As someone else pointed out .... From the BOM website... they are making it up as they go along..
http://www.bom.gov.au/climate/glossary/soi.shtml
Southern Oscillation Index
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 below −8 often indicate
El Niño episodes. These negative values are usually accompanied by sustained warming of the central and eastern tropical Pacific Ocean, a decrease in the strength of the Pacific Trade Winds, and a reduction in winter and spring rainfall over much of eastern Australia and the Top End. You can read more about historical El Niño events and their effect on Australia in the
Detailed analysis of past El Niño events.
Sustainted positive values of the SOI above +8 are typical of a
La Niña episode. They are associated with stronger Pacific
trade winds and warmer sea temperatures to the north of Australia. Waters in the central and eastern tropical Pacific Ocean become cooler during this time. Together these give an increased probability that eastern and northern Australia will be wetter than normal. You can read more about historical La Niña events and their effect on Australia in the
Detailed analysis of past La Niña events.
The
ENSO Wrap-Up includes the latest 30-day SOI value, as well as other information on indicators of El Niño and La Niña events.
The graph below shows monthly values of the SOI in recent years.
-8 does not fit the agenda so lets make it -7
http://www.bom.gov.au/climate/enso/index.shtml#tabs=SOI
Sustained positive values of the SOI above +7 typically indicate La Niña, while sustained negative values below −7 typically indicate El Niño. Values of between about +7 and −7 generally indicate neutral condition
