Twin cyclones spawned by monsoon trough

An active monsoon trough near northern Australia is the cause of the two cyclones now threatening the mainland.

Alan Sharp of the Bureau of Meteorology says the sudden flurry of cyclone activity is related to a tropical weather phenomenon known as the Madden-Julian Oscillation (MJO).

The MJO is a "pulse" of cloud and rainfall that moves east along the equator on a 30 to 60-day cycle.

When this pulse nears the equator north of Australia, as it has recently, it can push the monsoon trough south.

This dramatically increases moisture in the atmosphere around northern Australia and increases the chances of tropical low pressure systems and cyclones forming.

"While we're in that favourable area we will generally get a few cyclones forming and the monsoon trough will become stronger and affect Darwin and the Top End more," Mr Sharp said.

"In the quieter phase, the monsoon trough retreats and the probability of cyclones is lower."