Understanding Climate Risk

Science, policy and decision-making

Global warming breaks link between SAM and ENSO

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Global warming has caused SAM and ENSO to divorce according to Guojian Wang and Wenju Cai, published in Nature Science Reports on June 20. This is having major impacts on Australia and has contributed to the warm and dry conditions over the southern part of the continent since the late 1960s.

SAM is the Southern Annular Mode surrounding Antarctica, a band of wind and water that distributes hot, high pressure and cold, low pressure lobes around the Southern Ocean. This transfers atmospheric mass (pressure) between the mid and high latitudes. The positive phase is highly correlated with a positive phase of ENSO (the El Niño-Southern Oscillation), La Niña. A positive phase of SAM forces westerlies further south in autumn-winter, but in summer allows the easterly trades greater access, bringing in more moisture from the tropics and enhancing La Niña summer rainfall.

Both ozone depletion and global warming increase the positive phase of SAM, strengthening westerlies on the south and weakening them further north. More so, it is imposing itself on the relationship between SAM and ENSO, and the breaking of this relationship is the subject of Wang and Cai’s (W&C) paper. The effect is greatest during summer, and W&C say that it is also a major contributor to the breaking down of the autumn break, which has had a huge impact of winter grain production n southern Australia.

The relationship between warming and SAM is quite strong across 31 models for the 21st century, with a correlation of 0.59. The strength of the correlation between ENSO and SAM during the 20th century has waxed and waned, being weak in the 1940s and strong early and late in the 20th century. With the global warming signal taken out (top panel), the relationship between ENSO and SAM is strong but with it in, they depart in the late 1960s (lower panel).

Evolution of correlations among ENSO (i.e., Niño3.4), GMT, and the SAM in the 20th Century. Evolution of correlation between ENSO and GMT (red curve), GMT and the SAM (blue curve), and ENSO and the SAM (green curve) are shown using, (a), time series linearly detrended within each of a 31-year sliding window, and (b) raw time series. The correlation coefficient required to reach statistical significance at the 95% confidence level by Student’s t test is indicated by horizontal dashed lines.

Evolution of correlations among ENSO (i.e., Niño3.4), GMT, and the SAM in the 20th Century. Evolution of correlation between ENSO and GMT (red curve), GMT and the SAM (blue curve), and ENSO and the SAM (green curve) are shown using, (a), time series linearly detrended within each of a 31-year sliding window, and (b) raw time series. The correlation coefficient required to reach statistical significance at the 95% confidence level by Student’s t test is indicated by horizontal dashed lines.

The kicker is that the influence of warming is not gradual, instead it switches on abruptly (circled) in the lower panel. This shows quite clearly that the evolution of climate change is non-linear and not gradual, something I have been banging on about for quite some time. W&C also show that warming is not superimposed on variability in this case, but independent of it. This is completely consistent with the statistical analyses I have been using to separate climate variability in temperature from the warming signal.

So what does this mean for Australia’s climate? It means that an overwhelmingly positive SAM is keeping the westerlies south and contributing to our drier autumn winters and delivering weather typical of the Riverina to southern Victoria according to Cai. Recovery of the ozone layer and reduction in greenhouse gas emission would stabilise this process, rather than continuing to send it south.

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