Every two to seven years, trade-winds in the Pacific Ocean slow down or reverse their direction — no one is sure why. But when the trade winds slow down, everything changes. Water temperatures become warmer in the eastern Pacific and colder in the west. Nutrient upwelling slows, and fish populations become much smaller along the Pacific coast of South America. Rainfall follows the warmer water, causing flooding in Peru and drought in Indonesia and Australia. Because these changes can be highly destructive, advance warning of El Niño’s approach is important for emergency preparation. NOAA satellites are constantly collecting information on sea surface temperatures around the globe. NOAA also operates a network of buoys that measure temperature, currents, and winds in the tropical Pacific Ocean.