Help Needed ... turn on circulation pump when furnace in AC mode?

Short version: I have a pump that simply plugs in to an outlet and is on non-stop. It draws ~120w. I need it to only be on when the furnace is on, and in 'AC' mode. How can I make this happen?

Long version: We've recently finished building our home in the SF Bay Area (temperate climate) and rather than build a full AC system in (which is fairly uncommon around here), we ran two copper coils into the ground and have created a "poor mans" ground loop cooling system. We've got a water-to-air radiator in the outlet of the furnace before it runs into the ducts, and a pump that pumps the water in a loop throughout the system. Obviously we only want the pump running when the furnace is in AC mode...

(Don't ask how well it works yet ... we just finished plumbing it last night and honestly its not hot enough around here to do any good right now...)