Darwin weather characteristics

Jude Scott, Media and Communications Manager, Bureau of Meteorology

The dry season officially ends on 30 September and although temperatures and humidity are on the rise across the Top End, it’s not unusual to see some drier weather pushing in from the south during late August and September.

The build-up is the transition period between the last weeks of the dry and the onset of the first monsoon (characterised by widespread, significant rainfall over at least a few days, with a marked drop in temperature) in the Top End.

There are two phases to the build-up:

• during the first phase, we see more clouds in the sky, humidity on the rise and warmer overnight temperatures (from late August and into October)
• the second phase is where the storm clouds start to build and we see fairly regular showers and storms in the late afternoon and early hours of the morning (from mid-October to December)

September is usually the first month we feel build-up conditions kicking in, with the warmer nights (average overnight temperature in Darwin of 23°C versus 20.3°C in August and 19.3°C in July) and more humidity in the afternoons (47% relative humidity at 3pm versus 40% in August and 37% in July).

Sea breezes during August and September can still be quite refreshing — sea breezes are the result of hot air rising over the land and cooler air from the ocean being drawn in across the coast. However, as the sea surface temperature increases during the build-up months, sea breezes become warmer and more humid.

As clouds build during the day across the Top End during the build-up, we often see rainfall in the afternoon and evening. However early build-up thunderstorms tend to form inland, and can struggle to reach coastal areas, which can be frustrating for many Darwinites.

Climate outlook

Two key climate influences can affect the weather conditions in the Top End during the build-up:

• Indian Ocean Dipole (IOD): The sea surface temperatures in the Indian Ocean impact rainfall and temperature patterns in Australia. The IOD is currently negative, which means there is an increased chance of above average rainfall in the Top End during the build-up months
• El Niño/La Niña: 2015/16 saw a strong El Niño, with lower than average rainfall and higher than average temperatures across most of the NT. This year, expectations are that we will move into a neutral-to-La Niña phase, which increases the chances of higher than average rainfall during the early wet season for the NT.

The Madden-Julian Oscillation (MJO) plays a role in shorter term weather outlooks (weekly to monthly timescales), and can enhance rainfall in the Top End when in the northern Australian region.