Water masses govern to a large extent the circulation in an ocean basin away from the immediate vicinity of the surface. It is essential to initialize any ocean model with the temperature T and salinity S prevalent during the time of the model run. Unfortunately, it is very expensive to collect data on water masses in the ocean because of the large ship times needed to probe the deep parts of the ocean basin. However, the deep water masses do change very slowly, over time scales of hundreds of years and therefore it is possible to regard them as quasi-steady. Thus, appeal can be made to T and S data archived at national archives such as the National Oceanographic Data Center (NODC) and FNOC. These historical data can then be analyzed to derive gridded T and S fields for model initialization. Levitus  has done so at 1 degree resolution over the global oceans at 33 depths ranging from the surface to about 5000 m depth. There are enough data in the archives to derive meaningful temperatures over the upper 1000 m on a seasonal time scale and in the upper 200 m or so at monthly time scales. Data on salinity are more sparse. There are other T & S data bases, but none of these are so readily available for the general public. Provided is a program to access a Levitus gridded data base and extract any given region delineated by longitude and latitude and plot the results using GMT. These fields can also be animated.
(See exercise 7.)