9.6: Boundary Currents
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Boundary currents: currents associated with gyres flow around the periphery of an ocean basin. Boundary currents are ocean currents with dynamics determined by the presence of a coastline.
Two distinct categories of boundary currents:
• western boundary currents
• eastern boundary currents
Figure 9.10. Speed of currents measured by drifting devices (annual average in cm/sec)
Western Intensification of Boundary Currents
Wind blows westward along the inter tropical convergence zone at the equator, causing western intensification.
• Wind blowing across the oceans mounds water on the western side of ocean basins-up to 2 meters.
• The mounding of water is caused by converging equatorial flow and surface winds.
• The Coriolis effect is most intense in polar regions, so current flowing eastward near the poles is more dissipated than currents flowing westward at the equator.
• The higher side of a mound is on the western side of the ocean basins, having a steeper slope and therefore faster moving.
Eastern boundary currents (EBC) are slow (a few miles or km per day), wide (less than 600 miles [1000 km]), and shallow (less than .3 miles [.51 km])
Examples: Canary, California, Benguela, Peru
EBCs form along the cool and dry east side of ocean basins.
Western boundary currents (WBC) are fast (many miles or km per day), narrow (less than 60 miles [100 km] wide), and deep (up to 1.3 miles [2 km])
Examples: Gulf Stream, Brazil, Kuroshio, E. Australian, Agulhas.
WBCs form along the warm and wet west side of ocean basins.
Figure 9.11. The California Current is an eastern boundary current; part of the Northern Pacific Gyre. The Kuroshio Current near Japan is a western boundary current.
Gyres and boundary currents are large scale, but are also complex. Boundary currents change constantly (called meandering) producing spinning cone-shaped masses of water - spinning off of larger boundary currents.
Satellite temperature data of the ocean surface reveals the spreading and mixing of surface waters as currents move from one region to another, gaining intensity and dispersing energy as they move. The temperature data reveals large spinning eddies in portions of the ocean basins along the margins of major currents.
Figure 9.12 illustrates large eddy currents forming in the surface waters of the southern Atlantic Ocean west of southern Africa as revealed by satellite ocean surface temperature data. The eddy currents form as a part of the meandering processes that dissipate energy in the ocean waters. This meandering creates warm- and cold-core rings of swirling currents (Figure 9.13).
Figure 9.12. Large eddy currents in the South Atlantic Ocean revealed by surface temperature data.