By Andrew Pressly, Education and Engagement Coordinator at FMM
Like air in the atmosphere, ocean water is in perpetual motion. This movement of water forms what are known as currents. Currents can be local, affecting only a limited region, or global, encompassing several oceans. The movement of water around the globe can dramatically affect the climate, local ecosystems, and the transfer of heat. A variety of factors drive currents but the two strongest forces are wind and water density. Wind is the primary controlling force of surface currents while deep ocean currents are density driven.
Winds are one of the main controlling factors for surface currents. Specifically, large scale surface currents are driven by the major wind belts surrounding the planet. These large wind belts continuously blow in the same direction which keeps water moving in the same general direction. Surface currents are responsible for transferring heat from the tropics toward each of the poles. The transfer of heat from the equator toward the poles has direct effects on the local and global climate. This is seen within the Gulf Stream originating in the tropical Caribbean.
Running along the east coast of the United States before turning toward Europe, the Gulf Stream carries a tremendous amount of warm water northward. The heat stored in the Gulf Stream is what keeps most of Northern Europe significantly warmer than other locations that are equally far north. In fact, England is the distance from the equator as parts of Canada; however England experiences a much warmer climate year round. If the Gulf Stream did not carry warm water toward Europe, England and its neighboring countries would have a much colder climate similar to Canada. As the warm water from the Gulf Stream flows north to the Norwegian Sea, the colder, denser water surrounding the North Pole sinks and moves south along the bottom of the ocean. This process is what forms deep ocean currents.
Deep Ocean Currents
These currents take place at a depth below 100 meters and are not driven by wind but rather density. Changes in water density, resulting from the variability of the temperature and salinity of water, form the process known as thermohaline circulation. Surface currents carry water to cold regions like the North Atlantic Ocean. In this area the water becomes cold and dense. As the ocean water freezes salt is left behind increasing the saltiness and density of the water further. The salty-cold-dense water sinks toward the ocean floor and surface water moves in to replace it. This surface water in turn becomes cold and salty and sinks, driving what is known as the global conveyer belt. The global conveyer belt is a system of interconnected deep and surface currents that circulate water through the oceans on a 1000 year time frame. While this global circulation is slow, it is critical for maintaining a stable climate system on Earth. Unfortunately, this global circulation can be disrupted. Some research suggests that global warming will lead to melting sea ice and glaciers, and increased rainfall in the Northern Atlantic. This increase in warm freshwater into the surface of the ocean could slow or halt the global conveyor belt. If the conveyor belt was to stop it could result in drastic temperature changes in Europe and other places around the world.
Surface and deep ocean currents work together to create the world as we know it today. It is important that sailors and scientists can continue to monitor and understand these currents because of their impact on the climate, biological ecosystems, global economy, and much more.