The density of seawater, as well as the effects of temperature, salinity and pressure, are key parameters in the understanding of global ocean circulation, and also in the potential effects of global warming on sea levels. Density increases with depth due to the compression from the weight of the water above. Density will decrease with an increase in temperature – so as the oceans warm, they will expand due to increased kinetic energy between the molecules causing the volume of water to expand. Global warming will also release additional water into the oceans from melting land-ice. Warm water (being less dense) will remain above colder, denser waters apart from in situations where upwelling occurs such as around seamounts.
Movement of, and mixing between water masses of different densities, temperatures and salinities occurs in a 3-dimensional fashion and a number of studies have been undertaken to map the process. Long-term observations are needed to help capture the variations that happen over long-time scales (decades rather than years), as well as those variations that happen on an annual basis.
Density can be calculated from salinity (itself derived from conductivity (C) and temperature (T) measurements made at different depths (D). CTD instruments can be deployed vertically from vessels, or integrated into autonomous underwater vehicles (AUV), profiling floats and gliders to provide data that captures both vertical and horizontal variations, and these are able to be deployed from a variety of vessels of opportunity as well as from dedicated oceanographic and survey craft