We seem to have caught a bunch of oceanic engineering proposals recently. These are all a long way from implementation or even a convincing design principle. Engineering is like that to begin with, and the ocean is noted for sinking human schemes as fast as they are launched.
However, it is fairly easy to agree that mixing warm surface waters along the equator with deeper and much colder waters would be beneficial for other reasons besides lowering the intensity of tropical cyclones. The tough trick is to do all this at a profit.
The idea of simply floating an enclosure formed perhaps of a curtain wall down to 200 meters seems plausible. Except that warm water is lighter that cold water and it would create a pressure head that would blow the curtain wall apart. It would be likely a difficult engineering challenge for that reason alone.
I think that it will be necessary to build stiff structures able to withstand a surprising amount of hydraulic pressure whatever plan is used. These structures must also be able to manage marine fouling. In short, it is a sticky engineering problem.
My own thought toward this problem has been posted on before, but it is a good time to revisit the problem. I believe that the best and most probable way we have to accomplish some mixing is to develop a protocol for building a long vertical pipe that allows deep cold water to rise directly through the pipe to the surface. Such a flow, once initiated becomes powerful and fully sustaining.
Prior work suggested that it may be used to draw of power. Thus such a robust large diameter vertical pipe is source of power, cold water injection onto the surface itself and forcible mixing causing a drop in temperature. A secondary effect is an enriched ecosystem at the surface down stream from the outlet.
The engineering problem reduces to building stackable rings, each individually able to have neutral to positive buoyancy. This can be achieved by the expedient of oil filled bladders held in U shaped collar that is open to the sea from the underside. Such a design can be made uniform and linked together with high pressure oil lines running the full length of the tube.
In theory such a stack of sufficient diameter to resist bending, can be driven down almost to the sea bed far offshore.
At the present, this is a steel and concrete proposition. For that reason, turbines at the outflow need to be designed in to produce power that can at least support maintenance. These will be likely horizontal low RPM systems able to use a small hydrostatic head.
Cable stays and positive buoyancy will keep this in position and on station fairly easily.
There may be better solutions, but this seems at least practical for now.