How to close a gap above patio doors?

If we sit on a dining room chair and look up at the top of our closed and locked patio doors, we can see day light through the gap between the doors and the frame (not a small one). When there is a breeze outside the curtains move and it can get quite chilly even with the heating on.

We only moved in recently and hadn't noticed it before, but now we are looking to reduce our heat loss. Can this sort of gap be fixed or do we need to replace the windows ?
thanks


Ian