Tropical cyclones can unleash extensive devastation, as recent storms that swept over Jamaica and the Philippines made unmistakably clear. Accurate weather forecasts that buy more time to prepare are crucial for saving lives and are rooted in a deeper understanding of climate systems. Driving this forward, researchers at the Institute of Science and Technology Austria (ISTA) and others have successfully identified a previously unknown cyclic climate pattern by historical reanalysis of datasets and satellite observations. The findings are published in PNAS.
Jiawei Bao still remembers coming home from middle school to catch the weather forecast on TV. It spanned from China’s northernmost province, Heilongjiang, to the southernmost province and tropical island, Hainan. In winter, the temperature between these regions can range from cold to balmy, varying by a staggering 50 degrees Celsius. “I was always fascinated by how such variations were predicted,” Bao recalls. This early curiosity led him to pursue a career in climate science.
Fast forward to 2025. Bao is now a postdoctoral researcher in Caroline Muller’s group at ISTA, where he uses physics and mathematics to address fundamental questions in climate science. He aims to understand climate processes and their societal consequences.
Bao has now identified a new type of oscillation in the tropics. The tropics-wide intraseasonal oscillation, or simply TWISO, is a repeating pattern that manifests itself over several weeks in tropical regions with rainfall, clouds, and wind. Together with Muller, Sandrine Bony from the Centre National de la Recherche Scientifique (CNRS) at France’s Sorbonne University, and Daisuke Takasuka from Japan’s Tohoku University, he published the findings in the journal PNAS.
The ABCs of atmospheric circulation
Changes in the large-scale atmospheric circulations form an essential component of this newly identified oscillation. While atmospheric circulations and oscillations are complex, they influence our daily lives in the form of wind, weather fluctuations, and seasonal changes. In extreme cases, their impact becomes apparent in tropical storms, such as hurricanes, typhoons, and cyclones. These storms can have devastating effects, as shown by several recent examples—Hurricane Melissa in the Caribbean and typhoon Kalmaegi, which caused severe damage and fatalities in the Philippines before hitting Vietnam.
“Atmospheric circulation refers to the large-scale movement of air that redistributes energy, momentum, and mass from one specific location to another,” explains Bao. In tropical regions, he notes, the Hadley circulation is the primary north-south circulation pattern. It features rising air at the equator and sinking air in the subtropics. By contrast, the Walker circulation is the dominant west-east circulation pattern in the equatorial Pacific, with rising air over the western Pacific and the Maritime Continent (Indonesia, Malaysia, and Pacific islands), and sinking air over the eastern Pacific.
Oscillation as a giant pendulum
Besides atmospheric circulations, climate oscillations also play a crucial role in the climate of tropical regions. Oscillations are fluctuations of climate systems, ranging in duration from a couple of weeks to even millennia. “An oscillation is like a giant pendulum that swings back and forth. When it swings one way, it might bring warmer and wetter conditions. When it swings the other way, it could bring cooler and drier weather,” Bao explains.
Oscillations often trigger extreme weather conditions. A prime example is the El Niño-Southern Oscillation, which oscillates over timescales of two to seven years and causes extreme weather in various parts of the globe during its different phases.
Bao and his colleagues have now detected a new oscillation system called “tropics-wide intraseasonal oscillation.” Dubbed TWISO, it is an oscillation that has always been present but that went unrecognized—until now.
TWISO: Long present, newly identified
The scientists identified TWISO using satellite observations and the reanalysis of datasets, developed and maintained by leading research institutions and shared openly with the global scientific community.
For example, Bao used the ERA5 dataset, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). It represents the fifth generation of global climate and weather reanalysis dating back to 1940. Additionally, data satellite observation data from NASA’s Clouds and the Earth’s Radiant Energy System (CERES) contributed to the findings.
“TWISO is a natural phenomenon that has always been present but it was only recently identified in our paper through analysis of historical observations and reanalysis data,” Bao says. “The defining feature of TWISO is its tropics-wide coherence. It represents a large-scale oscillation encompassing the entire tropical belt, with variations occurring on intraseasonal timescales of about 30 to 60 days.”
The tropical atmosphere’s pulse
During each oscillation cycle, different components of the tropical climate system, including atmospheric temperature, ocean surface temperature, winds, and radiation, vary in a synchronized manner. In that sense, TWISO can be viewed as the “pulse” of the tropical atmosphere.
One of TWISO’s main elements is the variation of convection—the process by which heat is transferred through a fluid—over the “warm pool.” The warm pool is a region covering the western Pacific and the Maritime Continent where sea surface temperatures are the highest on Earth. It is a hotspot of intense and persistent thunderstorms that tightly link the ocean and atmosphere.
“We found that convection in this region goes through strong cycles of intensification and weakening, which play a central role in setting the rhythm for the entire tropical climate system to oscillate together,” says Bao.
Basis for better weather forecasts?
Bao and his colleagues note that the effects of TWISO on regional weather are still uncertain. Like other oscillations, TWISO represents a deviation of the normal state, which can often lead to extreme weather events. Bao highlights that during a specific phase of TWISO, sea surface temperatures increase, raising the likelihood of cyclone formation.
Given the annual threats posed by tropical storms, accurate weather forecasting is crucial for saving lives and livelihoods, planning evacuations, and preparing disaster responses. Nonetheless, predicting tropical weather one to two months in advance remains a major challenge. Since TWISO follows consistent patterns over 30 to 60 days, it presents an opportunity to enhance predictability within this timescale.
“By understanding TWISO, we could improve our ability to predict when tropical cyclones are likely to form, allowing us to issue earlier warnings and help to minimize the risks and damage they cause. We plan to address this in future research,” Bao says.
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