THE GROWTH OF CONVECTIVE PLUMES AT SEA-FLOOR HOT-SPRINGS

The growth of buoyant plumes in the presence of stratification (N) and rotation (f) is studied and illustrated with a number of numerical ex periments of convection induced by a localized source of buoyancy at t he lower boundary of a linearly stratified fluid. The presence of stra tification constrains the convection in the vertical giving rise to an equilibrium-spreading layer which receives the rising mass of plume f luid; the plume can be divided into an upper, mass-source driven antic yclone and a lower, buoyancy-source (F) driven cyclone. With N/f large , the plume's rise-height is set by the classical non-rotating scaling l(N) = (F/N-3)(1/4). Physically motivated scaling laws invoke angular momentum constraints and indicate the fundamental role played by rota tion, which sets the scale l(f) = (F/f(3))(1/4). The lateral spread of the upper-level anticyclone is constrained by rotation: for times gre ater than f(-1) the anticyclone grows laterally at a rate which is ess entially independent of N, and given by l(f)(ft)(1/3); the ratio of th e lateral scale of the anticyclone to its vertical scale (aspect ratio ) is proportional to N/f. The cyclone's lateral scale is l(f), and the strong cyclonic flow scales like fl(f). An enhanced lateral mixing is suggested to occur in the cyclone along slanted angular momentum and isopycnal surfaces, which become closely aligned. On a much longer tim e scale, the scaling suggests that the lateral growth of the upper lev el anticyclone is arrested by its interaction with the lower level cyc lone; a baroclinic instability is expected to detach the anticyclone f rom the source after a time of order ft similar to 100N/f.