Here's why:

* Acceleration is the rate of change of velocity. Velocity has both speed and direction.

* Slowing down means the speed is decreasing.

* Since the object is heading northward, its velocity is also northward.

* To slow down while heading northward, the object needs to have an acceleration that opposes its northward velocity. This opposing acceleration is directed southward.

Think of it this way: if you're driving north and you want to slow down, you press the brake pedal. This applies a force (and thus acceleration) in the opposite direction of your motion (southward).