A weather system perspective on winter-spring rainfall variability in southeastern Australia during El Niño

The El Niño Southern Oscillation (ENSO) is typically associated with below-average rainfall in the winter-spring season in southeastern Australia. However, there is also a large case-to-case variability pointing to the non-linear relationship of El Niño strength and the impact on east Australian rainfall. Despite recent progress in understanding the linkage of remote climate drivers and this variability, the dynamical processes by which the drivers transmit their influence on rainfall are not fully understood. With this study, we aim to advance the dynamical understanding by relating patterns of monthly rainfall anomalies over southeastern Australia to a novel dataset of objectively identified weather systems derived from ERA-Interim reanalyses.

We find 4 rainfall anomaly patterns in the austral winter-spring season (JJASON) with above-average rainfall (Cluster 1), below-average rainfall (Cluster 2), above-average rainfall limited to the East Coast (Cluster 3) and above-average rainfall limited to the South Coast (Cluster 4) in southeastern Australia. Changes in the frequency of weather systems explain partly the rainfall anomalies in the clusters. Results indicate a significant increase of weather system activity in Cluster 1 and a weakening of weather system activity in Cluster 2. In Cluster 3, enhanced blocking favors the development of cut-off lows on its northeastern flank leading to increased rainfall along the East Coast. Positive rainfall anomalies along the South Coast are associated with frontal rainfall due to an equatorward shift of the midlatitude storm track (Cluster 4). Most of the rainfall is produced by warm conveyor belts and cut-off lows but the contributions strongly vary between the clusters. We further find that anomalies in rainfall result from changes in rainfall frequency more than in rainfall intensity. By calculating backward trajectories of warm conveyor belt and cut-off low rainfall, we point to the importance of moist air masses from the Coral Sea and the northwest coast of Australia for wet months. Air parcels, that end up in WCB or cut-off low rainfall, reach southeastern Australia from the dry remote areas to the north and not as one would expect from the Southern Ocean.