I assume the function you've given is a surface where the magnitude of the velocity is constant. You need to also say how the speed changes when you move away from the surface.
So you'll need a function of the form:
To get this function, you'll probably want to simplify the equation for your surface, then write an expression that is constant on your surface and increases when decreases.
The direction at (x,y,z) will be (y,-x,0), which will make the wind go in a circle clockwise around the z-axis. It needs to be a unit vector, so:
and to get your vector field, just multiply speed by direction.
Post again if you're still having trouble.