Tropical Climate Update

First tropical cyclone for Australian region in 2017-18 expected

The first tropical cyclone in the southern hemisphere for the 2017-18 season, tropical cyclone Cempaka, was named by the Indonesian meteorological agency overnight on 27 November, and is currently located south of the Indonesian island of Java.

Cempaka is embedded in a weak tropical trough which extends to a second tropical low further to its west. This second low is forecast to merge with Cempaka and slowly strengthen as it moves in a southerly direction. Current forecasts suggest this system may track towards the far western coast of Western Australia at tropical cyclone strength, but is unlikely to make landfall at tropical cyclone intensity.

Further information on any tropical cyclones in the Australian Region can be found on the Bureau's Current Tropical Cyclones page.

Monsoon trough developing in southern hemisphere

Cempaka is embedded in a distinct low-pressure trough in the near-equatorial region of the southern hemisphere, readily seen on recent weather charts. The trough in the southern hemisphere is the southern component of the twin near-equatorial trough set up which is apparent either side of the equator. This is a typical transitional weather pattern seen at this of the year when neither a northern or southern hemispheric monsoon flow is prevalent.

While there are some monsoonal characteristics in this southern hemisphere trough, including some wind flow feeding into it from the northern hemisphere, it is currently a relatively weak feature. However, there are some broadscale factors which may invigorate and strengthen the trough in the coming week. One is the Madden–Julian Oscillation (MJO) which is expected to move into the region in the coming days. The second is a wind surge currently moving southwards through the South China Sea and predicted to move into the southern hemisphere. This pattern is a common precursor to monsoon trough development as the MJO and wind surge can provide rotational energy to intensify the trough. The tropical cyclone activity forecast in the region may assist in dragging any monsoon trough further south, but models currently indicate the monsoon trough is unlikely to initiate monsoon onset for northern Australia in the next week or two.

Madden–Julian Oscillation expected to strengthen

The Madden–Julian Oscillation (MJO) has been weak (or indiscernible) in the last week but the MJO signal has recently shown signs of strengthening. Most international models agree that the MJO will strengthen further and move over the western Maritime Continent during the next seven days. Most models indicate the MJO will progress further east over the central and eastern Maritime Continent (MC) in the following week. Some models expect the signal will still be strong enough to discern from background weather.

When the MJO is over the MC at this time of the year, convection and rainfall typically becomes enhanced over the northern and equatorial Indian Ocean, with a lesser impact on equatorial parts of the MC. If the MJO continues its progression into the central and eastern MC, broadscale westerly flow would typically develop over the western MC and extend as far south as northern Australia. However, typically the likelihood of increased rainfall occurs closer to Indonesia, and generally does not have a significant effect on Australian rainfall. Night-time temperatures are typically above-average for much of northern and eastern Australia.

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

La Niña ALERT: Tropical Pacific near La Niña thresholds

The tropical Pacific Ocean is approaching La Niña thresholds. If the current progression continues, and thresholds are exceeded for the duration of the southern summer, 2017-18 will be considered a La Niña event. As a result, the ENSO Outlook status has been upgraded to La Niña ALERT. This indicates there is approximately a 70% chance of La Niña forming in late 2017—three times more likely than in an average year. All international climate models indicate further cooling of the tropical Pacific Ocean is likely, however if La Niña does develop, it is expected to be weak and short-lived.

See the Bureau's current ENSO Wrap-Up for more information.