Defining Climate Terms
Seasonal climate and risk information for Victorian cropping regions
Rainfall Decile
A ranking of historical rainfall data into ten even groups for a defi ned rainfall period (e.g. monthly, yearly).
For 100 years or rainfall records, Decile 1 would be the 10 driest years on record, Decile 2 would be the next 10 driest years on record, and Decile 10 would be the 10 wettest years. Or Decile 1 = lowest 10 per cent of records and Decile 10 = highest 10 per cent of records.
See the chart on the right for example: Ouyen recorded 325mm of rainfall for 2009. This ranks as a Decile 5 for that period. The growing season rainfall (GSR) was 231mm, Decile 6. In November 68mm was recorded, Decile 10 - one of the wettest November rainfall months in the historical data set.
Ouyen Rainfall Deciles 2009
Deciles generated by ‘Rainman’, developed by QLD DEEDI
Anomaly
An anomaly is the amount above or below the long-term average.
ENSO (El Nino-Southern Oscillation)
The El Nino Southern Oscillation (ENSO) describes the ocean and atmospheric circulation of the Pacifi c Ocean. La Nina, El Nino and Neutral are the three phases. In La Nina years, an average to above average growing season is more likely. In El Nino years, the chances of a below average to average growing season is more likely.
Indicators of the ENSO phase include:
- SST (Sea surface temperature anomalies)
- Sub-surface sea temperature anomalies
- SOI (Southern Oscillation Index)
- Trade Winds (strength and direction)
- Cloudiness (near the dateline)
Characteristics of La Nina
- Cooler than average SST in the Pacifi c Ocean along the equator.
- Cooler than average sea temperatures to depth along the equator.
- Positive SOI, where pressure at Tahiti is greater than Darwin.
- Strong easterly trade winds along the equator.
- Below average cloudiness near the dateline.
Characteristics of El Nino
- Warmer than average SST in the Pacifi c Ocean along the equator.
- Warmer than average sea temperatures to depth along the equator.
- Negative SOI, where pressure at Tahiti is lower than Darwin.
- Weak easterly trade winds along the equator.
- Above average cloudiness near the dateline.
El Nino-Southern Oscillation Circulation
www.bom.gov.au/lam/Students_Teachers/elnanim/elani.shtml
Pacific Ocean sub-surface temperature anomaly
Southern Oscillation Index (SOI)
The SOI is calculated by measuring the air pressure at Tahiti and subtracting the air pressure at Darwin. The SOI index measures the strength and direction of the Southern Oscillation.
- Sustained negative values (lower than -5) often indicate El Nino episodes.
- Sustained positive values (greater than +5) often indicate La Nina episodes.
- Near zero values indicate Neutral episodes.
The July/August SOI for many Victorian locations has skill in forecasting the rainfall outlook for spring. If the SOI is consistently positive, the chance of spring rainfall is higher. If the SOI is consistently negative, the chance of spring rainfall is lower.
Southern Oscillation Index - ending April 2010
Sea Surface Temperatures (SST) - Pacific Ocean
The SST chart (16 November 2009) demonstrates warmer than average water along the equator in the Pacifi c Ocean and cooler than average water north of Australia. This is characteristic of El Nino conditions.
Pacific Ocean SST (El Nino) November 2009
www.osdpd.noaa.gov/ml/ocean/sst/anomaly.html
Indian Ocean (IOD+ve or IOD-ve)
The Indian Ocean Dipole (IOD) has three phases, positive (+ve), negative (-ve) and neutral. Measurements for IOD classifi cation occur in the eastern Indian Ocean (off Sumatra) and in the western Indian Ocean (along the equator near Kenya and Somalia).
When the eastern Indian Ocean waters are cold and the western Indian Ocean waters are warm an IOD+ve event occurs, as shown in the map on the right. This can correlate with less growing season rainfall in Victoria.
When the eastern Indian Ocean waters are warm and the western Indian Ocean waters are cold an IOD-ve event occurs. This can correlate with higher growing season rainfall in Victoria.
IOD events generally develop in late winter/spring. In 2006, 2007 and 2008, IOD+ve events occurred.
Positive IOD event September 2006
