Ocean and atmospheric circulation play an essential role in sustaining life by moderating climate over much of Earth's surface. An important part of the circulation of heat and freshwater and other sea water constituents are ocean surface currents. Their strength and variability play a role in weather and climate, impact environments for all life on Earth.
Global surface current patterns are driven by the wind, impacted by the barriers to flow provided by the land masses and the rotation of the earth, and ultimately derive their energy (like the wind) from the sun.
Two circulation patterns dominate the ocean: wind-driven currents in the upper ocean and the circulation in the deep ocean. Wind-driven currents are maintained by momentum transferred by the winds to the ocean surface. Ocean the wind sets the surface waters in motion as a current, the Coriolis force, the density distribution of sea water, and the shape of the ocean basin modify the speed and direction of the current.
Western boundary currents such as the Gulf Stream are among the fastest surface currents in the ocean. Western boundary currents flow toward the poles, northward in the Northern Hemisphere and southward in the Southern Hemisphere along the western boundaries of the ocean basins. Water moving in these currents transport large quantities of heat from tropics to mid-latitudes.
Eastern Boundary currents, such as the California current are slower, shallower, and wider than the western boundary currents. Similar to the return flow in a household heating system, these currents transport colder waters into the tropics where they are heated and transported poleward in the western boundary currents.
Ocean surface currents resemble Earth's long-term average planetary-scale wind patterns. Surface currents form gyres roughly centered in each ocean basin. Viewed from above, currents in these subtropical gyres flow in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere.
How do we gather and use ocean surface current data?
Ocean surface current research focuses on six threads: Older methods include tracking drifting objects (flotsam) and shift drift data (from navigation logs). Newer methods include satellite derived currents, surface current following drifters, surface feature tracking, and high frequency radar studies.
The following satellites and the instruments they carry are used for this research:
- TOPEX/Poseidon and Jason-1 measure ocean surface topography and circulation response to winds. These measurements allow scientists to study the ties between the oceans and atmosphere, to improve global climate forecasts and predictions, and to monitor events such as El Niño conditions and ocean eddies.
- QuikSCAT uses the on-board SeaWinds instrument to observe wind speed and direction. The QuikSCAT mission seeks to acquire all-weather, high-resolution measurements of near-surface winds over the global oceans. SeaWinds data is also combined with measurements from scientific instruments in other disciplines to help us better understand the mechanisms of global climate change and weather patterns.
- The SeaWiFS instrument observes how subtle changes in ocean color signify various types and quantities of marine phytoplankton (microscopic marine plants). The large-scale patterns formed by drifting concentrations of phytoplankton show scientists the location and movement of ocean surface currents.
- Terra's MODIS instrument monitors large-scale changes in the biosphere that yield new insights into the workings of the global carbon cycle and global heat transport via the ocean. MODIS can measure the photosynthetic activity of phytoplankton to yield better estimates of how much carbon is being absorbed and used in plant productivity. Coupled with the sensor's surface temperature measurements, MODIS' measurements of the biosphere are helping scientists track the sources and sinks of carbon dioxide in response to climate changes. Scientists in physical oceanography use surface temperature measurements and the location of phytoplankton to identify the location and movement of ocean surface currents.
How does this research impact our lives?
For centuries, people have used ocean surface currents to explore the world and transport good to market. Today we use them to take the most efficient path to save fuel in the shipping industry, to win a sailboat race, and to track pollution such as oil spills or assist in search and rescue operations. Ocean surface currents contribute to studies of severe weather such as hurricanes, short-term climate phenomena such as El Niño and long-term climate variability.