Stabilised frequency of extreme positive Indian Ocean Dipole under 1.5 °C warming

Decreased Indian Ocean Dipole variability under prolonged greenhouse warming

The Indian Ocean Dipole (IOD) is a major climate variability mode that substantially influences weather extremes and climate patterns worldwide. However, the response of IOD variability to anthropogenic global warming remains highly uncertain. The latest IPCC Sixth Assessment Report concluded that human influences on IOD variability are not robustly detected in observations and twenty-first century climate-model projections. Here, using millennial-length climate simulations, we disentangle forced response and internal variability in IOD change and show that greenhouse warming robustly suppresses IOD variability. On a century time scale, internal variability overwhelms the forced change in IOD, leading to a widespread response in IOD variability. This masking effect is mainly caused by a remote influence of the El Niño-Southern Oscillation. However, on a millennial time scale, nearly all climate models show a long-term weakening trend in IOD variability by greenhouse warming. Our results provide compelling evidence for a human influence on the IOD.

Sea level extremes and compounding marine heatwaves in coastal Indonesia

Low-lying island nations like Indonesia are vulnerable to sea level Height EXtremes (HEXs). When compounded by marine heatwaves, HEXs have larger ecological and societal impact. Here we combine observations with model simulations, to investigate the HEXs and Compound Height-Heat Extremes (CHHEXs) along the Indian Ocean coast of Indonesia in recent decades. We find that anthropogenic sea level rise combined with decadal climate variability causes increased occurrence of HEXs during 2010-2017. Both HEXs and CHHEXs are driven by equatorial westerly and longshore northwesterly wind anomalies. For most HEXs, which occur during December-March, downwelling favorable northwest monsoon winds are enhanced but enhanced vertical mixing limits surface warming. For most CHHEXs, wind anomalies associated with a negative Indian Ocean Dipole (IOD) and co-occurring La Niña weaken the southeasterlies and cooling from coastal upwelling during May-June and November-December. Our findings emphasize the important interplay between anthropogenic warming and climate variability in affecting regional extremes.

Identifying Metocean Drivers of Turbidity Using 18 Years of MODIS Satellite Data: Implications for Marine Ecosystems under Climate Change

Turbidity impacts the growth and productivity of marine benthic habitats due to light limitation. Daily/monthly synoptic and tidal influences often drive turbidity fluctuations, however, our understanding of what drives turbidity across seasonal/interannual timescales is often limited, thus impeding our ability to forecast climate change impacts to ecologically significant habitats. Here, we analysed long term (18-year) MODIS-aqua data to derive turbidity and the associated meteorological and oceanographic (metocean) processes in an arid tropical embayment (Exmouth Gulf in Western Australia) within the eastern Indian Ocean. We found turbidity was associated with El Niño Southern Oscillation (ENSO) cycles as well as Indian Ocean Dipole (IOD) events. Winds from the adjacent terrestrial region were also associated with turbidity and an upward trend in turbidity was evident in the body of the gulf over the 18 years. Our results identify hydrological processes that could be affected by global climate cycles undergoing change and reveal opportunities for managers to reduce impacts to ecologically important ecosystems.

The Indian Ocean Dipole and El Niño Southern Oscillation as major drivers of coral cover on shallow reefs in the Andaman Sea

Shallow reefs are a major feature of coral assemblages in the Andaman Sea. At Phuket, Thailand sheltered reefs are dominated by massive corals, together with an increasing abundance of branching species during favourable growth conditions. The growth of coral on these reefs is moderated by long-term increases in sea temperature and relative sea level but fluctuating decadal/intradecadal climate processes of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD), which modulate sea level and temperature, are the main drivers of coral cover. In this study, the contribution of these two climate processes was identified and also quantified. Over a 34-year study of fluctuating coral cover, the three major reductions in cover in 1997, 2010 and 2019 were linked to overlapping positive IOD (pIOD) and El Niños in 1997 and 2019, and with an El Niño alone in 2010. Combined pIOD and El Niño depressed sea level was the major factor in reducing cover in 1997 while El Niño extreme sea temperatures were responsible for large reductions in 2010. In 2019, a bi-phasic pIOD and El Niño resulted in lowered cover at a time of both decreased sea level and high sea temperature. Under global warming scenarios, it is projected that extreme pIODs, such as those seen in 1997 and 2019, will occur more frequently while El Niño frequencies will continue to increase even after global mean temperature stabilization. In these circumstances, and with steadily rising background sea temperatures, the future risks to the shallow reefs of the Andaman Sea are substantial, despite any temporary respite gained from climate related or land subsidence sea-level rise. Such findings have wider implications for all reefs affected by climatic-driven sea-level depressions, particularly those around Indonesian shores where similar El-Niño-related reductions in coral cover have been reported.

Coupling of Indo-Pacific climate variability over the last millennium

The Indian Ocean Dipole (IOD) affects climate and rainfall across the world, and most severely in nations surrounding the Indian Ocean^1-4. The frequency and intensity of positive IOD events increased during the twentieth century⁵ and may continue to intensify in a warming world⁶. However, confidence in predictions of future IOD change is limited by known biases in IOD models⁷ and the lack of information on natural IOD variability before anthropogenic climate change. Here we use precisely dated and highly resolved coral records from the eastern equatorial Indian Ocean, where the signature of IOD variability is strong and unambiguous, to produce a semi-continuous reconstruction of IOD variability that covers five centuries of the last millennium. Our reconstruction demonstrates that extreme positive IOD events were rare before 1960. However, the most extreme event on record (1997) is not unprecedented, because at least one event that was approximately 27 to 42 per cent larger occurred naturally during the seventeenth century. We further show that a persistent, tight coupling existed between the variability of the IOD and the El Niño/Southern Oscillation during the last millennium. Indo-Pacific coupling was characterized by weak interannual variability before approximately 1590, which probably altered teleconnection patterns, and by anomalously strong variability during the seventeenth century, which was associated with societal upheaval in tropical Asia. A tendency towards clustering of positive IOD events is evident in our reconstruction, which-together with the identification of extreme IOD variability and persistent tropical Indo-Pacific climate coupling-may have implications for improving seasonal and decadal predictions and managing the climate risks of future IOD variability.

Global heat stress on health, wildfires, and agricultural crops under different levels of climate warming

The effects of heat stress are spatially heterogeneous owing to local variations in climate response, population density, and social conditions. Using global climate and impact models from the Inter-Sectoral Impact Model Intercomparison Project, our analysis shows that the frequency and intensity of heat events increase, especially in tropical regions (geographic perspective) and developing countries (national perspective), even with global warming held to the 1.5 °C target. An additional 0.5 °C increase to the 2 °C warming target leads to >15% of global land area becoming exposed to levels of heat stress that affect human health; almost all countries in Europe will be subject to increased fire danger, with the duration of the fire season lasting 3.3 days longer; 106 countries are projected to experience an increase in the wheat production-damage index. Globally, about 38%, 50%, 46%, 36%, and 48% of the increases in exposure to health threats, wildfire, crop heat stress for soybeans, wheat, and maize could be avoided by constraining global warming to 1.5 °C rather than 2 °C. With high emissions, these impacts will continue to intensify over time, extending to almost all countries by the end of the 21st century: >95% of countries will face exposure to health-related heat stress, with India and Brazil ranked highest for integrated heat-stress exposure. The magnitude of the changes in fire season length and wildfire frequency are projected to increase substantially over 74% global land, with particularly strong effects in the United States, Canada, Brazil, China, Australia, and Russia. Our study should help facilitate climate policies that account for international variations in the heat-related threats posed by climate change.

Future climate change likely to reduce the Australian plague locust (Chortoicetes terminifera) seasonal outbreaks

Climate is a major limiting factor for insect distributions and it is expected that a changing climate will likely alter spatial patterns of pest outbreaks. The Australian plague locust (APL) Chortoicetes terminifera, is the most economically important locust species in Australia. Invasions cause large scale economic damage to agricultural crops and pastures. Understanding the regional-scale and long-term dynamics is a prerequisite to develop effective control and preventive management strategies. In this study, we used a 32-year locust survey database to uncover the relationship between historical bioclimatic variables and spatial seasonal outbreaks by developing two machine learning species distribution models (SDMs), random forest and boosted regression trees. The explanatory variables were ranked by contribution to the generated models. The bio-climate models were then projected into a future climate change scenario (RCP8.5) using downscaled 34 global climate models (GCMs) to assess how climate change may alter APL seasonal distribution patterns in eastern Australia. Our results show that the model for the distribution of spring outbreaks performed better than those for summer and autumn, based on statistical evaluation criteria. The spatial models of seasonal outbreaks indicate that the areas subject to APL outbreaks were likely to decrease in all seasons. Multi-GCM ensemble means show the largest decrease in area was for spring outbreaks, reduced by 93-94% by 2071-2090, while the area of summer outbreaks decreased by 78-90%, and 67-74% for autumn outbreaks. The bioclimatic variables could explain 78-98% outbreak areas change. This study represents an important step toward the assessment of the effects of the changing climate on locust outbreaks and can help inform future priorities for regional mitigation efforts in the context of global climate change in eastern Australia.

Increased drought events in southwest China revealed by tree ring oxygen isotopes and potential role of Indian Ocean Dipole

The highlands in southwestern China experience pronounced fluctuations in the hydroclimate with profound impacts on agriculture and economics. To investigate the drought history of this region beyond instrumental records, a tree ring cellulose oxygen isotope (δ¹⁸O_c) chronology was developed for the period 1733-2013 using samples collected from six Larix trees in the low-latitude highlands (LLH) of southwestern China. The analysis revealed that δ¹⁸O_c is significantly correlated with the rainy season (May-October) precipitation and relative humidity, as well as drought severity. The δ¹⁸O_c chronology accounts for 46% of the observed variance in the rainy season precipitation and it was subsequently used to reconstruct precipitation. The reconstructed precipitation reveals an apparent drying trend since 1840, accompanied by increasingly frequent drought events since 1970. Interdecadal variability is also present, characterized with two distinct wet periods in 1740-1760 and 1800-1900 and two drier periods in 1760-1800 and 1900-2013. On the interannual timescale, the LLH precipitation was modulated collectively by the El Niño-Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD). There appears to be an enhanced precipitation-IOD relationship since 1970 in response to the increase in positive-IOD events, implying an increasing likelihood of drought for the southwest China LLH.