The direction of the velocity will be towards east.
Explanation:
When an object moves in a circle then its velocity is towards the tangent to the circular path and its acceleration is always towards center of that circular path. If we observe the figure attached, when the airplane is at the top its acceleration will be towards south (towards the center of circle) then its velocity direction will be towards east (tangential to circular path).
Although the wave slows down, its frequency remains the same, due to the fact that its wavelength is shorter. When waves travel from one medium to another the frequency never changes.