PANS-OPS Vol 1 S4 Ch 7 is quite specific:
7.4 MISSED APPROACH PROCEDURE WHILE CIRCLING
7.4.1 If visual reference is lost while circling to land from an instrument approach, the missed approach specified for that particular procedure must be followed. The pilot will make an initial climbing turn toward the landing runway and overhead the aerodrome. At this point, the pilot will establish the aircraft climbing on the missed approach track.