Tropical Climate Update

Transitional weather pattern continues across tropics

As we move into the austral summer, the focus of low-pressure activity will shift predominantly to the southern hemisphere, leading to the development of a monsoon trough. In a typical year, this monsoon trough moves south to northern Australian latitudes towards the end of December, heralding the start of the Australian summer monsoon. We have yet to see any monsoonal development across the southern hemisphere this wet season.

Meanwhile in the northern hemisphere, tropical rainfall decreases, particularly over the Indian subcontinent and South-East Asia regions. Similarly, tropical cyclone activity over the western North Pacific region declines significantly after November. This is all part of the annual cycle which sees changes in broadscale wind patterns as the sun predominantly affects one hemisphere in preference to the other. The current transitional wind pattern is highlighted by low-pressure troughs in both hemispheres across the near-equatorial zone.

Madden–Julian Oscillation moves into Pacific Ocean

With no monsoon trough in either hemisphere acting as a focus for tropical weather, it is often atmospheric tropical waves such as the Madden–Julian Oscillation (MJO) which influence the likely location of enhanced tropical weather. This was evident during much of November, with enhanced tropical rainfall and cyclone activity associated with the passage of the moderately strong MJO pulse observed across the western North Pacific and South-East Asia region. In the past week, tropical weather eased over these regions as the MJO pulse moved further east over the Pacific Ocean and strengthened.

At this time of the year, an MJO pulse moving into the central Pacific Ocean is typically associated with suppressed tropical cloudiness and rainfall over India and South-East Asia, while above-average rainfall develops between the tropical Date Line and central Pacific Ocean. Westerly winds also typically develop across the tropical Pacific in this scenario, disrupting the usual trade wind regime and potentially assisting the development of El Niño. Nearly all climate models surveyed by the Bureau forecast the MJO pulse to continue moving east at similar strength in the coming week.

Tropical rainfall and cyclone activity across the Maritime Continent in recent weeks was also associated with Equatorial Rossby wave activity—a tropical wave related to the MJO that moves in the opposite direction. This wave is typically identified by a pair of lows straddling the equator. In the past week, an Equatorial Rossby wave aided development of a pair of tropical lows between the Date Line and Papua New Guinea; the northern hemisphere low of the pair subsequently developed into tropical cyclone Man-Yi. An Equatorial Rossby wave remains active over the tropics near Indonesian longitudes, increasing the likelihood of enhanced rainfall in that region in the coming days. 

See the Bureau's current MJO monitoring for more information on the MJO.

Tropical cyclone activity across northern hemisphere eases

After reaching typhoon intensity (equivalent to an Australian category 3 or stronger tropical cyclone) during the past week in the tropical western North Pacific, Man-Yi moved into a region of unfavourable environmental conditions, well to the east of the Philippines, and is expected to dissipate in the coming days.

The tropical low previously known as Samuel tracked westwards across the Philippines last week, before intensifying prior to landfall on the southern Vietnamese coast on 26 November. The re-named storm, Usagi, briefly attained typhoon strength before weakening slightly at the time of landfall. Damaging winds, heavy rainfall and mudslides were associated with this storm, with multiple fatalities reported and significant agricultural damage. Usagi has since been downgraded to a tropical depression.