Update: it appears that the September values of the Tahiti-Darwin SOI (+25) and Klaus Wolter Multivariate ENSO Index (MEI) are at historical levels when compared to previous Septembers, with the SOI only showing a higher September value (+29.7) back in 1917!
From the Australian Bureau of Meteorology, the trace is now at a comparable level to the 2008 La Niña, though the onset is more rapid:
Issued on Wednesday 13 October
The La Niña in the Pacific remains a moderate to strong event. The Southern Oscillation Index (SOI) value of +25 for September was its highest monthly value since 1973 [Ryan Maue adds here: the September value of +25 is the highest value for that month since 1917, the only other time it has been exceeded according to the Tahiti-Darwin SOI data back to 1876], while central Pacific Ocean temperatures are comparable to those observed during previous moderate events, such as 2007 and 1998. Long-range models surveyed by the Bureau suggest that this La Niña will persist into at least early 2011.
ENSO indicators remain firmly at La Niña levels. The central Pacific Ocean is more than 1°C cooler than the long-term mean at the surface, while temperatures below the surface are up to 5°C cooler than normal. The SOI remains above +20, which places it in the top 5% of observed values. Although trade winds are stronger than normal over the western Pacific and cloudiness over the central and western Pacific continues to be suppressed, a strong Madden Julian Oscillation (MJO) may weaken some of these indicators over the coming fortnight.
La Niña periods are generally associated with above normal rainfall during the second half of the year across large parts of Australia, most notably eastern and northern regions. Night time temperatures are historically warmer than average and Tropical Cyclone occurrence for northern Australia is typically higher than normal during the cyclone season (November-April).
A negative IOD event is also underway in the Indian Ocean. Negative IOD events are typically associated with above average rainfall over large areas of southern Australia during spring. IOD events usually decay in the months of November and December with the onset of the Australian monsoon.
Click image to go to BOM page or see our WUWT ENSO/SST page here
h/t to Geoff Sharp