During El Nino, the trade winds die down. Warm water moves toward the coast of Peru (white arrows on the sea surface). As a result of this shift, convection patterns in the atmosphere over the Pacific Ocean are altered. El Nino conditions cause:
- Floods and mudslides in Peru and the Southwestern United States
- Increased disease associated with flooding
- Massive fish kills along the Peruvian coastline because nutrient upwelling fails to occur
- Drought in Australia and Indonesia causing crop failures and wildfires
- Disruption in marine ecosystems
- Decreased hurricane activity in the North Atlantic and increased tropical storm activity in the Pacific Ocean
During normal conditions, trade winds blow toward the west across the Pacific Ocean. Warm water is pushed toward Indonesia and Australia. Upwelling occurs along the Peruvian coast. Normal rainfall and climate patterns are observed.
The oscillation between normal and El Nino conditions has a natural period of about 3-4 years. El Nino conditions occur irregularly approximately 2-7 years. La Nina usually, but not always, follows an El Nino year.
La Niña conditions are the result of unusually strong trade winds. Warm water is pushed past the normal condition and unusually cold ocean temperatures persist in the equatorial Pacific, as compared to El Niño, which is characterized by unusually warm ocean temperatures in the equatorial Pacific. (La Niña is also sometimes called El Viejo.) La Nina results in:
- Increased hurricane activity in the North Atlantic
- Drier conditions in Peru
- Stronger upwelling along Peru
- Warmer than normal winter temperatures in southeastern United States and cooler than normal winter temperatures in the northeastern United States
