The Stability of Buoyancy-driven Coastal Currents

Griffiths, R.W. and Linden, P.F., 1981. The stabil i ty of buoyancy-dr iven coastal currents. Dyn. Atmos . Oceans, 5: 281--306. Buoyancy-dr iven boundary currents were generated in the labora tory by releasing buoyan t fluid f rom a source adjacent to a vertical boundary in a rota t ing container . The boundary removed the Coriolis force parallel to it, a l lowing the buoyan t fluid to spread in a current along the boundary . Use of a cylindrical boundary and a line source that released fluid un i formly around the c i rcumference enabled an ax isymmetr ic (zonal) current to be produced. With the con t inuous release of fluid f rom the source, the current grew in width and depth unti l it became unstable to non-ax isymmetr ic disturbances. The wavelength and phase velocit ies o f the disturbances were consistent wi th a mode l of barocl inic instabil i ty of two-layer f low when fr ict ional dissipation due to Ekman layers is included. However , when the current only occupied a small f ract ion of the total depth, baro t ropic processes were also thought to be impor tan t , with the growing waves gaining energy f rom the hor izonta l shear. In o ther exper iments , gravity currents were produced by a poin t source adjacent to ei ther a zonal (circular) or a meridional (radial) vertical boundary . The currents were also observed to become unstable to the same upst ream breaking waves as those on the cont inuous zonal current . Finally, some compar isons are made with oceanic coastal currents. 1. I N T R O D U C T I O N Buoyancy-driven coastal currents occur in many parts of the world oceans. A particularly striking example is the East Greenland Current in which cold, fresh polar water flows southward from the Arctic Ocean along the east coast of Greenland. It occupies a wedge-shaped region some 200 km in width, with the maximum depth (200 m) at the coast. The primary driving force appears to be the density difference between the polar water and the water of the Norwegian Sea. Where land masses are not present, the southward spreading of the less dense polar water is inhibited by the effects of the Earth's rotation. Southward motion induces an east--west flow to conserve angular momentum, and this flow, in turn, produces a north--south Coriolis force which opposes the buoyancy force. Consequently, further southward motion is inhibited. However, at a meridional barrier such as the 0 3 7 7 0 2 6 5 / 8 1 / 0 0 0 0 0 0 0 0 / $ 0 2 . 5 0 © 1981 Elsevier Scient if ic Publishing Company