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BackgroundEkman Transport (Image credit: NASA. Studying Earth's Environment From Space) Two major forces drive ocean currents: pressure gradients and wind stress. Pressure gradients can be caused by differences in sea surface height, similar to water piling up on one side of a bucket, or by density differences, similar to the gas from dry ice flowing off the stage at a concert. These gradients have many causes, such as, wind blowing over water near a boundary, density differences based on temperature and salinity changes, or changing water surface slopes due to the tides or waves. Wind generated currents are the result of friction between the water surface and the overlying atmosphere. When the wind blows, it drags surface waters along with it. As surface waters move in response to the wind stress, energy is transferred to greater depths, creating motions well below the ocean surface. Near-surface waters move to the right of the wind (Northern Hemisphere). This phenomenon is called an Ekman Spiral, in honor of Vagn Walfrid Ekman, the scientist who first predicted this flow pattern theoretically. The spiral is caused by the rotation of the Earth, or the Coriolis effect. Averaged over the depth of the spiral, the transport moves 90o to the right of the wind direction in the Northern Hemisphere (90o to the left of the wind in the Southern Hemisphere). The wind’s strength and latitude (the higher the latitude, the stronger the Coriolis effect) determine the depth of the Ekman Spiral, but is typically about 100 meters. The major ocean currents, however, do not result directly from the wind. In the North Atlantic, for example, the Trade winds force the Ekman transport towards the north, while the Westerlies force it towards the south. In the ocean region separating these major wind belts, surface waters converge to create a slight mound in the ocean surface. Geostrophic Flow and Ocean Gyre Circulation (Used with permission of the American Meteorological Society) Like a high-pressure system in the atmosphere, the waters beneath the mound tend to flow AROUND the high in a clockwise direction. This concept is called geostrophy. Because of the shape and rotation of the Earth, the peak of the mound is closer to the western side of the ocean basin than to the eastern side. This results in much stronger currents on the western side of ocean basins. This happens in every ocean basin and the currents are known as western boundary currents. In the subtropical North Atlantic, this current is called the Gulf Stream. Henry Stommell was the first physical oceanographer to show theoretically how the combination of the earth's rotation, wind and friction form western boundary currents. In coastal regions, where the water depth is much less than in the open ocean, ocean currents are slowed by friction from contact with the sea floor and the coastline. These interactions change the balance of forces and can result in surprisingly complicated flow patterns in coastal regions. Scientists, applying new technology, are beginning to understand these coastal currents. Currents like the Gulf Stream and some coastal currents meander like a lazy river passing through hilly terrain. Meanders are commonly caused by the deflection of the current from seafloor bathymetry, projections of the shoreline or random instabilities within the current. If the flow in the main current becomes very curvy, a meander may be cut off, creating an eddy. False Color SST (AVHHR) image of the western North Atlantic (Image credit: NOAA) Currents are usually described in terms of both horizontal and vertical variations. For example, current patterns in a 1000 m deep water column that stretches over several thousands kilometers, such as the Gulf Stream, will be described very differently from current patterns associated with a 10 m deep estuary that is only a few kilometers long. As a result, oceanographers use local boundaries (i.e., coastline, shelf break, ocean surface, ocean bottom) to help define the degree of spatial and temporal variability that they are able to resolve. The multi-layered oceanSurface: The surface of the ocean is loosely defined as the vertically well mixed portion in the upper part of the water column. HF Radar maps, as well as some of the ADCP measurements, on this site represent surface currents. Surface current data from drifters are also available. Middle: The middle water column lies below the surface layer but does not include the near-bed region. ADCP measurements displayed on this site depict middle water column currents. Bottom: The bottom, or benthic, boundary layer includes the near-bed region in which frictional forces caused by contact with the seabed exert a strong influence on current patterns. Bottom current observations are derived from ADCPs and fixed-point measurements within the benthic boundary layer. Because the thickness of the water column varies widely from the deep ocean to coastal regions, graphics presented on this site contain information on total water column depth. Therefore, the 3-layer description of the water column must be viewed in the context of the location the measurement is made. The Gulf Stream System1769 Franklin-Folger chart of the Gulf Stream (Image credit: NOAA) The Gulf Stream System originates in the tropical North Atlantic, born of two other currents. One current, known as the North Equatorial Current, moves east of the Bahamas. The other moves through the Caribbean Sea, travels through the Gulf of Mexico as the Loop Current, and then speeds up in the Straits of Florida. When the two parts join, this western boundary current, the Gulf Stream, travels north along the coast of the southeaster United States. The Gulf Stream veers off the North American continent near Cape Hatteras and travels across the Atlantic Ocean. This section of the Gulf Stream has many meanders that periodically "pinch off" to form eddies. Because of their persistence, these eddies are commonly referred to as Gulf Stream rings. Some Interesting Gulf Stream System Facts
How to Read a Current VectorLatest SEACOOS Surface Current measurements Reading a current vector is similar to a wind barb but a little different than reading the color dots on a temperature map. A current vector is a type of measurement that includes both quantity and direction, and the speed and direction of ocean currents are visually depicted by arrows. The length of the arrow is directly proportional to the speed and the arrow always points in the direction the current is moving. Some of our graphic displays, such as HF Radar data, depict many vectors on a single map. Because the flow speed can vary significantly from region to region and through time, each graphic has a legend depicting a reference vector that is labeled with the speed it represents. All vectors on the map that are shorter than the reference vectors indicate slower speeds and vice versa. Scientists measure currents in units of meters per second (m/s) or centimeters per second (cm/s). Currents not only have variable strengths, but they travel in a certain direction. To distinguish which way the current is moving, oceanographers divide the current into east/west, north/south and up/down components. Note that currents can flow vertically (up/down), not just horizontally (east/west or north/south). Data Gathering and Quality ControlOceanographers use various techniques to measure current speed and direction. One traditional way is to use a drifting buoy and track its progress over time. Drifter tracking is now conducted largely by satellite. Another very common way is to use current meters mounted directly on fixed platforms or ships to measure the speed and direction as the water flows past a fixed point. Satellite altimeters detect large-scale currents by measuring sea-surface height (SSH) and relate slopes in SSH to the current strength and direction. Coastal currents can be measured by fixed current meters attached to moorings on the continental shelf. As you can imagine, it would take thousands of current meters to resolve the current patterns throughout and area as large as the continental shelf. Land based radars provide a way to measure surface currents at many points throughout the continental shelf simultaneously. This technology represents the first of its kind to resolve the spatial complexity of water movement in coastal regions. Layer Descriptions
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