Development of a point plume in the presence of background rotation

Abstract

This paper describes a laboratory study on the evolution of a point turbulent plume placed at the free surface of a homogeneous fluid layer in the presence of background rotation. It is shown that the plume initially evolves as if there is no rotation. However, the rotational effects become important after the plume descends a vertical distance hc1≈3.3(B/Ω3)1/4 for a normalized time Ωtc1≈2.4, whence the vertical descent rate of the plume is reduced while maintaining approximately the same lateral growth rate. Here Ω is the rate of background rotation and B is the specific buoyancy flux of the plume. The rotational effects inhibit the lateral growth of the plume at a time Ωtc2≈5.5, when the maximum plume width is bc≈1.4(B/Ω3)1/4. Thereafter, the vertical descent continues and the plume evolves into a cylindrical shape while developing a cyclonic circulation in and around it, except near the plume front. Upon reaching the bottom surface after traveling a fluid depth of H, the plume deflects, propagates horizontally, and becomes unstable breaking up into anticyclonic eddies. Studies carried out for the case of H<hc1 show that this instability is initiated at a horizontal length scale proportional to the Rossby deformation radius of the deflected flow, and hence it is of baroclinic type. These eddies appear to align vertically with the cyclonic eddies formed by the barotropic instability of the surface rim current, thus producing heton-like structures. The influence of the diameter d0 of the plume on the flow evolution is also studied, and it is shown that plumes with aspect ratio h/d0<12 (where h is the vertical extent) can be approximated as point plumes. Scaling arguments are advanced to explain the results. Some geophysical applications of the study are also discussed.