Dynamics and Predictability of El Niño–Southern Oscillation: An Australian Perspective on Progress and Challenges

Abstract

El Niño and La Niña, the warm and cold phases of El Niño–Southern Oscillation (ENSO), cause significant year-to-year disruptions in global climate, including in the atmosphere, oceans, and cryosphere. Australia is one of the countries where its climate, including droughts and flooding rains, is highly sensitive to the temporal and spatial variations of ENSO. The dramatic impacts of ENSO on the environment, society, health, and economies worldwide make the application of reliable ENSO predictions a powerful way to manage risks and resources. An improved understanding of ENSO dynamics in a changing climate has the potential to lead to more accurate and reliable ENSO predictions by facilitating improved forecast systems. This motivated an Australian national workshop on ENSO dynamics and prediction that was held in Sydney, Australia, in November 2017. This workshop followed the aftermath of the 2015/16 extreme El Niño, which exhibited different characteristics to previous extreme El Niños and whose early evolution since 2014 was challenging to predict. This essay summarizes the collective workshop perspective on recent progress and challenges in understanding ENSO dynamics and predictability and improving forecast systems. While this essay discusses key issues from an Australian perspective, many of the same issues are important for other ENSO-affected countries and for the international ENSO research community.

Impact of ENSO on dependence between extreme rainfall and storm surge

“…This result is supported by the recent finding that the Central Pacific ENSO events (e.g. La Niña events) are more impactful over Australia than the Eastern Pacific events (typically extreme El Niño events) (Santoso et al 2019). However, increased dependence is obtained during El Niño in northern Australia near the Gulf of Carpentaria and the southwest corner of Western Australia from both observed and ROMS modelled surge data (figures 2(d) and 3(d)).…”

“…The significant impact on the La Niña phase of ENSO on the dependence is due to the fact that the Central Pacific ENSO events (e.g. La Niña events) are more impactful over Australia than the Eastern Pacific events (typically extreme El Niño events) (Santoso et al 2019), highlighting the importance of ENSO event diversity for evaluating impact of ENSO on coastal flood risk over countries like Australia. The contribution of each phase of ENSO to dependence was subsequently explained by the synchronicity of correlations for rainfall and storm surge with respect to ENSO.…”

On the Delayed Coupling Between Ocean and Atmosphere in Recent Weak El Niño Episodes

“…In addition, recent studies (Choi et al, ; Xiang et al, ) suggest that the change in mean state toward a stronger zonal SST gradient may be responsible for the more frequent occurrence of CP relative to EP El Niño. However, both EP and CP El Niño are expected to generate enhanced convection near the Date Line (e.g., Ashok et al, ; Santoso et al, ; Xiang et al, ), and so the modulating influence from the zonal SST gradient pattern (Figure ) identified here is distinct from the influences described in those studies. This suggests that the zonal SST gradient pattern deserves attention separate from any of its potential influences on EP or CP ENSO.…”

The End of an El Niño: A View from Palau

“…For these reasons, we believe the intraseasonal SSHAs to be Rossby waves. It is worth noting that while 1997/98 and 2015/16 were considered to be eastern Pacific El Niños by their large SSTs in the Niño 3.4 region, the 2002/03 event was a central Pacific El Niño with a different spatial pattern (Santoso et al, 2019). Furthermore, the El Niño event in 2009/10 had a higher value on the Niño 3.4 index, a similar wind stress curl anomaly in January, and shoaling near Palau compared to the El Niño in 2002/03, but it did not have strong, intraseasonal variability as assessed by < (η 120H ) 2 > .…”

“…Each ENSO event is unique, and there is debate about the initialization, termination, and impacts of different events (Clarke, 2014;Santoso, 2019). Therefore, all observations of El Niño and associated events improve our understanding of both their predictability and impact.…”