Milking the Weather - June 2010 Edition
Seasonal Outlook
Summary as of 23 June, 2010
http://www.bom.gov.au/climate/enso/
Tropical Pacific continues to cool, odds firm for a La Niña event in 2010.
The majority of climate models suggest a continued cooling of the central Pacific ocean to beyond La Niña thresholds before the end of winter.
Sea surface temperatures have continued to cool and are now generally cooler than average in areas east of the date line (see right).
The Southern Oscillation Index (SOI) (pressure difference between Tahiti and Darwin) has been consistently positive since April (see below), trade winds have strengthened and sea sub-surface temperatures have also cooled. These indicators are consistent with the developing stages of a La Niña event.
Historically, about 35~40 per cent of El Niño events are followed by a La Niña within the same year.
Above: Sea surface temperatures have cooled in the central equatorial Pacific. A continued cooling of sea surface temperatures will increase the chance of a La Niña in 2010.
http://www.osdpd.noaa.gov/ml/ocean/sst/anomaly.html
SOI (Southern Oscillation Index)
SOI is in positive territory, currently around + 8.0. Sustained positive values can be an indicator of La Niña. http://www.bom.gov.au/climate/enso/
The SOI is calculated from monthly or seasonal changes in air pressure difference between Tahiti and Darwin. The SOI index measures the strength and direction of the Southern Oscillation.
- Sustained negative values often indicate El Niño episodes.
- Sustained positive values, La Niña episodes.
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Near zero values, Neutral episodes.
30 Day Moving SOI
What is La Nina?
La Niña refers to the extensive cooling of the central and eastern Pacific Ocean. In Australia (particularly eastern Australia), La Nina events are associated with increased probability of wetter conditions. Characteristics of La Niña are:
- SOI: Sustained positive values
- SST: Warmer sea temperatures north of Australia, cooler than normal central and eastern tropical
- Pacific Ocean Trade Winds: Stronger than normal (easterly) trade winds across the Pacific Ocean
- Cloudiness: Increased cloudiness over tropical Australia, Papua New-Guinea, Indonesia
