TWISO, a Newly Recognized Climate Pattern from the Tropics

Amid the rising temperatures of tropical oceans, scientists have unveiled a new phenomenon known as the TWISO climate pattern, a giant-scale oscillation that affects the entire tropical belt. This finding is not only academically relevant but could also be key to predicting extreme weather and managing disasters in tropical countries such as Indonesia.

The tropical region is the most volatile climate zone, serving as the site for the emergence of El Niño and La Niña phenomena, as well as tropical cyclones whose impacts can extend to subtropical areas.

Tropical cyclones can cause widespread damage, as clearly demonstrated by Cyclone Kalmaegi, which claimed hundreds of lives in the Philippines in the middle of this month.

Accurate weather forecasts that provide more time for preparation are crucial for saving lives and are rooted in a deeper understanding of climate systems.

To further this goal, researchers at the Institute of Science and Technology Austria (ISTA) and other institutions have identified previously unknown cyclical climate patterns in the tropics through historical reanalysis of data sets and satellite observations. The findings were published in the journal PNAS on Monday (November 24, 2025).

Jiawei Bao, a postdoctoral researcher at ISTA, is the first author of this report. This research was conducted in collaboration with Caroline Muller, also from ISTA, Sandrine Bony from the National Center for Scientific Research (CNRS) at Sorbonne University, France, and Daisuke Takasuka from Tohoku University, Japan.

So far, researchers have identified several patterns or atmospheric circulations along the equator that occur over the course of days to months. For example, the equatorial Rossby waves, Kelvin waves, and Madden-Julian Oscillation (MJO).

Rossby waves move from east to west and vice versa. Meanwhile, Kelvin waves move from west to east. The MJO is a wave cycle that moves eastward across the Earth every 30-40 days.

The activity of these three waves can create conditions that support the widespread growth of rain clouds, thus often associated with extreme rainfall.

In addition, in tropical regions, there is a recorded pattern of large-scale atmospheric circulation, namely the Hadley circulation, which is the main north-south climate circulation pattern. This pattern features rising air at the equator and descending air in the subtropics.

There is also the Walker circulation, which is a dominant west-east circulation pattern in the equatorial Pacific, with rising air over the western Pacific and Maritime Continent (Indonesia, Malaysia, and the Pacific islands), and descending air over the eastern Pacific.

Large-scale atmospheric circulation changes form an important component of the intraseasonal oscillation across the tropical region, or briefly TWISO (Tropics-Wide Intraseasonal Oscillation), which was recently identified by Bao and his team. This phenomenon is described as a recurring climate pattern that occurs over several weeks in the tropical region.

This wave also affects weather fluctuations and seasonal changes. "Atmospheric circulation refers to the large-scale movement of air that redistributes energy, momentum, and mass from one specific location to another," explained Bao.

In addition to atmospheric circulation, climate oscillations also play a significant role in the climate of tropical regions. Oscillation refers to fluctuations in the climate system, with durations ranging from several weeks to thousands of years.

"The oscillation is like a giant pendulum swinging back and forth. When it swings in one direction, it can bring warmer and wetter conditions. When it swings in the other direction, it can bring cooler and drier weather," explained Bao.

So far, Oscillation often triggers extreme weather conditions. A prime example is the El Niño-Southern Oscillation (ENSO), which oscillates over a period of two to seven years and causes extreme weather in various parts of the world during different phases.

In addition to ENSO, there is the TWISO oscillation system in the tropical area. "We have identified a significant TWISO in observational data and satellite reanalysis. This oscillation has a period of 30-60 days and manifests itself in various variables involving convection, radiation, surface flux, sea surface temperature (SST), and large-scale circulation," wrote Bao and the team.

Scientists identified TWISO using satellite observations and reanalysis of datasets, which were developed and managed by leading research institutions and shared openly with the global scientific community.

"TWISO is a natural phenomenon that has always existed, but was recently identified in our paper through historical observation analysis and reanalysis of data," said Bao.

"The characteristic of TWISO is its coherence that encompasses the entire tropical region. This phenomenon represents a large-scale oscillation that covers the entire tropical belt, with variations occurring on an intraseasonal time scale of about 30 to 60 days," he stated.

During each oscillation cycle, various components of the tropical climate system, including atmospheric temperature, sea surface temperature, winds, and radiation, fluctuate in a synchronized manner. In this context, TWISO can be viewed as the "pulse" of the tropical atmosphere.

One of the main elements of TWISO is convection variation, which is the process of heat transfer through fluids, over the "warm pool." The warm pool is an area that encompasses the western Pacific Ocean and the Maritime Continent with the highest sea surface temperatures on Earth.

This region is a hotspot for intense and continuous thunderstorms that closely connect the ocean and the atmosphere. "We found that convection in this area experiences strong cycles of intensification and weakening, which plays a central role in regulating the rhythm for the entire tropical climate system to oscillate together," said Bao.

The findings of Bao and the team also indicate a connection between TWISO and MJO. One of the hypotheses regarding this connection is that TWISO is driven by the nonlinear interaction between MJO and large-scale atmospheric disturbance circulation.

The issue is that a research team from Pusan National University, South Korea, in a study published in Communications Earth & Environment in August 2025, found that the way the MJO propagates changes as sea surface temperatures (SST) in the tropical oceans warm unevenly.

According to this study, the MJO moved faster in the Indian Ocean and Maritime Islands during the period of 2003–2022 compared to the previous decade. In contrast, in the western Pacific, the propagation of the MJO slowed during the same period.

The main factors driving these changes are stronger moisture gradients, “pre-moistening” ahead of the convection center, and increased atmospheric stability in the upper troposphere. In the western Pacific, horizontal moisture weakens, vertical air motion decreases, and atmospheric stability is lower, which slows the MJO.

Kyung-Ja Ha, the lead researcher from Pusan National University, stated that understanding how ocean warming affects the MJO is crucial for improving the accuracy of medium-term ("sub-seasonal") climate models.

"By improving the way climate models capture the influence of ocean warming on MJO behavior, rainfall forecasts and drought risk could become significantly more accurate," he stated.

The discovery of changes in the movement of the MJO could have consequences for extreme weather. Since TWISO is a major intraseasonal oscillation involving large-scale circulation, it is highly likely that these ocean climate changes "reset" the pulse of the broader tropical climate.

By improving the way climate models capture the influence of ocean warming on MJO behavior, rainfall forecasts and drought risk could become significantly more accurate.

The combination of MJO and TWISO dynamics during global warming could be a new key for intraseasonal extreme weather forecasting.

Bao and his colleagues noted that the impact of TWISO on regional weather is uncertain due to limited data. However, like other oscillations, TWISO is a deviation from normal conditions, which can often lead to extreme weather events.

At a certain phase of TWISO, sea surface temperatures rise and increase the likelihood of cyclone formation. "The intraseasonal oscillation of large-scale circulation identified here challenges the conventional view of the Hadley cell as a stable system over shorter time scales. This highlights the dynamic interaction between moist convection and large-scale circulation," wrote Bao.

TWISO is expected to have significant implications for tropical weather and climate. First, oscillations in large-scale circulation could drive coordinated responses across all tropical climate systems. As a result, many processes become variable, which enhances intraseasonal predictability.

Secondly, strong oscillations can drive the system away from its average state, leading to significant deviations from typical conditions and increasing the likelihood of extreme events.

In addition, the intraseasonal variability in average tropical atmospheric peak radiation indicates that TWISO modulates the tropical energy balance. By modulating energy flux, TWISO affects the exchange with areas outside the tropical zone, which has the potential to influence global weather and climate patterns.

Considering the annual threats posed by tropical storms, accurate weather forecasting is crucial for saving lives and livelihoods, planning evacuations, and preparing disaster responses.

Nevertheless, predicting tropical weather one to two months in advance remains a significant challenge. Since TWISO follows a consistent pattern over 30-60 days, this provides an opportunity to enhance predictability within that timeframe.

"By understanding TWISO, we can enhance our ability to predict when tropical cyclones are likely to form, allowing us to issue early warnings and help minimize the risks and damages they may cause. We plan to discuss this in upcoming research," Bao stated.