and S that maintains a well-defined
plume stem with its accompanying large lateral property gradients. Initial parameter sensitivity experiments made with a three-dimensional time-dependent
hydrodynamical model of buoyancy-driven plumes in sheared, stratified cross
flows have shown a number of effects on plumes caused by differences in rotation
rates, turbulent mixing intensity, and cross-flow strength. While the context
of the work here is for hydrothermal plumes rising several hundreds of meters
into the benthic ocean as the result of chronic releases of magmatic heat from
vents along submarine ridge crests, the model has considerable generality. Important
features of the model include: inflow and outflow boundaries that allow passage
of fluid, heat, and salt without the development of unrealistic along-stream
boundary layers; bottom Ekman boundary layers for velocity, temperature, and
salinity; time- and space-dependent turbulent mixing; and the use of an advection
scheme for and S that maintains a well-defined
plume stem with its accompanying large lateral property gradients.
The model shows that most of the cross-flowing fluid encountering the stem
is entrained into it on the upstream side of the plume. In the parameter regime
examined, little entrainment into the stem occurs on the lee side of the stem.
The rising column of fluid also deflects some cross flow around it, thus acting
in part like an obstruction. The result is vertical counterrotating vortices
on each side of the plume stem, long identified in studies of plumes and jets
as the 
z couplet. The rising
column of fluid also leads to internal waves downstream at or above the level
of neutral buoyancy.
Distributions of velocities around the heat source have properties, in a general sense, like those earlier observed for jets injected into cross streams in nonrotating environments: vorticity couplets in all three coordinate directions develop in the plume stem, then follow the plume to its level of neutral buoyancy, and ultimately decline in strength with downstream distance. Rotation, as expected, breaks the cross-stream symmetry or antisymmetry of the distributions.
Intensity of turbulent mixing changes width and wispiness of plumes, with higher
stem viscosity/diffusivity resulting in steadier plumes with smaller downstream
lateral spread. Though the source is steady, turbulent mixing coefficients of
reduced size allow oscillations in and w
at the buoyancy frequency, which are suppressed when the strength of turbulent
mixing is increased.
For fixed buoyancy and increasing cross-flow strength, model plumes encompass
instances of plume bifurcation. When the ratio R of maximum upward velocity
to cross-flow strength was 2.8, the plume had a columnar void just downstream
of the plume stem but none at greater distance. When R = 1.0, bifurcation
was incomplete but vertical sections at increasing distance from the stem showed
vertically bimodal 
distributions. When R = 6.6, no plume bifurcation was observed. Initial
experiments are too few in number to identify mechanisms that cause bifurcations,
but results suggest that this model is a tool that can contribute to that understanding.
Acknowledgments. Support for this work comes from the NOAA VENTS program. Encouragement to pursue this modeling work by VENTS colleagues is appreciated. Contribution 1687 from NOAA/Pacific Marine Environmental Laboratory.
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