A recent study published in Nature Geoscience provides groundbreaking insights into long-term changes in tropical weather patterns that are leading to an increased frequency of extreme weather events such as heat waves and heavy rainfall in the Indo-Pacific. These changes are possibly driven by global warming, among other factors.

The paper, titled "Indo-Pacific regional extremes aggravated by changes in tropical weather patterns," employs a recently proposed methodology that characterizes occurrence trends of weather patterns using atmospheric analogs, which are linked to the concept of recurrences in dynamical systems theory.

Unlike previous approaches, which have often focused on shifts in average behavior, the method used in the study can identify occurrence trends for each daily weather pattern, thereby enabling a direct study of their association with extreme events—something that was previously unachievable.

Thanks to this methodology, it was possible to identify the emergence of new large-scale atmospheric patterns, which are exacerbating regional weather extremes.

The study, led by doctoral student Chenyu Dong and Assistant Professor Gianmarco Mengaldo from the College of Design and Engineering (CDE) at the National University of Singapore (NUS), and a collaborative team of international scientists, uses advanced reanalysis datasets to analyze the tropical Indo-Pacific region's evolving weather systems.

The researchers found that since the 1990s, previously rare weather patterns have become more common, while some others that were once prominent have nearly disappeared. These changes are linked to shifts in the Pacific Walker Circulation, a key driver of tropical weather and climate, whose future changes remain highly uncertain in current climate models.

Detecting long-term trends in the tropical Indo-Pacific has consistently been a challenge, especially on a daily time scale, due to the confluence of several modes of variability that tends to overshadow trend signals. This study is one of the first to investigate long-term changes in tropical weather patterns and their relationship with extreme events on a daily time scale.

"Critical changes in tropical weather patterns are significantly aggravating regional extremes, namely heat waves and extreme precipitation, in the tropical Indo-Pacific region. Our study is one of the first to disentangle trend vs. variability in the tropics, an aspect that has been historically challenging.

"We show that the changes identified cannot be fully explained by interannual modes of variability, and a possible culprit is anthropogenic global warming, though the influence of other factors may play a role.

"Further in-depth analyses are required to better inform climate modeling and climate adaptation strategies, especially in the tropical Indo-Pacific, where climate models still struggle to provide reliable projections.

"For Singapore, and other countries in Southeast Asia, improving climate projection capabilities and better understanding how tropical dynamics and regional extremes are evolving is of vital importance. This study is one step towards this direction," said Asst Prof Mengaldo from the Department of Mechanical Engineering at CDE, NUS.

The study shows that new large-scale atmospheric configurations (or weather patterns) that were rare before the 1990s have emerged, while some others that were prominent have disappeared. These emerging weather patterns manifest as a stronger Pacific Walker circulation (or Walker cell) and are associated with wetter and warmer conditions in Southeast Asia and drier conditions in the equatorial Pacific.

The emerging patterns cannot be explained by interannual modes of natural variability, such as the El Niño Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO), but they are instead likely driven by long-term trends from the 1940s to the present.

These trends and shifts of large-scale atmospheric dynamics in the tropical Indo-Pacific may be caused by global warming and other factors. Although these identified emerging patterns may be driven by the combined effect of different factors (excluding known modes of inter-annual variability), the implications for current and near-future climate are critical.

The emerging weather patterns are also strongly linked to increased regional weather extremes, namely heat waves and extreme rainfall. In certain regions, these extremes are up to four times more frequent than climatology when associated with emerging weather patterns.

For example, several regions, including parts of Indonesia, Singapore, South India, the Philippines and the western Pacific, exhibit markedly increased frequency of heat waves compared with climatology. The South China Sea and its surrounding areas, including Vietnam and the Philippines, the Malay Peninsula, Singapore, the tip of South India and a portion of the Indian Ocean off the coast of Australia, exhibit considerably increased frequency of extreme rainfall.

This increase in extreme weather is noteworthy, given that such changes are associated with long-term climate trends in a region that is highly vulnerable to weather extremes.

These findings are significant in the context of climate change as they reveal that new and emerging weather patterns are contributing to increasingly severe weather in a region home to more than a billion people, as well as unique and vulnerable ecosystems.

The increased occurrences of heat waves and extreme rainfall can lead to acute heat distress and flooding, respectively. With extreme weather events posing severe socio-economic and environmental challenges, understanding these changes is critical for improving climate models and informing future climate adaptation strategies.

This study was conducted by an international team of climate scientists from leading institutions, including NUS, Institut Pierre-Simon Laplace (IPSL), Uppsala University, Stockholm University, University of Cambridge, Columbia University, World Meteorological Organization (WMO), and the Center for Climate Research Singapore (CCRS). The team is committed to advancing climate research to better understand the impacts of a changing climate on regional weather patterns and extremes.

"The emergence of new tropical weather patterns is a key signal of how anthropogenic climate change is altering atmospheric dynamics on a daily scale. Our findings show a significant increase in heat waves and extreme precipitation in the Indo-Pacific, which may have profound consequences not only for the region but for global climate as well.

"This shift in weather patterns challenges our previous understanding of tropical variability and highlights the urgency to improve climate projections and preparedness for extreme events in vulnerable regions," said Dr. Davide Faranda, Research Director at the Laboratoire de Science du Climat et de l'Environnement (LSCE) of Institut Pierre-Simon Laplace (IPSL), French National Center for Scientific Research (CNRS).

"Heat waves and extreme rainfall are two weather extremes that require careful and advance planning from policymakers to mitigate their effects. For instance, more frequent heat waves may lead to high peaks in electricity demand with possible power outages, many heat-related illnesses that would need enough hospital beds, and crop failure that could threaten food security.

"More frequent extreme rainfall may lead to floods, which in turn are a direct threat to human life, buildings and infrastructure. Extreme rainfall may also lead to crop failure, contamination of drinkable water, and landslides.

"Southeast Asia is a relatively research-scarce region in terms of extreme weather, and further efforts are required to better prepare policymakers and local communities for a changing climate," said Asst Prof Mengaldo.