THE CLIMATE KELPIE BLOG: Why the late shift in the winter 2020 rainfall forecasts?

Posted by BCG on 24th August 2020

Every week the Bureau of Meteorology releases updated climate outlooks for the weeks, fortnights, months and seasons ahead (Figure 1). The climate outlooks show the odds of the upcoming period being wetter or drier, and warmer or cooler, than usual.

Figure 1. The rainfall outlook for August to October 2020 (chance of above median rainfall, issued 30 July). Source: Bureau of Meteorology.

These outlooks are different to weather forecasts. While weather forecasts can tell you what the temperature will be tomorrow and how much rain to expect, climate outlooks cannot be this specific. This is because the further we look ahead, the more that small or random changes in the atmosphere can amplify into different weather patterns. That is why they are expressed as a likelihood (or probability), rather than a certainty, of wetter or warmer conditions ahead.

The purpose of climate outlooks is to tip the odds in your favour, which means that in the long term, making multiple small decisions based on the outlooks gives you an edge.

Levels of accuracy

Before making plans based on the outlooks, it is important that you check how accurate the outlook has been in the past.

The Bureau publishes charts showing the historical accuracy of the model. By re-running the current climate model every month since 1991, we can show the percentage of the time that the outlook would have favoured the correct outcome. Green areas indicate that the outlook has been correct more often than not, with darker greens (higher percentages) indicating areas with a higher level of outlook accuracy.

During winter, green covers much of Australia (Figure 2), indicating that there would have been good agreement between the forecasts and the observed rainfall. Past accuracy changes with every outlook period, so make sure you check the past accuracy is above 50 per cent before using the outlooks in your decision making.

Figure 2. Forecast accuracy for August to October (as of 17 July). Use forecasts with an accuracy of greater than 50 per cent for decision making. The darkest green indicates the greatest accuracy for the time period. Source: Bureau of Meteorology.

Historically accurate outlooks are the sign of a good model, and good models respond to changes in climate drivers.

Changes in 2020

The outlook for winter 2020, which was released on 4 June 2020, saw the chance of wetter conditions ease back by around 10 to 15 per cent from the outlook of a week before.

It is reasonable to ask how such a rapid change in the outlook can occur. Large changes in the seasonal outlook from week to week are rare, as the key drivers of our climate generally change slowly. However, in the case of the winter 2020 forecast, while the warmth in the oceans to the northeast and northwest remained, the atmosphere above the Southern Ocean underwent an abrupt shift.

The winter outlook changed because of the influence of a short-term climate driver: a rapidly developing positive  Southern Annular Mode (SAM), which persisted until the third week of June.

SAM events at this time of year can only be forecast around two weeks ahead at best, and typically interact with the impacts of longer-term drivers such as  El Niño and La Niña (ENSO) and the Indian Ocean Dipole (IOD). In this case, the rapidly evolving and very strong SAM event was captured by the models very late in May and has certainly been a key player in Australia’s dry June.

While the interplay of climate drivers is complex and less predictable in late autumn and early winter, these three drivers have also been behind the wetter than average start to 2020 in southeast Australia (Figure 3).

Figure 3. Rainfall deciles for January to June 2020 showed above-average rainfall for much of southeast Australia. Source: Bureau of Meteorology.

While it is unusual for climate drivers to change abruptly, you can check on their current status via the latest Climate Driver Update (formerly the ENSO Wrap-Up) which is published every second Tuesday at 3pm. This goes into much greater depth on the current climate drivers and their recent changes.

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Bureau of Meteorology climate outlook

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