Wednesday, August 5, 2009

El Niño slowly consolidating in the Pacific

Bureau of Meteorology, 5 August 2009

Atmospheric indicators are increasingly showing patterns typical of a developing El Niño event. These indicators are driven by warm conditions in the tropical Pacific Ocean. If these warm conditions persist, as forecast by leading climate models, 2009 will be considered an El Niño year.

Pacific Ocean surface temperatures continue to exceed El Niño thresholds. As a result, cloud patterns, Trade winds and rainfall along the equator have all shown signs of responding to the warmer ocean conditions. The Southern Oscillation Index (SOI) has fallen over the past two weeks, and is now near zero: persistent negative values are a feature of El Niño events.

The last month has seen below average rainfall across much of eastern Australia with particularly dry conditions through Queensland. El Niño periods 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.

The most recent value of the Indian Ocean Dipole (IOD), as measured by the Dipole Mode Index (DMI), is near zero. The Bureau's POAMA model suggests the DMI may increase over the coming months, consistent with the developing El Niño event.

See IOD forecast, DMI values.

In Brief
  • The sea surface remains significantly warmer than the long-term average across most of the tropical Pacific Ocean, exceeding El Niño thresholds in central to eastern areas.
  • A large amount of the tropical Pacific sub-surface water is also warmer than the long-term average, particularly in the east.
  • The latest 30-day SOI value is close to zero, the monthly value for July was +2. Persistent negative values are a feature of El Niño events.
  • Trade winds have been consistently weaker than normal across most of the equatorial Pacific in recent months, particularly in the west.
  • Consistent with an emerging El Niño, cloudiness near the date-line has generally been greater than normal over the past month.
  • Six of the seven leading international climate models surveyed by the Bureau predict the tropical Pacific to continue to warm and to remain above El Niño thresholds for the remainer of 2009.

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