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Planning for the northern rains

Planning for the northern rains

The Bureau's northern rainfall onset outlook lets businesses and communities in northern Australia know the chances of an early or late start to the summer rains.

If you're a grazier, a mango grower, a sugarcane farmer or a fisherman in northern Australia, the annual swing from the dry season (during the cooler months, May–September) to the wet season (October–April) dominates your annual planning cycle. While October marks the beginning of the wet season, the rain doesn't just start falling on 1 October. Rather, it begins gradually with the occasional afternoon shower, and then 'builds up' to more frequent rains as the season progresses. For many businesses in the north, it's the timing of the shift from dry to wet that can make a big difference to the bottom line.

For instance, if a grazier can increase stocking rates earlier, more cattle will put on more weight before they're ready for market. For a mango grower, knowing the rains could be early enables picking to start before heavy drops mark the fruit and reduce the quality. And for a sugar farmer, knowing there will be drier ground means less chance of machinery bogging in the paddocks and sugar going unharvested.

Storm front over Muirhead, NT, December 2013 - by Louise Denton Photography

Storm front over Muirhead in the Northern Territory

To assist agriculture in the north, Bureau researchers, supported by the Managing Climate Variability research and development programme, looked at how predictable rainfall was at the start of the northern wet season. In particular, they focused on the very first rains to fall. After consultation with industry, this threshold was defined as the first 50 mm accumulated after 1 September, which is roughly the amount required to stimulate plant growth after the dry season.

For most locations, the first 50 mm of rainfall comes long before the start of the northern Australian monsoon. The monsoon involves a wind change from the southeast to the west or northwest, drawing warm moist air in from the tropics, bringing heavy, consistent rainfall to a broad region.

Research suggested that our climate model has good accuracy when it comes to the first northern rains. The physics-based model that the Bureau uses for our climate outlooks is a world leader in capturing and forecasting tropical events such as the Madden–Julian Oscillation (a pulse of cloud and wind that typically strikes Australia's tropical regions every 30–60 days).

As a result of this successful research, we have enhanced the Bureau's climate outlooks service with a northern rainfall onset outlook, focused solely on northern Australia. This outlook is updated every month from June through to August, on the same day as we issue the official Climate Outlook.

The northern rainfall onset outlook gives the likelihood, or probability, of the first 50 mm of rain after 1 September arriving earlier than normal. If that probability is greater than 50 per cent—as is the case for most regions in 2016—then the odds favour an earlier than normal onset date. The 2016 outlook is driven by the likely conditions in the tropical Pacific and Indian oceans later in the year. A greater than normal chance of La Niña in the Pacific and a persistent negative Indian Ocean Dipole, means an early start to the first northern rains is likely. While it's not certain the tropical Pacific Ocean will reach La Niña thresholds, it is still expected to increase the moisture fed into northern Australia. Although this doesn't guarantee an earlier-than-normal start to the wet season, it at least gives mango growers (for example) a heads-up on the heightened risk of early rains while picking (which can mark the fruit).

Map showing the June outlook for the 2016–17 northern rainfall onset

Map showing the June outlook for the 2016–17 northern rainfall onset

Back in 2010, when the research was first underway, a strong La Niña climate pattern also pushed the odds towards an earlier than normal start, with probabilities of 70-plus per cent that the first rains would be early. And sure enough, they came on early and strong leading into one of the wettest seasons on record.

Of course, these are odds; much like a horse in a race, they don't guarantee a certain win every time, but they can give you a good idea of the likelihood of a certain outcome—in this case the chance of an early or late onset to the northern rains. We've made a video about how you can use climate outlook odds in your favour, or you can read our blog article 'Don’t get the butterflies: using a climate outlook is simple'.

The northern rainfall onset outlook includes maps that show the average timing of the northern rainfall onset across all years:

Map of median dates for northern rainfall onset, 1960–2009

Map showing the median date of the northern rainfall onset, 1960–2009 (grey area = insufficient data)

In coastal Queensland and the western Top End, this tends to be in October; while the far west of Western Australia typically doesn't pick up its first 50 mm until mid-January. Of course these are just the average times—El Niño or La Niña can sway things later or earlier, so we have also included maps of typical El Niño or La Niña onset dates.

The outlook also provides the average probability for several key agricultural regions in the north, presenting the chance of an early onset as an arrow on a dial. This means users in these regions can get an instant read on the current outlook for their area, and even compare it to the previous month.

Regional outlook dials for the northern rainfall onset outlook

It's not always going to be as clear-cut as in 2010 in the tropics, but by following the odds over the long term, this service will help those in the north plan intelligently. When used in conjunction with other Bureau information (e.g. our ENSO Wrap-Up, Climate Outlooks and model summary) and combined with on-farm information and a risk management overlay, the northern rainfall onset outlook will further the potential of agriculture in the tropics—not to mention help plan a few of those fishing trips!

To see the latest northern rainfall onset outlook, go to www.bom.gov.au/climate/rainfall-onset.

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