Milking the Weather - March 2011 Edition
To wait for the autumn break or have we had it?
Rod Eldridge, DPI Colac
Following the wet spring and summer of 2010 / 2011, a question many farmers may now be thinking is, what does the autumn hold for sowing and establishing pastures? Over recent years we have seen a noticeable decline in autumn rainfall, so can we expect the rains to continue or should we think about the drying trend? An understanding of shorter term climatic variability as opposed to longer term climate change can help.
Long term autumn rainfall trends
Figure 1 shows the autumn rainfall (1900 – 2010) and five year rolling average for Victoria. The long term average (1900 – 2010) autumn rainfall for the area is 152mm, however for annual average rainfall for the period 1996 to 2010 was only 119mm.
A recent study by the BOM and CSIRO (Timbal et al. 2010), found that rainfall across southeastern Australia (including Victoria) for the 13 year period 1997 to 2009 was the lowest on record. The rainfall deficit for 1997 – 2009 was 45 per cent greater than the previous 13 year rainfall low which ran from 1933 to 1945 and was known as the Word War II drought.
Figure 1: Autumn rainfall and five year rolling average of autumn rainfall (1900 -2010) for southern Australia (131.5o - 137.5oE, 35.5o-38.5oS)
Timbal also found that approximately 52 per cent of the total annual rainfall decline between 1997-2009 occurred during autumn. Autumn rainfall for this period was 25 per cent below the long term autumn average and part of a long-term declining trend which commenced in the 1970’s. This decline was unprecedented in the rainfall records and was found to be clearly outside the inherent natural variability experienced over the long term rainfall record.
Will the current weather continue?
The very wet year of 2010 was attributable to the La Niña event in the Pacific Ocean, with warm water off the Queensland coast. This also coincided with a negative dipole event in the Indian Ocean, with warm water off the coast of north west Australia. Both of these events can be associated with average or above average rainfall over the spring period. There have been four occasions since 1958 when these two events have coincided, (1964, 1971, 1974 and 1975).
Over summer and autumn, the warm waters associated with these events generally break down as the oceans cool and the La Niña weakens. They have less influence on our rainfall over autumn.
Recently we have been experiencing sub-tropical like weather, humid and with substantial rain events from the north. As the season progresses it’s useful to watch for the reversion to conditions which generally bring the winter rains. This will be the movement of the sub tropical ridge (ie location of the high pressure systems) to its more northward position, and the penetration of cold fronts from the southwest further northwards onto the mainland (Southern Annular Mode).
Is the break important this season?
Presently a number of the global climate models are predicting average to slightly wetter conditions for autumn. As we enter the growing season it is useful to be aware of what the global climate models are predicting and look for any trends across a number of models. Also bear in mind that the reliability of these models improves as the year progresses.
Should the current warmer sea surface temperatures persist longer into autumn it could lead to continuation of reasonable rainfall events over the coming months, and we may not see or need a traditional autumn break. The combination of stored soil moisture, relatively mild daytime temperatures and moderate evaporation rates for this time of year could provide good conditions for the establishment of pasture, and possibly some early sowing. However there could also be the possibility of some poorly drained soils getting too wet.
