Exploring Our Fluid Earth

Water Layers and Currents

Water forms layers in the ocean because of differences in density that are due to differences in temperature and salinity. These water layers are not stable. Instead, water layers are constantly moving and interacting. A current is a moving mass of fluid—either a gas or a liquid. Both moving masses of air and moving masses of water are called currents: wind currents and water currents. Ocean currents refer to the movements of water masses in the ocean. The ocean can be thought of as a collection of many currents moving in different directions and at different levels. Ocean circulation is the total movement of all ocean currents.

Gravitational Circulation and Currents

The forces that move water to produce ocean currents are caused by density, gravity, winds, and the rotation of the earth. A gravitational current is a sinking mass of water that moves toward the ocean floor; its movement is called gravitational flow.

Thermohaline Water Flow and Global Ocean Circulation

The world ocean and atmosphere regulate heat transport across the planet through large scale ocean and atmospheric circulation, keeping earth’s temperatures relatively constant. In general, warm water is transferred from the equator to the poles and cooler water from the poles is transferred back towards the equator.

The Modeling Thermohaline Water Flow Activity was an oversimplified model of global ocean circulation, with several limitations. Water at the bottom of the ocean is very cold and salty, even at the equator. This very cold and salty water forms near the poles through the processes of sea ice formation and density changes. When sea surface ice begins to form, salts are excluded from the developing ice. The additional salt increases the density of the water below the ice. Once this cold very salty water is formed, because of its density, it sinks until it hits the ocean floor. This water flows along the bottom of the ocean toward the equator under warmer water layers. Unlike in the Modeling Thermohaline Water Flow Activity, large quantities of warm water are only on the surface of the ocean. This is because the sun can only heat the top layer of water. Although hydrothermal vents on the ocean floor emit very hot water, they do not release the amounts of hot water demonstrated by the model.

Models provide a way to understand systems that are too complex to observe directly, but they can oversimplify or focus on only a limited aspect of a complex concept. Scientists can use models to help make predictions about the natural world. However, it is important to acknowledge the model’s limitations. Motion of water in the ocean is affected by many factors. Thermohaline water flow plays a role in, but does not completely explain, global ocean circulation. Global ocean circulation is the result of many factors including density, wind, waves, tides, and the shape of the ocean floor.