UNSTEADY, TURBULENT CONVECTION INTO A HOMOGENEOUS, ROTATING FLUID, WITH OCEANOGRAPHIC APPLICATIONS

Turbulent convection into a homogeneous, rotating fluid has been gener ated in laboratory tanks, for both laterally confined and unconfined d omains. When a given experiment was in a solid-body rotation, a source located at the top surface of the water column was activated to relea se denser saltwater into the underlying, less-dense fluid of total dep th H. As a result, a downward propagating 3D turbulent front was forme d. Eventually, at a transition depth z(c), rotational effects dominate d the turbulence and many quasi-2D vortices were generated, which then penetrated downward beneath the upper 3D turbulent layer. Measurement s in the confined experiments gave z(c) almost-equal-to (12.7 +/- 1.5) (B0/f3)1/2; the mean diameter (D(v)) of the quasi-2D vortices as D(v) almost-equal-to (15.0 +/- 1.5)(B0/f3)1/2, their downward speed of pro pagation (u(c)) as u(c) almost-equal-to (1.0 +/- 0.1)(B0/f)1/2, and th e maximum swirl velocity (u(v)) of an individual vortex as u(v) almost -equal-to (4.0 +/- 0.4)(B0/f)1/2 (where B0 is the surface buoyancy flu x and f is the Coriolis parameter). All are in agreement with scaling predictions presented here and in a number of previous publications. I n the unconfined experiments, when z(c) < H, the vortex columns of the type discussed above ''filled out'' after reaching the bottom, took o n a conical shape, and then underwent a collective baroclinic instabil ity with the resultant larger-scale vortices propagating away from ben eath the source. The velocity field of the vortex columns extended thr oughout the water column and many vortices surrounded the source. The measured diameter (D) of the circle of maximum velocity was consistent with the scaling D/H almost-equal-to (5.2 +/- 1)(Ro)1/2, where Ro* = (B0/f)1/2/fH = (B0/H-2f3)1/2 is a natural Rossby number of the flow b ased on the characteristic vortex velocity. When z(c) > H, and the rad ius of the source (R) was of the same order as the fluid depth, the tu rbulent layer contacted the bottom directly without forming small-scal e vortices, spread horizontally, and eventually baroclinic vortices fo rmed at the edge of the spreading front or gravity current. The diamet er of these vortices was consistent with the scaling D/H almost-equal- to (7.0 +/- 1) (Ro)2/3. Using an extension of the model of O. M. Phil lips, it is suggested that this result should be modified by multiplyi ng by a factor (R/H)1/3 when R/H is large. When z(c) = H, then (Ro)1/ 2 = 0.28, and this constitutes a transition Rossby number between the two regimes. These more extensive results are in agreement with previo us theoretical predictions and limited experimental measurements by th e authors. Results from the present study have been applied to convect ive events observed in the Golfe du Lion and the Arctic and a reasonab le agreement found, especially when the finite size of the source was taken into account. Furthermore, it is concluded, based on the scaling results found here, that fluid of a density large enough to constitut e ''bottom water'' can only be produced under circumstances that limit mixing with ambient fluid, for example, in an intense vortex column i n deep water or under an extensive region of cooling in shallow water. Arguments favoring these possibilities are presented.