Year-to-year changes associated with El Niño-Southern Oscillation (ENSO) and its impact on Australia and other parts of the world, both vary on interdecadal time-scales in the observational record and in the BMRC global coupled atmosphere-ocean general circulation model (CGCM). A key and active area of research is aimed at assessing the predictability of such changes. A century-long integration of the CGCM and four sets of decadal-long perturbation experiments are used to show that if these interdecadal changes are predictable then the level of predictability is low.

Previous work demonstrated that an index for the Interdecadal Pacific Oscillation (IPO) - which is closely related to the interdecadal component of an index for the Pacific Decadal Oscillation (PDO) - is statistically linked to changes in ENSO vigor and ENSO’s impact on Australian climate during the twentieth century.

We will show that the earlier observational analyses and the low predictability of both ENSO vigor and ENSO’s impact on Australia in the CGCM can be explained qualitatively without the need for predictability on decadal or longer time-scales as follows:

(1) Changes in both ENSO vigor and the relative frequency and magnitude of El Niño and La Niña events in a given interdecadal period are essentially unpredictable.

(2) There is a nonlinear relationship between ENSO (as measured by NINO4 or the Southern Oscillation Index for example) and All-Australia rainfall. A large La Niña sea-surface temperature (SST) anomaly is closely linked to a large Australian response (i.e., Australia usually becomes much wetter), whereas the magnitude of an El Niño SST anomaly is a poorer guide to how dry Australia will actually become. Australia tends to dry out during El Niño events, but the degree of drying is not as tightly linked to the magnitude of the El Niño SST anomaly. This relationship is clearly evident in both observational records and the CGCM. It indicates that forecast information should not include the generalization that “the larger the El Niño SST anomaly, the greater the risk of severe Australian drought”. This nonlinearity is important in helping to determine the character of the modulation of ENSO’s impact on Australian rainfall. However, we will show that apparent modulation of ENSO teleconnections can occur in other regions without the need for nonlinearity, and again without the need for predictability on decadal or longer time-scales. (3) Indices of the IPO or PDO are good proxies for the (largely unpredictable) interdecadal component of ENSO variability. The magnitude of the correlation coefficients between the IPO index and the interdecadal component of traditional ENSO indices are large in both the observations and the CGCM.

Note that other aspects of Indo-Pacific climate do exhibit predictability on multiyear and longer time-scales, and they will be described. This study therefore highlights that while improved understanding of interdecadal variability in ENSO and the Indo-Pacific generally has the potential to improve our ability to predict climate, care is needed in the development of statistical forecast systems seeking to exploit observed inter-relationships.