Saturday, August 22

...of droughts and flooding rains.

We have had some unusually heavy frosts and strong winds recently, which are sucking moisture out of the soil. There is still good pasture so no feed worries as the lambs arrive (165 as at last week from 128 ewes which gives a lambing percentage at birth of a cracking 130%). Not worried about rainfall yet but as most of Australia lies within the globe’s arid belt where rainfall is low and unpredictable the weather is always on our minds. Between 1864 and 2002 Australia suffered from 14 major droughts and in one of the worst in 1895 to 1903 the sheep population halved to 53 million head.

The single most important influence on Australian rainfall is the El Nino Southern Oscillation phenomenon (ENSO). El Nino and its opposite twin La Nina are anomalous warming and cooling, respectively, of the ocean water masses that result in changes in atmospheric circulation patterns with a subsequent effect ion rainfall amounts and distribution. The Southern Oscillation Index, shown to July this year above, is generally negative during El Nino events and positive in La Nina floods.

If an El Nino event is to develop it normally starts between March and May signaled by a falling SOI, usually to about -10. Parts of eastern Australia then begin to experience lower than average seasonal rainfall in autumn and late winter. As high pressure systems build into early spring there are fewer low pressure systems able to deliver rain and late in the calendar year the drought intensifies through summer. The effect normally breaks down in March, the SOI rises sharply and there is heavy rainfall often causing serious flooding.

Because ENSO events persist they can be predicted some months ahead. So we are watching the SOI carefully on the Bureau of Meteorology site and keeping our fingers crossed that we get some spring rains to get us through the summer.

No comments: