There are two basic circulation systems in our oceans. This section focuses on density-driven circulation. The ocean is a single body of water, but it is not homogeneous.
Oceanographers define distinct water masses based on their physical and chemical characteristics.
Temperature and salinity are the primary components used to recognize a particular water mass. When water warms up, its molecules vibrate and move around more. Denser water masses sink below lighter ones. A water mass seeks its appropriate density level and then spreads laterally. When one water mass sinks, another rises elsewhere.