Yesterday the ABC hyped up their climate alarmism to another new level with their uncritical and unabashed reporting of a claim by the CSIRO that “Extreme La Niña events … will almost double in frequency as the climate warms”.
“Lead author Dr Wenju Cai, chief scientist at Australia’s CSIRO Oceans and Atmosphere Flagship, says their work shows La Niña events will occur every 13 years compared with a past frequency of one every 23 years.”
This is the paper:
Increased frequency of extreme La Niña events under greenhouse warming, by Wenju Cai et al., published yesterday.
Time for a reality check.
The authors say they used climate data from 1900 to 2005, and 21 climate models to predict conditions for 2006-2099, and that an extreme La Nina is defined by Central Pacific sea surface temperature anomalies of more than 1.5C below normal. They claim that an increase in severe El Ninos will lead to an increase in following extreme La Ninas.
In the paywalled article I suspect the Central Pacific region they use is actually the Nino 4 region. In this analysis I use data from the Nino3.4 region, which is the overlap between Nino 3 and Nino 4, covering Latitudes 5 degrees South- 5 North and Longitudes 170 degrees West- 120 West. This is the most common data region used. I downloaded data from http://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Nino34/ and calculated monthly anomalies from the 1961-1990 means. There are data from 1870, however I chose to use data from 1876 to match Southern Oscillation Index (SOI) data.
Here are the results:
Fig.1: Nino 3.4 anomalies. Note 1900 & 2005 limits, and +/- 1.5C thresholds.
By screening for events of +/- 1.5 or more, we remove the clutter and identify extreme events:
Fig.2: Nino 3.4 data exceeding +/- 1.5C
The paper claims that the incidence of extreme La Ninas will increase from one per 23 years to one per 13 years. While there are more extreme La Ninas in the last 45 years, I count seven La Ninas from 1900 to 1999, which is one per 14 years. There were three very high El Nino peaks since 1970, but there are clusters of extreme El Ninos in the first and last thirds of the record. So possibly the claim for increased La Nina frequency was for an increase in the frequency of abrupt swings from El Nino to La Nina.
Fig.3: 12 monthly change in Nino 3.4 anomalies. +/- 3C is the threshold for swings from extreme El Nino to extreme La Nina.
Fig.4: Removing the clutter, change exceeding +/- 3C.
There we have it. The extreme changes since 1900 have all been in the last 45 years. Is this due to Greenhouse warming or natural climate change? Could it have anything to do with the Interdecadal Pacific Oscillation? Or is it an artefact of my arbitrary choice of extreme threshold?
More importantly, does the Southern Oscillation Index (SOI) tell the same story?
SOI data are from the BOM website.
Fig.5: 12 month running mean of the SOI inverted. Threshold is +/- 8. Note the historical rises and falls.
Fig.6: Nino 3.4 and 12 month inverted SOI match fairly well, although SOI values lag by up to 2 years.
Fig.7: El Nino and La Nina conditions per SOI criteria (+/- 8). An extreme ENSO event might be +/- 16, although I have not seen that mentioned anywhere.
Again note the clusters of El Ninos, and the spread of La Ninas, in small groups with large gaps between.
Fig.8: 12 month SOI change exceeding +/- 16. Horizontal lines indicate the threshold for an annual swing of +/- 24 units, which is associated with some dramatic weather events.
I left all of the changes >16, to show the historical spread. Note there were three extreme La Nina (< -24) changes from 1876- 1916, and three from 1960- 2000, and four from 1973- 2014. There is no unusual trend.
How does this correspond with the observed rainfall record, especially for South East Australia, which is predicted to receive more extremes of rain and drought due to greenhouse warming?
Fig. 9: Number of months of severe deficiency.
Fig. 10: Number of very wet months.
Not very alarming.
Queensland is especially susceptible to ENSO events.
Fig. 11: The match for Queensland wet years is better.
Fig. 12: But not for droughts!
Where are the extreme El Ninos? Call me underwhelmed.
Depending on the index used, the criteria used, and the length of the record used, you can say we’ve had an increase in extreme ENSO swings, or no noticeable change other than a long period (70 to 90 years?) cycle of more and less extreme changes.
My money’s on the latter, but Time will tell.