A review of the El Niño Southern Oscillation impacts on Australian climate

In this study we review the current knowledge of the impacts associated with the El Niño Southern Oscillation (ENSO) in Australia. Among the major large-scale modes of variability, ENSO is the dominant phenomenon influencing seasonal mean rainfall and temperature, producing a spatially coherent pattern across nearly two-thirds of Australia. Its influence typically amplifies during multi-year events and varies on multidecadal cycles. When co-occurring with other climate modes of variability, such as Indian Ocean Dipole, Southern Annular Mode and Madden–Julian Oscillation, ENSO’s combined influence can explain about 50% of seasonal rainfall variability in parts of eastern and northern Australia during spring.

The main large-scale mechanisms that explain ENSO’s influence in Australia are (1) via changes in tropical atmospheric circulation associated with the Southern Oscillation and its related changes in sea level pressure, (2) through the modulation of the Pacific South American (PSA) pattern, and (3) indirectly via changes in the Indian Ocean sea surface temperatures (SST), which trigger Rossby wave trains to the Australian extra-tropics. These mechanisms affect the intensity and persistence of weather systems that control rainfall, particularly in eastern Australia. Their impacts are further modulated by local SST and land-atmosphere processes that can alter evaporation, humidity and moisture advection inland, thereby modulating rainfall response during ENSO events.

Although most studies published in the literature have focused on addressing the El Niño impacts, it is the La Niña phase of ENSO that produces a more consistent and arguably more societally impactful change across Australia. The ENSO-Australian rainfall relationship is asymmetric and stronger for La Niña. It is also the Central Pacific-type of ENSO event that typically produces stronger impacts on Australia. We will discuss the links between ENSO diversity, weather patterns, and associated extreme events, such as droughts and floods.

In a warmer climate, the ENSO-Australian rainfall relationship is projected to intensify by about 10-20%, consistent with many other regions across the globe. ENSO-driven precipitation and surface temperature variability is projected to strengthen in September to November over southeastern and southern Australia, while the largest changes are projected to occur during the warm season from December to February over most of western and northern Australia.

Despite considerable improvements in ENSO predictability and seasonal outlooks over the past four decades, predicting its impacts remains challenging because of large internal atmospheric variability. In addition, the observed cooling trend in the Pacific Ocean directly challenges the accuracy of El Niño-like warming projections in a future warming climate. These evolving ENSO features highlight the need for strategic research, sustained in situ monitoring, reduced model biases, and improved understanding of the anthropogenically induced changes in Pacific temperatures to support adaptation strategies.