Today, the American South West is in the grip of the worst drought for 500 years. Scientists are starting to understand how extreme weather patterns of the past can repeat in the future. Climatologists have begun to understand the way pressure and ocean temperatures oscillate in partly predictable ways that govern our climate and our weather. USGS Scientist Greg McCabe explains two newly recognised ocean oscillations - the Pacific Decadal Oscillation and the Atlantic Multi-decadal Oscillation. He demonstrates how a combination of the two, affects weather, and specifically drought, in the United States.