(At least) in Release 2014b, the contourf - command returns a Contour object, which may be filled or not, but there is no such thing as a "FaceAlpha"-property. The question is, how to make these filled areas nonetheless appear semi-transparent ???
(I have already tried to work with the returned contour matrix cc and fill-commands, but it is hard to do this if - as here is the case - the borders of the axes are touched.)