From AMS Glossary
Ocean currents with dynamics determined by the presence of a coastline.
They fall into two categories: 1) western boundary currents, which are narrow, deep-reaching, and fast-flowing currents, not unlike jet streams, associated with current instability and eddy shedding; and 2) eastern boundary currents, which are shallow, cover a wider region, are of moderate strength, and are often associated with coastal upwelling and a subsurface countercurrent along the continental slope. Both are integral parts of the circulation in oceanic gyres. The rotation of the earth causes an accumulation of energy on the western side, which has to be dissipated in boundary currents; this gives the western boundary currents typical widths of 100 km and typical speeds of 2 m s-1 and causes them to shed eddies frequently to increase the dissipation of energy. No similar requirement of energy dissipation exists on the eastern side, so eastern boundary currents can be broad and slow. Their special character as a boundary current results from coastal upwelling, which brings the thermocline to the surface and as a result produces a temperature front and an associated geostrophic maximum in the current speed, known as the coastal jet. Because of the upwelling, eastern boundary currents are atmospheric heat sinks. Western boundary currents are atmospheric heat sinks if they move cold water toward the equator, which occurs in the subpolar gyres, and atmospheric heat sources where they move tropical water into temperate regions, as in the subtropical gyres.