The answer must vary between locations.
Over land and particularly outback australia i would take it to be moisture inflow from the Indian Ocean and Arafura sea.
Over the oceans is different again. In the tropical pacific it can be seen that the ocean sst's drive the clouds as shown in earlier posts in this thread.
With a increase in cloud cover we also see increase in ssts (generally) . I can only assume it is the sst's driving the cloud formations. Makes no sense for it to be the other way. So when we see a decrease in cloud cover we also see a decrease in sst's
