An elegant answer! I like it better than mine.
Mine was similar - it relates to airmass and earth. At the pole, the earth isn't moving at all. At the equator, it's fair belting round. Therefore as the air moves North or South, the ground under it slows down or speeds up "relatively", with the airmass thus acquiring an apparent east/west velocity relative to the earth.
And then the water goes down the plug'ole and the experiment is over.