ENSO update

Strong indicators of El Niño persist

Indicators suggest an El Niño event is developing across the Pacific Basin. Conditions have reached a point that, should they persist at such levels through the remainder of the southern winter and into spring, 2009 will be considered an El Niño year.

Leading climate models indicate that warming of the Pacific will continue for the next few seasons, with very little chance of the current development stalling or reversing.

Continuing El Niño signals include central Pacific Ocean surface temperatures around 1°C above average, and supporting sub-surface temperatures up to 4°C warmer than normal. Trade winds remain weaker than average, and there is an emerging signal of enhanced cloudiness near the date-line

Conversely, the Southern Oscillation Index (SOI) rose over the past week to near zero. However, this would appear to be a response to local weather conditions near Darwin and Tahiti, rather than a long-term climate signal, and hence the SOI is likely to fall again in the weeks ahead.

El Niño events are usually (but not always) associated with below normal rainfall in the second half of the year across large parts of southern and inland eastern Australia.

After many weeks of positive values, the most recent value of the Indian Ocean Dipole (IOD), as measured by the Dipole Mode Index (DMI), was slightly negative. In the past, positive IOD values have been associated with drier conditions through south east Australia in winter and spring.

• The Pacific Ocean sea surface is significantly warmer than the long-term average across most of the tropical Pacific, with El Niño thresholds having been reached in central to eastern areas.
• A large amount of the sub-surface water of the tropical Pacific is also warmer than the long-term average, particularly in the east.
• The latest 30-day SOI value is +1, while the monthly value for June was −2.
• Trade winds remain weaker than normal across the western equatorial Pacific.
• Consistent with an emerging El Niño, cloudiness near the date-line has recently increased.
• All international climate models predict the tropical Pacific to continue to warm and to be above El Niño thresholds throughout most of the second half of 2009.