Final answer:
The ad campaign is using a fixed interval message reinforcement strategy.
Step-by-step explanation:
The ad campaign described in the question is using a fixed interval message reinforcement strategy. In a fixed interval schedule, reinforcement is delivered after a fixed amount of time has passed since the last reinforcement, regardless of the number of responses.
By buying a constant amount of GRPs throughout the year but increasing the GRP total in the weeks leading up to the most heavy purchasing period, the strategy aligns with a fixed interval schedule, where the reinforcement (i.e., increased GRPs) is delivered after a specific time interval or period.