Tropical Climate Update

Tropical cyclone activity continues over Indian Ocean

Tropical cyclone Dahlia was the first tropical cyclone for the 2017-18 Australian region tropical cyclone season. It briefly peaked at category 2 intensity but was downgraded to a tropical low on 3 December after encountering unfavourable upper-level wind shear and cooler ocean temperatures. Currently, the remnants of ex- tropical cyclone Dahlia lie to the west of the Pilbara coast of Western Australia. The system is not expected to re-intensify.

In the northern hemisphere, tropical cyclone Ockhi, which attained the status of 'very severe cyclonic storm' (equivalent to a category 3 tropical cyclone using the Australian classification system), is located in the eastern Arabian Sea. This makes it the strongest storm in the North Indian Ocean for 2017, and the strongest in the Arabian Sea since 2015. Ockhi is the first tropical cyclone in the Arabian Sea in 2017—typically, only one tropical cyclone affects the Arabian Sea each year. Ockhi is expected to slowly weaken in the coming days as it moves towards the northwest coast of India.

Madden–Julian Oscillation over the Maritime Continent

The tropical cyclone activity in the Indian Ocean coincided with a moderate to strong pulse of the Madden–Julian Oscillation (MJO) which moved eastwards across the Maritime Continent during the last week. The MJO signal is predicted by most climate models to remain weak to moderate over the eastern Maritime Continent and western Pacific Ocean in the coming week. Typically, there is enhanced convection and rainfall over the Maritime Continent and far northern Australian when the MJO moves across this region at this time of the year. The broadscale wind pattern across far northern Australia tends to be westerly in this scenario but is typically not associated with the onset of the Australian monsoon.

Across the broader region of the Indian and northwest Pacific oceans, a double near-equatorial trough (NET) pattern is still apparent (see chart here). While a trough remains in the near-equatorial northern hemisphere, it is unlikely that a strong cross-equatorial flow emanating from the South China Sea will become established; this flow is typically a necessary component for Australian monsoon onset. Instead, the energy associated with the wind passing over the South China Sea is drawn into the northern hemisphere trough, potentially invigorating it and any tropical lows embedded within. The northern NET usually dissipates during December, facilitating cross-equatorial flow into the southern hemisphere and possible Australian monsoon onset. Current forecasts suggest the northern NET will not break down in the next fortnight.

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

La Niña in tropical Pacific Ocean

The Bureau's ENSO Outlook has been raised to LA NIÑA status, indicating that the tropical Pacific has reached La Niña levels. Climate models suggest this La Niña will be weak and short-lived, persisting until early southern autumn 2018. Most oceanic and atmospheric indicators show clear signs of La Niña. These include sea-surface temperature (SST) anomalies over the central tropical Pacific Ocean, the Southern Oscillation Index and cloudiness near the Date Line.

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. Impacts associated with the current late-developing and relatively weak La Niña are not expected to be typical. 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 indicate above-average rainfall for December to February for northern Australia.

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