Weekly Tropical Climate Note

Positive Indian Ocean Dipole weakens further

The positive Indian Ocean Dipole (IOD) continues, but has weakened further in the past week. The strength of the current event is reflected in the Bureau's rainfall outlook for December, which shows a high likelihood of below-average rain over northern Australia, apart from parts of the far northwest. While some subtle changes have been observed across the tropical Indian Ocean in the last week or so, the latest gradient-level wind analysis pattern shows the evolution towards a southern hemisphere summer pattern is well behind schedule. There is still no tropical trough south of the equator—a feature that is usually well established by this time of the year and is a precursor to the formation of a monsoon trough in the southern hemisphere. This suggests there is a high probability of a delayed monsoon onset for Australia in 2019–20.

The IOD typically dissipates as the southern hemisphere monsoon trough develops, so the breakdown of the current IOD event is likely to be later than normal. Nearly all climate models indicate the positive IOD will continue until January 2020, although it is expected to be significantly weaker than its current level by that time. As the IOD weakens over the next month or so, its influence across Australia is expected to lessen. As a result, the rainfall outlook for early 2020 shows a reduced likelihood of drier than normal conditions over much of northern Australia.

Read more about the Indian Ocean Dipole

Weak Madden–Julian Oscillation to have minimal influence on Australian rainfall

The Madden–Julian Oscillation (MJO) was indiscernible during the past week. While most climate models indicate the MJO will remain indiscernible for the next fortnight, some models indicate it might strengthen over the western Indian Ocean in the coming days, before rapidly weakening in about a week or two as it encounters the influence of the positive IOD over the eastern Indian Ocean. This region presents a hostile environment to a pulse of enhanced weather, such as the MJO, due to cooler than average sea surface temperatures, dry easterly winds and widespread, descending air which inhibits deep cloud formation.

An MJO pulse over the western Indian Ocean at this time of the year typically decreases the chance of above-average rainfall across northern Australia and the Maritime Continent. However, if the MJO pulse remains weak or indiscernible, its influence on rainfall patterns over northern Australia is likely to be insignificant.

Read more about the Madden–Julian Oscillation

Typhoon activity in western North Pacific Ocean

Typhoon Kammuri (Tisoy) is the 16^th typhoon to form in the western North Pacific region, according to the Japan Meterological Agency (JMA). This is close to the 1981–2010 annual average of 15 typhoons. A typhoon is comparable in intensity to an Australian category 3 tropical cyclone, with mean winds in excess of 119 km/h.

Kammuri (Tisoy) made landfall on the northern Philippines in recent hours at its peak strength, with sustained mean winds in excess of 167 km/h (equivalent to an Australian category 4 tropical cyclone). Kammuri (Tisoy) is forecast to track westwards across the Philippines and weaken.

Another tropical low well east of the Philippines may strengthen to tropical cyclone intensity in the next 24-48 hours.

Tropical cyclone information for the western North Pacific region available at the Japan Meteorological Agency