Active weather is expected to affect northern Australia and parts of the Mascarene Islands in the south Indian Ocean this week, with the monsoon trough a triggering factor in both cases.
The monsoon trough is an area of the Intertropical Convergence Zone that interacts with the larger scale monsoon circulation. This trough is marked by an area of relative minima in sea level pressure, as well as a local maximum of vorticity (a measure of the spin of the atmosphere).
An area of disturbed weather developed along the monsoon trough east of northern Madagascar on Thursday 11 January. The depression drifted south-westwards, quickly gaining enough organisation to be defined as a tropical cyclone by Friday, and a severe tropical storm by Saturday.
Favourable atmospheric and oceanic conditions enabled the cyclone, called Belal, to enter a phase of rapid intensification on Saturday – defined as an increase in wind speeds of 30 knots across a 24 hour period. Belal is expected to make landfall, or pass close to, the island of Réunion during Monday afternoon. Météo-France issued a red cyclone warning for the island, noting that Réunion has not experienced such a threat since Cyclone Firinga in January 1989.
Belal is expected to remain a strong storm through the coming week, passing south of Mauritius on Tuesday, before drifting eastwards towards the island of Rodrigues later in the week.
Elsewhere, the monsoon trough is expected to lie across northern Australia this week, resulting in moist north-westerly airflow. While the Australian monsoon is typically expected to be weakened during an El Niño event (due to the focus of rainfall across the western Pacific being shifted eastwards with the displacement of the warmer waters), an active phase of the monsoon is expected this week.
Between Monday and Saturday, a broad swath of the northern half of the Northern Territory and northern Queensland are expected to see severe thunderstorms bring rainfall totals in excess of 100mm, with parts of the Northern Territory worst affected – with potential rainfall totals of 300mm possible here locally, comfortably exceeding the normal January rainfall in places. Beyond this week, there is a chance of the area of heavy rainfall spreading southwards to affect southern parts of the Northern Territory.
