Weekly Tropical Climate Note

El Niño ALERT—dry start to wet season likely

The chance of El Niño developing has increased in recent weeks, with the Bureau's ENSO Outlook raised to El Niño ALERT last week. El Niño ALERT means there is approximately a 70% chance of El Niño occurring in 2018. The tropical Pacific Ocean has warmed in recent weeks and models predict further warming of the ocean is likely. The Southern Oscillation Index has been at or near El Niño thresholds for much of the last couple of months.

After several weeks when the Indian Ocean Dipole (IOD) index was above the positive IOD threshold of +0.4 °C, the index returned to neutral levels this past week. This may only be a temporary fluctuation, however, due to cooling by a tropical cyclone off eastern Africa. A majority of models surveyed by the Bureau suggest IOD index values are likely to return to neutral by November.

El Niño and positive IOD conditions at this time of the year increases the likelihood of a drier than usual start to the northern wet season for Queensland and the Northern Territory of Australia. Tropical cyclones are typically fewer in number with El Niño, consistent with the recently released Tropical Cyclone Outlook for 2018-19.

Twin tropical cyclones over the North Indian Ocean

For the first time since the start of the satellite era (1979 for the North Indian Ocean), simultaneous tropical cyclones were observed in the Arabian Sea and the Bay of Bengal. Cyclones in the North Indian Ocean are relatively rare. Typically only 2-3 cyclone develop across the Bay of Bengal and Arabian Sea each year, although 5 tropical cyclones have been observed across the region so far this year.

Tropical cyclone Luban developed over the Arabian Sea early on 9 October, followed just a few hours later by Titli, over the Bay of Bengal. The Indian Meteorological Department classified both as a Very Severe Cyclonic Storm, which is comparable to a category 3 severe tropical cyclone in the Australian region. Titli crossed the coast near Palasa, Andhra Pradesh, in northeast India, on 11 October, generating widespread flooding which led to landslides and local reports of multiple fatalities. Luban weakened to the equivalent of an Australian category 1 storm prior to landfall on the south-eastern Yemeni coast on 14 October and brought heavy rain to the region. Both systems have now weakened to below tropical cyclone intensity.

Madden–Julian Oscillation weakens over Indian Ocean

A moderately strong pulse of the Madden–Julian Oscillation (MJO) moved over Indian Ocean longitudes in the past week. In the last few days, this pulse started weakening, and most international models predict it will become indiscernible in the coming days. However, some of these models also indicate a rapid redevelopment of another MJO pulse over African or American longitudes in about a week's time.

With the MJO over the Indian Ocean during the last fortnight, rainfall was below average over South-East Asia and the western North Pacific Ocean: regions which had previously been very active with respect to rainfall and tropical cyclone activity. The focus of tropical activity shifted to the northern Indian Ocean, highlighted by the previously unseen occurrence of twin cyclones across adjacent seas of the North Indian Ocean. If a new MJO pulse develops across African or American longitudes, rainfall would typically remain below average over South-East Asia and the western North Pacific region. Tropical cyclone activity might again be a feature over the North Indian Ocean in this scenario.

If an MJO pulse does redevelop over African or American longitudes, trade winds over the tropical Pacific Ocean are likely to remain weaker than usual. There has been a pronounced warming in the tropical Pacific Ocean in recent weeks coinciding with the weaker trade winds. A continuation of this situation would potentially further assist El Niño development.

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

Product code: IDCKGEW000

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