How a climate conundrum at the heart of the Pacific is challenging disaster preparedness in Australia
- Briefing Note 499
An ongoing enigma at the heart of the Pacific is confounding predictability of climate extremes at seasonal and longer timescales. Australian emergency management and other exposed sectors of the economy rely on seasonal forecasts for efficient allocation of limited resources. It is safe to say the summer of 2023-2024 did not pan out as expected, and this has undermined confidence in climate predictions.
Flash drought conditions across eastern Australia in late winter and early spring of 2023 increased societal angst about the coming summer, especially given the last time similar climate conditions aligned was during the Tinderbox Drought of 2017-2019 culminating in the disastrous Black Summer bushfire season of 2019-2020. Early onset of bushfire activity had emergency services on edge, preparing for a potentially catastrophic bushfire season (Rabehaja and Browning 2023); instead, most of Australia experienced a summer of above average rainfall and for some parts of Queensland and the Northern Territory, the worst flooding in memory.
The Australian Bureau of Meteorology have been unjustly criticised for their September declaration of El Niño that spooked many farmers into reducing heard numbers leading to a crash in livestock prices (Fowler 2024). According to the National Oceanic and Atmospheric Administration (NOAA), El Niño had already been underway since June when the Niño 3.4 sea surface temperature (SST) anomaly index exceeded their >0.5C threshold. Based on Niño 3.4 SST, 2023 ended up as the second strongest El Niño in the past 20 years. However, it was short lived. By mid-summer the eastern Pacific had already begun to cool, with most models forecasting a return to La Niña later in 2024. This rapid transition back to La Niña caught many by surprise, especially after the recent prolonged triple-dip La Niña spanning 2020 to 2022.
At seasonal and longer time scales relevant to emergency planning and preparedness, climate predictability comes mostly from ocean temperatures. The El Niño Southern Oscillation (ENSO) is the leading mode of global climate variability affecting most regions through complex teleconnection patterns. Even the extraordinary high global temperatures experienced over the past 10 months are partially due to El Niño related atmospheric feedbacks (Schmidt 2024; Timmerman 2024). Researchers have been puzzled to observe that SST near the epicentre of ENSO in the eastern Pacific show a long-term cooling trend over the past 30-years, this trend contrasts with almost all other ocean areas and is not simulated by most climate models (Cuff 2023). Unaccounted SST trends in the Niño region are having confounding implications for global climate predictability (e.g. Henson 2023).
Long term cooling trend
Climate models have long projected a shift towards a more El Niño like mean state, however observations indicate the opposite is happening (Figure 1; Wills et al. 2022). In 1997 Cane et al. reported that SST in the eastern Pacific had been cooling while most of the world’s oceans warmed. Recent studies confirm a continuation of the La Niña like trend (e.g. Seager et al. 2019; Dong et al. 2023), with increasing debate about the cause—especially as to whether it is cyclic as part of natural Pacific decadal variability, or is a trend being driven by global warming. This question is difficult to answer as high-quality observations are not long enough to study historical variability at multidecadal timescales, and there is generally poor agreement with existing models. Either way, this discrepancy with models needs to be reconciled if predictability is to be improved.
Several candidate mechanisms include tropical pacific winds, clouds, convection, and ocean upwelling. All these processes are not fully resolved in most global climate models (e.g. Lee et al. 2022). Presumably, improvements in model resolution and skill, particularly around representing clouds and convection, will help to reduce discrepancies.
One intriguing mechanism proposed by Dong et al. (2022) is that the eastern Pacific cooling trend is being driven by meltwater from Antarctic glaciers. As glacier melt accelerates under a warmer climate, cool fresh water is released into the Southern Ocean. The waters around Antarctica, south of the Pacific, also show a cooling trend that is not simulated by climate models (Figure 1; Wills et al. 2022).When Antarctic meltwater is included in calculations, climate models start to show more realistic cooling in the tropical Pacific (Dong et al. 2022). Models with meltwater also simulate increased low-level cloud which reduces the overall rate of near-term global warming compared with existing models that do not simulate eastern Pacific cooling. The corollary: near-future warming projections by current global climate models may be overestimated.
Compared to Greenland and the Arctic, Antarctica has been slower to respond to increasing global temperatures. However, the continent is warming and some large-scale changes, such as collapse of the West Antarctic ice sheet, are now considered inevitable (e.g. Naughten et al. 2023). The extraordinary 2023 decline in Antarctic sea-ice extent (Browning 2023) illustrates just how quickly climatic changes can happen; changes that will likely impact the global climate systems in ways that are difficult to anticipate.
Implications for climate predictability
SST in the eastern tropical Pacific have a dominant role in causing year-to-year variability of the climate system via teleconnections to many parts of the globe including Australia. If the observed cooling is part of natural Pacific decadal variability, and therefore cyclic, it means that existing climate projections could be correct in predicting a more El Niño like future climate state. For Australia this would mean more frequent and severe droughts, bushfires, and heat extremes. Alternatively, if the observed cooling is a trend driven by Antarctic meltwater (and/or other global warming related factors) then this will need to be incorporated into climate models such that they project a more La Niña like future. For Australia this would mean wetter conditions, with less drought but increased risk of flood and storm; it could also mean a slower rate of global temperature increase and relatively lower risk from heat extremes.
Many parts of Australian society, especially emergency services, rely on seasonal predictions for preparedness and efficient allocation of limited resources. There is also increasing consideration of multidecadal climate projections in resilience planning and decision making across the private sector and all levels of government. Most available projections are derived from models that simulate a warming trend in the eastern Pacific, which may be erroneous.
Open discussion around knowledge gaps can initially diminish public confidence in climate science, but it also motivates research efforts. The eastern Pacific SST enigma is now considered one of the most important questions in climate science (Lee et al., 2023); resolving the mechanism will be crucial to improving predictability at seasonal to multidecadal timescales. Consequently, efforts are being directed towards developing more accurate climate models that are better at simulating cloud cover, ocean currents, winds and melting Antarctic glaciers.
References
Browning, S., (2023). Wind Anomalies Drive 2023 Antarctic Sea Ice Deficit. Risk Frontiers Briefing Note 487. https://riskfrontiers.com/insights/wind-anomalies-drive-2023-antarctic-sea-ice-deficit/
Cane, Mark A. et al. (1997). Twentieth-Century Sea Surface Temperature Trends.Science275,957-960(1997). DOI:10.1126/science.275.5302.957
Cuff, Madeleine (2023). Something strange is happening in the Pacific and we must find out why. https://www.newscientist.com/article/mg25934500-100-something-strange-is-happening-in-the-pacific-and-we-must-find-out-why/
Dong, Y., Pauling, A. G., Sadai, S., & Armour, K. C. (2022). Antarctic ice-sheet meltwater reduces transient warming and climate sensitivity through the sea-surface temperature pattern effect. Geophysical Research Letters, 49, e2022GL101249. https://doi.org/10.1029/2022GL101249
Dong, Y., Polvani, L. M., & Bonan, D. B. (2023). Recent multi-decadal Southern Ocean surface cooling unlikely caused by Southern Annular Mode trends. Geophysical Research Letters, 50, e2023GL106142. https://doi. org/10.1029/2023GL106142
Fowler, E., (2024). Bureau of Meteorology’s botched weather call crushes Elders’ earnings. https://www.afr.com/companies/agriculture/elders-shares-plunge-24pc-on-earnings-downgrade-20240408-p5fi5k
Henson, B. (2023). A mystery in the Pacific is complicating climate projections: The El Niño-Southern Oscillation, which has a huge influence on global weather patterns, isn’t behaving as computer models predicted. That’s puzzling scientists. https://yaleclimateconnections.org/2023/04/a-mystery-in-the-pacific-is-complicating-climate-projections/
Henson, B., (2023) A mystery in the Pacific is complicating climate projections. https://yaleclimateconnections.org/2023/04/a-mystery-in-the-pacific-is-complicating-climate-projections/
Lee, S., Karnauskas, K., Heede, U., and L’heureux, M. (2023) How the pattern of trends across the tropical Pacific Ocean is critical for understanding the future climate. https://www.climate.gov/news-features/blogs/enso/how-pattern-trends-across-tropical-pacific-ocean-critical-understanding
Lee, S., L’Heureux, M., Wittenberg, A.T. et al. (2022) On the future zonal contrasts of equatorial Pacific climate: Perspectives from Observations, Simulations, and Theories. npj Clim Atmos Sci 5, 82. https://doi.org/10.1038/s41612-022-00301-2
Naughten, K.A., Holland, P.R. & De Rydt, J. (2023) Unavoidable future increase in West Antarctic ice-shelf melting over the twenty-first century. Nat. Clim. Chang. 13, 1222–1228. https://doi.org/10.1038/s41558-023-01818-x
Rabehaja T., and Browning, S., (2023). An Eventful Start to the Fire Season on the East Coast. Risk Frontiers Briefing Note 492. https://riskfrontiers.com/insights/an-eventful-start-to-the-fire-season-on-the-east-coast/
Schmidt, G., (2024) Climate models can’t explain 2023’s huge heat anomaly — we could be in uncharted territory. Nature 627, 467. doi: https://doi.org/10.1038/d41586-024-00816-z
Seager, R., Cane, M., Henderson, N. et al. Strengthening tropical Pacific zonal sea surface temperature gradient consistent with rising greenhouse gases. Nat. Clim. Chang. 9, 517–522 (2019). https://doi.org/10.1038/s41558-019-0505-x
Timmerman, A., (2024) 2023 – a year to be remembered. https://ibsclimate.org/news/2023-a-year-to-be-remembered/
Wills, R. C. J., Dong, Y., Proistosecu, C., Armour, K. C., & Battisti, D. S. (2022). Systematic climate model biases in the large-scale patterns of recent sea-surface temperature and sea-level pressure change. Geophysical Research Letters, 49, e2022GL100011. https://doi. org/10.1029/2022GL100011
