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 Normal, is a display of the way convection might often occur at high latitudes. Here the water initially is assumed to be almost constant in temperature and salinity from top to bottom. At the times when the air immediately above is colder than the water, there is a transfer of heat from the water to the atmosphere. The surface waters cool, condense, become more dense and ultimately sink. Because the cooling can be very intense at high latitudes, the surface water can cool enough to sink to the bottom. Note in this animation that the convection is depicted to occur in a narrow, almost chimney like area. This is very much the way nature and deep convection behaves at high latitudes. Note later in this animation, the coldest water has made its way to the bottom and it appears the water is moving from right to left near the bottom. This depiction is meant to indicate a movement toward the tropics at these depths.
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 126.96.36.199.0