Understanding the variability of the density of ocean water is critical to understanding changes in the ocean's circulation, particularly those parts of the circulation that pertain to climate. In the tropics, the sun warms the surface water and causes that water to expand. Because the surface water is now less dense than the cooler water below, the warmest waters remain near the surface. Near the poles, the energy input by the sun is not as strong, and the surface waters are not warmed to the degree they are away from the poles. Here, it is the salinity of the water plays a critical role as to which water is found at the surface as the waters near the surface are not that much different in temperature to the water below. These animations highlight the crucial role of salinity in high latitude convection (upward and downward movement of water) and climate.
This animation, labeled Fresh, illustrates the condition where the water near the surface is assumed to be much fresher than the saltier water below. Now when a atmosphere cools the surface water, the water sinks, but it does not make it all the way to the bottom. The scenario displayed is one where the condensing effect of the cooling is not strong enough to overcome the effects that salinity has on the density of the water. The less saline the water, the less dense it is. A cold fresh layer of water is constrained near the surface. Sometimes, this layer can even freeze insulating the water from any further cooling by the atmosphere. Note that in this animation there is very little movement of the water at depth back toward the tropics.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 220.127.116.11.0