A drought is a long period when rainfall is below average for an area. El Niño and La Niña can cause drought
Normal conditions
Normally, the trade winds blow west, pushing warm water towards Australia. There’s low pressure over Australia with air rising and cooling so that water vapour condenses to form clouds. This brings precipitation to the western Pacific. There is high pressure over the eastern Pacific so air is sinking and warming up. This means no condensation so low precipitation.
El Niño conditions
In El Niño conditions, air pressure rises in the western Pacific and falls in the east. This causes the trade winds to reverse so that they now blow west to east. This pushes warm water towards South America meaning more air is rising creating low pressure. Low pressure over South America brings thunderstorms and flooding. Over Australia there is now high pressure so air is sinking and warming up. This means no/less condensation occurs bringing clear skies and no/less precipitation. This unusually dry weather can lead to droughts occuring in eastern Australia. El Niño events happen every 3-4 years and last for 9-12 months.
La Niña conditions
La Niña is when normal conditions become more extreme. Trade winds blow more strongly towards the west causing more warm water to build up near Australia. This causes heavy rainfall and floods in this area. In the eastern Pacific/South America, there is less rainfall than normal which can lead to droughts. La Niña events happen every 2-7 years.