To study currents, sound speed, and other properties of the water column, we use a CTD instrument that has sensors measuring Conductivity (which is converted to salinity -- the water's saltiness), Temperature, and pressure (which is converted to Depth). This instrument is lowered from the ship through the water column on a wire. Figure 1 shows a plot of temperature (blue), salinity (red), and sound speed (black) with depth taken with a CTD at about 27.0oN 95.6oW on 12 June 2005. Throughout the world's oceans, the pressure increases with depth, and the temperature generally decreases with depth. In the Gulf of Mexico, the salinity is generally relatively low near the surface. In places where circulation features, such as cyclonic and anticyclonic eddies, transport shelf water that has mixed with river water into deeper water, the salinity can be much lower than that shown in this figure. Salinity in the Gulf increases to a maximum, which typically occurs at 50-250 m, because of higher salinity water carried into the Gulf from the Atlantic by the Loop Current. The salinity then decreases to a minimum that occurs at about 500-1000 m; this minimum is from water formed in Antarctica that is transported into the Atlantic and then into the Gulf of Mexico in the Loop Current. Salinity then increases slightly below this. In deep waters of the Gulf, below about 1500 m depth, temperature and salinity have only very small changes.

Sound is used to study many aspects of the ocean environment. In SWSS, we use it to listen for and study the vocalizations of the sperm whale. Sound travels at speeds of about 1500 m/s, which is roughly 1 mile per second. The sound velocity depends on the temperature, salinity, and pressure of various areas of the ocean. It increases as the pressure, temperature, and salinity increase. The effects of pressure and temperature generally are more important than those of salinity. Note in Figure 2 that the sound velocity profile is similar in shape to the temperature profile. Note also that at depths below about 600 m the sound speed profile ceases to decrease and begins to increase even though the temperature is still decreasing--this is the effect of the increasing pressure on the sound speed. This high-low-high structure for sound speed creates the sound channel where sounds can travel long horizontal distances. This channel is not fully developed in this figure. Knowledge of the sound speed profile is important for tracking underwater sounds and estimating how far the sounds will travel.

To see the effects of pressure, students from the Toledo Middle School 7th grade science class in Toledo, Oregon, decorated standard styrofoam cups (Figure 2, upper panel). These were put in a cloth net bag and secured to the CTD frame by SWSS scientist Dr. Joel Ortega of Oregon State University (Figure 3). The CTD, its frame, and the cups were lowered to depths just below 1000 m (see Figure 1 for depth). When the CTD was recovered, the cups were removed from the bag and were found to have been shrunk by the immense pressure they experienced at depth (Figure 2, lower panel). The sperm whale dives to these depths and even deeper. The animal's body is specially adapted to withstand these pressures. So, unlike the cups shown here, when the sperm whale returns to the surface, it returns the same size it was when it dove down.