Weekly Tropical Climate Note

Transition period for tropical weather patterns

October to December are typically transition months for much of the tropics as wind and pressure patterns adjust to the movement of the sun, which since the equinox in late September is over the southern hemisphere. Tropical wind patterns reorganise ahead of the Australian monsoon which typically becomes active in the second half of December. Low-pressure troughs on both sides of the equator (double trough) are a common feature in the Indian Ocean at this time of the year. During the next month or two, the double trough set-up will breakdown and low pressure will dominate the southern hemisphere tropics and easterly trade-wind flow will prevail in the northern hemisphere.

This time of the year is also associated with a relative lull in tropical cyclones across the globe. The northern hemisphere tropical cyclone season is winding down and the tropical cyclone season in the southern hemisphere is in its early stages. The first tropical cyclone in the Australian region is usually associated with the monsoon onset in late December, although tropical cyclones have developed over the southern Indian Ocean and the South Pacific as early as the last week of October. Tropical cyclones have crossed the Australian coastline as early as mid-November, including a category 3 storm which made landfall on the Pilbara coast in Western Australia in 1955. The Australian tropical cyclone season runs from November through April.

Madden–Julian Oscillation to remain weak

The Madden–Julian Oscillation (MJO) has been weak (or indiscernible) since the beginning of November. Satellite imagery depicts broadscale convection over the northern Indian Ocean and western Maritime Continent, associated with non-MJO tropical wave activity in the region. These waves will weaken and move out of the area, and international models agree the MJO will remain weak this coming week. This means convection across this region is likely to decrease, returning to climatologically "normal" weather conditions

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

Late-developing La Niña remains possible

The El Niño–Southern Oscillation (ENSO) remains neutral but indicators including trade-wind strength and SSTs across the equatorial Pacific continue their trend towards La Niña thresholds. La Niña WATCH remains in place. La Niña WATCH means there is approximately a 50% chance of La Niña forming in late 2017.

International models suggest further cooling of the tropical Pacific is likely, with six of eight models suggesting cooling will reach and persist at levels long enough to be considered a La Niña event. If La Niña does develop, it is likely to be weak and short-lived. La Niña is typically associated with above-average rainfall during the northern wet season, average or above-average tropical cyclone numbers across the Australian region and an earlier onset of the Australian monsoon. The effects from a late-forming La Niña, such as may develop in the coming months, are less clear as this type of event is relatively rare. Near-average SSTs are expected to persist across waters to the north and west of northern Australia (atypical during La Niña), and current rainfall outlooks do not favour widespread above-average rainfall for November to January for Australia.

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

 

Product code: IDCKGEW000

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