polar region (at 53.5ºS), in the subtropics (30ºS), and in the tropics (10ºS). (Similar temperature profiles are obtained at locations in the northern hemipshere.)

Features to note are:

1. In the polar regions, the temperature does not vary much with depth. It is also much colder than 4ºC, the expected winter temperature at the bottom of a freshwater lake. In the Atlantic Ocean at 53.5ºS temperature varies between 2.4ºC at the surface and 0.9ºC at 2000 m depth, with a minimum of 0.4ºC near 100 m.
2. Below 1000 m, temperatures in the subtropics and tropics are only slightly warmer than in the polar regions. In the Atlantic Ocean they vary from about 3.8ºC at 1000 m to about 2.7ºC at 2000 m.
3. Above 1000 m temperatures increase rapidly towards the surface, to about 24ºC at 30ºS and to about 28ºC at 10ºS.
4. As the surface is approached, over the last few tens of meters temperature does not vary at all (in all climatic regions). The layer of constant temperature is caused by mechanical stirring of the water from wind waves (wind mixing); it is therefore known as the mixed layer. Its thickness varies with the seasons. In the example stations shown in the graph, the mixed layer is 40 - 50 m thick.