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Wednesday 15 February 2017

Floods, heat and bushfires

Major events have taken place across the continent over the past week with record rainfall in southern WA leading to widespread flooding, and a heatwave in eastern states that set new historic highs for length and intensity and led to catastrophic bushfires in NSW.

These events have been covered extensively in general media. AWN will provide an account of weather aspects of the event soon, when more information is available.

Tropical rain likely to contract towards the east

The Bureau of Meteorology's Weekly Tropical Climate Note, issued on Tuesday, points to a likely weakening of monsoonal wet weather over Australia's northwest and the NT Top End during the coming week as the Madden-Julian Oscillation (MJO) pulse moves eastwards, well out into the South Pacific Ocean. However, the Cape York Peninsula may still see above-average rainfall for a while.

Monsoonal conditions over northern Australia in recent weeks have greatly enhanced rainfall, especially in the WA Kimberley and northwestern part of the NT Top End, where many locations recorded over 200mm of rain in the past week alone. In the Maritime Continent - the area extending from the north Australian coast to the Malay Peninsula and including Indonesia, Borneo, New Guinea, the Philippines and the warm, shallow seas in between - heavy rain has caused landslides and deaths in Bali. Farther east, there has been persistent, heavy rain from Papua New Guinea through the Solomon Islands, Fiji and Tonga in the South Pacific.

These monsoonal conditions have been enhanced by a moderate to strong MJO pulse moving east over northern Australia as well as unusually strong westerly winds picking up moisture as they pass over the warm waters of the Maritime Continent into the monsoon trough. As the MJO moved into the Western and central Pacific during the past week, cloudiness and rainfall over Melanesia has been much more active than normal.

In the longer term, international models are predicting the MJO will maintain strength as it moves towards the Eastern Pacific during the coming week. Normally, when it reaches this area, cloudiness and rainfall over all northern Australia are suppressed, although small-scale weather systems can still produce localised significant rainfall.

Possible twitches of an El Niño, but too early to call

The BoM also released its ENSO Wrap-Up on Tuesday. While the El Niño–Southern Oscillation (ENSO - Video) remains neutral, there are some indications of a slow movement towards an El Niño. The Bureau says "In recent weeks, the central and eastern tropical Pacific Ocean has shown surface warming, and climate models suggest this warming is likely to continue during the southern autumn. In marked contrast to last year, western Pacific sub-surface temperatures are up to 5 °C warmer than at the same time last year, indicating La Niña-like conditions are unlikely in 2017."

The Bureau notes that at this time of year, both ENSO and the climate models that try to forecast it have their greatest variability, so forecasts need to be used with care. As if to underline that, the January Climate Model Summary had said "two models suggest a more rapid warming with El Niño thresholds exceeded by June. The remaining models maintain temperatures consistent with a neutral ENSO throughout the outlook period." The February Climate Model Summary is due out on 16 February.

The Bureau is therefore hedging its bets, saying that "either neutral or El Niño are considered the most likely ENSO state for the southern winter and spring." All should become clearer as we move out of the autumn predictability barrier which is typically the ENSO transition period.

Another climate driver important to us, the Indian Ocean Dipole (IOD - Video), is typically too weak to influence our climate until the end of April due to the presence of the Australian Monsoon. Early indications are that it will be neutral through late autumn into early winter.