The 2020 spring, what might we expect?
Author: Dale Grey (Agriculture Victoria) | Date: 23 Jul 2020
Take home messages
- Irrigated districts have had a great start to the season, with an early wet up to catchments.
- Forecasts remain for a wetter SNSW and for wetter Victorian catchments. Forecasts are split between wetter and neutral for the Victorian plains.
- The Pacific Ocean shows some chance of becoming a La Niña, where the Indian Ocean must do a 180-degree flip from its current condition to become a negative IOD (i.e. favourable to wet conditions).
Key Question 1: What factors have led to such a good start for much of this region in 2020?
The autumn of 2020 will be remembered for its timely break and high rainfall in places. The rainfall pattern changed in January after the long-term dry period during the 2019 spring and early summer. Summer storms were commonplace on the plains and in the catchments through to April. As a result, stored soil moisture in irrigated paddocks turned out to be both a blessing and a curse when it came to watering up and/or sowing. It was a blessing in the catchments as soils wet up and streamflow was primed to run off much earlier than normal however it added some challenges with the practices of watering up and/or sowing.
Catchment flow during autumn and early winter was higher than normal. Much of this rain was a result of summer weather events rather than the influence of any actual climate drivers. Lower pressure maintained itself in South-Eastern Australia for many of these months indicating plenty of low-pressure troughs to bring tropical moisture down. The one feature that was in play was very warm waters to the North West of Australia, evaporating more moisture and acting as a feed source to enable good rains.
Key Question 2: How is the forecast looking for the remainder of winter and spring?
A preponderance of wetter forecasts has been a feature of world climate forecasts since April, however many models started to forecast wetter in March. Despite models suggesting wetter conditions, average to drier conditions have occurred on the Riverine Plains and the catchments, after the big wet of April. It’s important not to focus too much on forecasts in autumn, as tantalising as they may be. History says it’s a time of the year where everything is in flux and rapid changes can happen with little warning. Since autumn, models have lost some, but not all, of their enthusiasm for a wetter spring. At the time of writing (10/7/2020) models are split between wetter and neutral being more likely for the Northern Victorian plains, but the majority have a wetter North East catchment. For southern NSW, the majority of models remain convinced that a wetter spring on the plains and in the catchments is more likely.
Key Question 3: Is there anything in particular that we need to be aware of that could affect the forecast?
A common feature of forecasts during this season is the number of them predicting La Niña or negative Indian Ocean Dipole (-IOD) conditions. Both phenomena increase the chances of spring rainfall being wetter. At the time of writing (10/7/2020), the majority of the world models think both can still occur. But like the promise of ‘free ice cream tomorrow’, eventually we need to see some evidence that the oceans and atmosphere are still interested in these events. As it currently stands, there is more evidence of a La Niña forming than a -IOD, but both seem to be getting a lack of support from the atmosphere above the ocean. In the Pacific Ocean we have cooler water to depth, an outbreak of cooler water at the surface, and decreased cloud at the Dateline. Yet the Southern Oscillation Index and the Trade winds show no real signs of being interested in La Niña. Without the atmospheric support the enthusiasm of the ocean will be in vain. In the Indian Ocean there is a large pool of warmer water with lower pressure and there is greater cloud off the African Coast. Off Indonesia, conditions are normal. To become a -IOD, Indonesia needs to warm up and Africa needs to cool down. This is a much larger task than what needs to happen in the Pacific Ocean area, given the former is halfway toward La Niña and the Indian Ocean is nowhere near -IOD. The models that give monthly outlooks have been doing better, in terms of successfully predicting the drier June and predicting a drier July. If this trend continues then there will be no free ice cream tomorrow and we will be living on good stored soil moisture and unfortunately, looking at reduced inflows when we need them in spring. Trade wind activity in the Pacific and the Indian Oceans holds the key to turning things towards what the models are predicting, or having it slip from view.
Trade winds that provide support for warmer oceans to our north west and north east, hold the key to wetter forecasts coming to fruition.
The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. The author would also like to thank Agriculture Victoria for their long-term support of this work.
Dale Grey (Seasonal Risk Agronomist)
PO Box 3100, Bendigo DC, Victoria, Australia, 3554
GRDC Project code: 9176117
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