As we have seen, surface ocean currents are the dominant sources of deep water masses. In fact, it is a little more complicated than this as other deep water masses also feed one another. However, in a generalized sense, the surface and deep ocean currents can be viewed as an integrated system known as the Global Conveyor Belt, a concept conceived by the brilliant Geoscientist Wally Broecker of Columbia University. Diagrams of the Global Conveyor Belt (GCB) are two dimensional and therefore simplified and do not, for example, include all of the intermediate water masses or surface water currents. However, the key of the Global Conveyor Belt concept is that it explains the general systems of heat transport as well as bottom water aging and nutrient supply in the oceans.
Global Conveyor Belt
Video: Coral Growth (00:58) This video is not narrated.
The following animation traces the path of water through the surface and deep ocean showing the dominant features of the GCB including formation of NADW in the North Atlantic.
Video: The Thermohaline Circulation - The Great Ocean Conveyer Belt (2:46) This video is not narrated.
The GCB shows the dominant source of deep water in the oceans as North Atlantic Deep Water and how this splits in two to flow into the Indian and Pacific Oceans. In these locations, upwelling of the deep water mass produces surface water currents that generally flow back towards the original source of deep water in the North Atlantic. For heat supply, the conveyor belt involves the transport of heat and moisture to northwest Europe by the Gulf Stream; this accounts for about 30% of the heat budget for the Arctic region, making the GCB extremely important for climate in the Arctic.
Because deep-water masses circulate very slowly, the GCB takes about 1500 years to complete, meaning that the oldest water in the oceans is about this age. In addition, because oxygen is gradually depleted in deep waters as they age, and because CO2 contents and nutrients conversely increase, the oldest water masses of the ocean in the North Pacific are among the most nutrient-rich, CO2 rich, and oxygen-depleted waters in the ocean. Conversely, the newly produced NADW waters are among the most nutrient depleted, CO2 depleted, and well-oxygenated waters in the world.
As it turns out, recent research on the detailed configuration of surface and deep currents shows that circulation is much more complex than the GCB. Floats deployed in the ocean don’t always follow expected pathways in the GCB model. Wind actually plays a more significant role in causing downwelling than previously thought. Moreover, mixing by small systems or eddies plays a large role in driving surface currents.
Check Your Understanding
Global conveyor belt question:
How long does it take water to circuit the Global Conveyor Belt?
Click for answer.
It takes 1500 years for water to circuit the Global Conveyor Belt
Earth is sometimes called “the blue planet” because it is largely covered by water—about 70 percent of Earth’s surface is ocean. The ocean contains most of the world’s water and is very effective at absorbing and storing energy absorbed from sunlight. The exchange of heat between the ocean and the atmosphere drives the water cycle and influences climate. For example, heating of the ocean leads to evaporation—the primary way that liquid water from Earth’s surface moves into the atmosphere as water vapor.
The Sun is Earth’s main source of energy; however, it heats the planet unevenly. Because Earth is spherical and rotates on a tilted axis, different areas of the planet receive more sunlight at different times of year. The tropics—the latitudes near the equator—receive the most energy from sunlight averaged over the year while polar areas receive the least. This differential heating of the planet creates circulation patterns in the atmosphere and oceans as thermal energy is redistributed.
Earth’s global ocean is in constant motion, distributing thermal energy and nutrients around the world. Although there are many named oceans, such as the Atlantic or the Pacific Oceans, all of them are connected to each other. As warm water moves from the tropics toward the poles, it cools down. Near the poles, the cold water sinks and flows through the ocean basins, eventually making its way back toward the surface. Global ocean circulation is a complex system of ocean surface currents, deep currents, eddies, and gyres that is driven by wind and by temperature and density differences in the ocean.
For many years, the large-scale sinking, rising, and flow of ocean water was known as thermohaline circulation because of the influence of water temperature, salinity, and density. However, recent data and research have determined that this “global ocean conveyor belt” model was oversimplified and did not adequately describe the complexities of ocean circulation. The newer meridional overturning circulation (MOC) model accounts for the many varied factors that impact ocean currents while addressing the complexities of ocean circulation through averaging ocean flows over long time periods.
The ocean stores and distributes energy, supports life, and drives weather and climate. This video illustrates key processes in the ocean that can be observed using many different datasets from instruments and models, such as CERES (Clouds and the Earth’s Radiant Energy System), OSTIA (Operational Sea Surface Temperature and Sea Ice Analysis), OSCAR (Ocean Surface Current Analyses Real-time), SeaWiFS (Sea-Viewing Wide Field-of-View Sensor), and MODIS (MODerate-resolution Imaging Spectroradiometer). For example, the CERES Heat Flux and OSTIA sea surface temperature maps show how the Sun warms Earth unevenly. OSCAR Ocean Currents shows the speed and motion of sea surface currents, which are driven by winds as seen in the animated wind map. The visualization of surface and deep currents uses ECCO2 (Estimating the Circulation and Climate of the Ocean, Phase II) data to give a sense of flows at the surface and at a depth of 2,000 meters. The SeaWiFS/MODIS map shows the concentration of chlorophyll in the ocean, which indicates nutrient distribution and biological productivity. The OSTIA sea surface temperature anomaly maps shows how unusually warm or cool water in the equatorial Pacific, which happens during El Niño and La Niña, can alter weather patterns and affect plant growth.