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An improved diagnostic for summertime rainfall along the eastern seaboard of Australia

The eastern seaboard of Australia (ESB) is a unique climate entity. A region of complex topography bound by the coastline and the ridge of the Great Dividing Range, the ESB exhibits distinctly different rainfall patterns to the rest of southeastern Australia. As a large percentage of the Australian population resides along the ESB, understanding current and future rainfall variability in this region presents an important and difficult challenge. This challenge is compounded by the inability of most general circulation models to properly resolve the region. This study presents a diagnostic for characterizing the variability in summertime rainfall along the ESB. This simple yet effective East Coast Flow Index (ECFI) infers easterly geostrophic flow from the meridional pressure gradient along the ESB, and vertical circulation from the upper‐level ω field. The ECFI outperforms the common indices for drivers of rainfall variability, achieving statistically significant correlations with ESB summertime rainfall for a range of different timescales. The ECFI is shown to have great potential as a predictor for statistically downscaling rainfall along the ESB as (1) it provides a physical link between large‐scale forcings and local precipitation responses, (2) is capable of capturing multiyear variability and (3) is realistically represented by general circulation models. While the ECFI was designed for the summer months (December–February) it is still capable of performing well in the other seasons, especially spring (September–November).