El Niño and La Niña are part of the ENSO (El Niño Southern Oscillation) a weather system which alternates between 3 states of El Niño, Neutral, and La Niña.

La Niña

During La Niña winds coming off the Andes push the warm surface waters to the West Pacific which strengthen the Trade wind. The sun warms up the surface waters off the coast of South America as the winds push it to the West Pacific they evaporate creating an overall low pressure in the West Pacific this rising air falls back down in the East Pacific which creates a high pressure system which pushes the trade winds even stronger across the Pacific until there is a stable locked in cycle of warm water being pushed westward. Usually this cycle is locked in once established for six months before the changing seasons can cause a reset. This causes a much stronger wet season for Tropical Regions in the West Pacific.

Neutral

During a neutral phase of ENSO there is no obvious leaning towards La Niña or El Niño. The Trade winds are neither pushing warm waters off the South American coastline nor is it pushing it East from Australia and the West pacific.

El Niño

During an El Niño phase the trade winds are reversed compared to the La Niña phase, this bring more wet weather to the East pacific and dryer weather to the West pacific as with the other phases they tend to get locked in once strongly established until a season change therefore during the summer in the southern hemisphere it will tend to be locked in.