La Niña forms when low atmospheric surface pressure over the warm water of the western Pacific Ocean is combined with high atmospheric surface pressure over the cooler waters of the eastern Pacific. Air flows from high-pressure areas to the low pressure areas. As the atmosphere moves from high to low pressure, the trade winds strengthen, which in turn, cause the upwelling of cold ocean waters. The cold water cools the air, resulting in lower surface pressure.
Colder than typical ocean temperatures prevent rain clouds from forming over the equatorial region of the eastern Pacific Ocean. Simultaneously, the rainfall in the western section of the equatorial Pacific Ocean is increased. The southwest region of North America may experience drought while Australia and Indonesia receive large amounts of rain. Cold La Niña winds occur about every three to five years and result in disruption of normal rainfall patterns.
La Niña can cause stronger than usual trade winds because of the increased difference between high- and low-pressure areas. The greater the pressure gradient, the stronger the trade winds become. The stronger trade winds cause an upwelling of colder ocean waters along the southern Pacific coast of North America. The upwelling of deep, cold waters cools the surface of the Pacific more, which in turn increases the difference between high- and low-pressure areas.
The equatorial region of the Southern Hemisphere may experience excessive rainfall and flooding during La Niña. La Niña during the winter season in the Northern Hemisphere results in greater than normal rainfall in southeast Africa and the northern part of Brazil. La Niña during the summer season in the Northern Hemisphere results in monsoon rains in India. The western coast of North and South America can experience low rainfall amounts or even drought conditions.