Climate variability in eastern Australia is strongly related to ENSO. In La Niña summers, eastern Australia typically experiences cooler wetter weather, whereas during El Niño events, summers are generally warmer and drier than average.
The relationship between ENSO and rainfall is also non-linear with greater differences in rainfall between La Niña and neutral events than between neutral and El Niño seasons as we can see in this figure.
Figure 1: The ENSO-Eastern Australia rainfall relationship. Rainfall totals are greater in La Niña seasons (very negative NINO3.4 index) than in neutral (near-zero NINO3.4) or El Niño (very positive NINO3.4) seasons
We investigated this non-linearity in the ENSO-rainfall relationship in more detail. Using a reanalysis we could show that this non-linearity is more strongly related to moisture availability over the southwest Pacific than it is to variability in winds.
Figure 2: There are stronger relationships between moisture availability and eastern Australia rainfall in negative NINO3.4 seasons (left column) than in positive NINO3.4 seasons (right column) as illustrated by the scatter plots (top row) and mapped correlations (bottom row).
Climate models have strong differences in their ENSO-rainfall relationships and these differences are strongly associated with varying moisture availability relationships in the models.
For a coupled model run that fails to reproduce the ENSO-rainfall non-linearity, an atmosphere-only version of that model also fails to capture the non-linearity. This shows that in order for the relationship between ENSO and rainfall in eastern Australia to be accurately reproduced we need models to have both realistic oceanic ENSO patterns and accurate atmospheric responses.
This work has important implications for studying rainfall variability and projections in eastern Australia as many models do not have a realistic teleconnection between ENSO and rainfall in this region.