Abstract
AbstractThe El Niño phase of the El Niño Southern Oscillation (ENSO) is typically associated with below‐average cool‐season rainfall in southeastern Australia (SEA). However, there is also large case‐to‐case variability on monthly time‐scales. Despite recent progress in understanding the links between remote climate drivers and this variability, the underlying dynamical processes are not fully understood. This reanalysis‐based study aims to advance the dynamical understanding by quantifying the contribution of midlatitude weather systems to monthly precipitation anomalies over SEA during the austral winter–spring season. A k‐means clustering reveals four rainfall anomaly patterns with above‐average rainfall (Cluster 1), below‐average rainfall (Cluster 2), above‐average rainfall along the East Coast (Cluster 3) and along the South Coast (Cluster 4). Cluster 2 occurs most frequently during El Niño, which highlights the general suppression of SEA rainfall during these events. However, the remaining three clusters with local above‐average rainfall are found in ∼52% of all El Niño months. Changes of weather system frequency determine the respective rainfall anomaly pattern. Results indicate significantly more cut‐off lows and warm conveyor belts (WCBs) over SEA in El Niño Cluster 1 and significantly fewer in El Niño Cluster 2. In El Niño Cluster 3, enhanced blocking south of Australia favours cut‐off lows leading to increased rainfall along the East Coast. Positive rainfall anomalies along the South Coast in El Niño Cluster 4 are associated with frontal rainfall due to an equatorward shift of the midlatitude storm track. Most of the rainfall is produced by WCBs and cut‐off lows but the contributions strongly vary between the clusters. In all clusters, rainfall anomalies result from changes in rainfall frequency more than in rainfall intensity. Backward trajectories of WCB and cut‐off low rainfall highlight the importance of moist air masses from the Coral Sea and the northwest coast of Australia during wet months.
Highlights
The rainfall variability in southeastern Australia (SEA) is notable for its magnitude compared to other places in the world that are located in similar climate zones (Nicholls et al, 1997)
The lower observed above-average rainfall pattern during El Niño is illustrated by a smaller percentage of El Niño months that fall into Cluster 1 (8.3%) compared to all months in full Cluster 1
El Niño–Southern Oscillation (ENSO) is the primary driver of interannual rainfall variability and its positive phase – El Niño – is typically associated with below-average rainfall in SEA, especially in the winter–spring season from June to November
Summary
The rainfall variability in southeastern Australia (SEA) is notable for its magnitude compared to other places in the world that are located in similar climate zones (Nicholls et al, 1997). Drivers of rainfall variability in Australia can be split into two categories based on their temporal and spatial extent: lower-frequency processes including both large-scale atmospheric and oceanic phenomena, and higher-frequency processes on the synoptic scale referred to as synoptic systems (Risbey et al, 2009b). This separation is not clear-cut, as large-scale drivers affect drivers on the smaller scales and viceversa (King et al, 2014). Extratropical cyclones and the passage of cold fronts influence rainfall along the southeastern coastal regions and cut-off lows and tropical–extratropical interactions manifested as northwest cloudbands contribute to rainfall more inland (Tapp and Barrell, 1984; Wright, 1997; Murphy and Timbal, 2008)
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