This is possibly the goto data that can be used all year round. The data is taken from measuring the pressure from Darwin and Tahiti and calculating the difference.
The graph basically says positive numbers means rain and negative is lack of rain. This graph for 2023 Oct is showing lack of rain.
I was asked for the formula, here it is.
It is calculated as follows: SOI = 10*(Pdiff - Pdiffav) / SD(Pdiff) where Pdiff = (average Tahiti MSLP for the month) - (average Darwin MSLP for the month), Pdiffav = long term average of Pdiff for the month in question, and SD(Pdiff) = long term standard deviation of Pdiff for the month in question.
The SOI is an indicator of low pressure. Low pressure pulls air towards it, and oushes it up. When air rises it cools and forms rain. Thus low pressure is synonymous with rain. It is so linked to rainfall that it correlates with the Gross Domestic Product GDP.
The Southern Oscillation Index, or SOI, gives an indication of the development and intensity of El Niño or La Niña events in the Pacific Ocean. The SOI is calculated using the pressure differences between Tahiti and Darwin. Sustained negative values of the SOI lower than −7 often indicate El Niño episodes.
The SOI often forms cycles and this is due to the regular cycle of High pressure systems moving from the West of the nation to the East. These high pressure systems tend to take a week. So the cycle dips down towards more negative when a High moves to the East, and as a void opens up, a low pressure is evident and thus the SOI climbs.
A High brings trade winds and the SOI can also be used to determine when these will arrive. When a High sits over SA the trade winds begin up the East Coast of Australia and thus rain for the Far North of QLD and some other exposed coastal regions.
Sometimes a High may stall, rush through or be pushed South over New Zealand, and thus the SOI stays more positive.
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