La Niña is a phase of the El Niño–Southern Oscillation (ENSO) and is associated with cooler than average sea-surface temperatures in the central and eastern tropical Pacific Ocean. La Niña phases typically occur every three to five years, with peak strength occurring during winter. The cool ocean water during a La Niña event affects weather patterns around the globe by altering air pressure and circulation patterns in the atmosphere.
During a La Niña event, areas of high pressure form over Australia and Indonesia, while low-pressure systems develop over the central and eastern Pacific Ocean near South America. This creates what is known as the Walker Circulation, which results in increased trade winds blowing from east to west across the tropical Pacific. These stronger than normal trade winds help to push warm surface water towards Indonesia, causing an increase in sea levels there. At the same time, they also cause cooler than average water to upwell along the coast of Peru and Ecuador, resulting in below average sea surface temperatures across much of the eastern Tropical Pacific.
The cooler than average ocean water during a La Niña event can have far-reaching impacts on global weather patterns. For example, it can lead to wetter than normal conditions across parts of Australia and Southeast Asia, as well as drier conditions across parts of South America. In North America, a La Niña event typically leads to increased precipitation along the West Coast of the United States and Canada but decreased precipitation across parts of the southern United States. Additionally, colder than average winter temperatures are often seen across North America during a La Niña event .