Spatial Characterization of Rainfall, Convective Available Potential Energy, and Low-Level Relative Humidity across ENSO Phase Intensities over the Indonesian Maritime Continent Based on ERA5 Reanalysis
Using ERA5 reanalysis data from 1995–2025, this study characterizes how rainfall, convective instability, and low-level humidity over the Indonesian Maritime Continent respond spatially to varying intensities of El Niño and La Niña, revealing uniform moisture-rainfall deficits and surpluses that scale with event strength, a non-monotonic west–east dipole in instability, and asymmetric spatial expansions or contractions of significant anomalies during strong events.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The skies above the Indonesian Maritime Continent, a vast archipelago stretching between the Indian and Pacific Oceans, are among the most active on Earth. This region acts as a giant engine for the planet's weather, where warm ocean waters constantly feed moisture into the atmosphere, fueling massive clouds and heavy rain. For decades, scientists have known that the rhythm of this rainfall is heavily influenced by a climate phenomenon called the El Niño-Southern Oscillation, or ENSO. This cycle involves shifts in ocean temperatures and wind patterns across the Pacific, which ripple outward to alter weather far away. When the cycle swings toward a warm phase, known as El Niño, the region typically experiences drier conditions. When it swings toward a cool phase, called La Niña, the skies tend to open up with heavier rains. However, while the general direction of these changes is well understood, the details of exactly how the atmosphere responds to different strengths of these events have remained somewhat fuzzy. Most previous studies treated El Niño and La Niña as broad categories, often missing the nuances that occur when these events are weak, moderate, or severe. Furthermore, researchers have rarely looked at rainfall, atmospheric instability, and low-level moisture together to see how they interact across the entire archipelago during these different intensities.
A team of researchers from the Sekolah Tinggi Meteorologi, dan Geofisika in Indonesia set out to fill these gaps by mapping the atmosphere's behavior across the full spectrum of ENSO intensity. They focused on three specific ingredients that determine whether it rains: the amount of precipitation, the energy available for storms to form, and the moisture content in the lower atmosphere. To do this, they turned to a comprehensive digital record of the weather known as ERA5, which reconstructs past atmospheric conditions using data from satellites and weather stations. They analyzed thirty years of data, from 1995 to 2025, sorting the months into six distinct groups: weak, moderate, and strong El Niño events, and the same three levels for La Niña. By comparing the weather patterns in these specific groups against the long-term average, they could see exactly how the atmosphere changed as the climate cycle intensified.
The study revealed that the response of rainfall and low-level moisture is remarkably consistent and predictable. As the El Niño phase strengthens from weak to strong, the region experiences a growing deficit in rain and moisture. Conversely, as La Niña intensifies, the surplus of rain and moisture grows larger. This relationship is monotonic, meaning the effect gets steadily stronger as the event gets more intense. During the strongest El Niño events, the dry conditions spread across a wide area, covering the waters around Kalimantan and the Java Sea. In contrast, the strongest La Niña events bring a deluge of moisture that blankets almost the entire domain, with the wettest conditions concentrated along the equatorial band from Sumatra to Papua. The researchers found that the area where these significant changes occur actually expands as El Niño gets stronger, but strangely, it contracts slightly as La Niña reaches its peak intensity, even though the amount of rain in those areas remains very high.
However, the story of atmospheric instability, measured by a value known as Convective Available Potential Energy, is quite different. This value represents the energy available to lift air and form thunderstorms. Unlike rainfall, which simply gets wetter or drier, this energy does not increase or decrease in a straight line. Instead, the researchers discovered that the energy rearranges itself across the map. During moderate and strong events, a distinct dipole pattern emerges, splitting the region into two opposing sides. On one side, primarily over northern Sumatra and the Malay Peninsula, the energy for storms increases. On the other side, stretching from Kalimantan through the Banda Sea, the energy decreases. This split becomes sharper and more defined as the ENSO event grows stronger. It appears that the atmosphere is not just getting more or less unstable overall, but is shifting the location of where storms are most likely to form.
To ensure these patterns were real and not just random noise in the data, the team applied two different statistical tests to every point on their map. They found that both methods agreed almost perfectly, confirming that the observed patterns of rain, moisture, and storm energy are robust and reliable. The study also highlighted a limitation in how these results should be read: because strong La Niña events are rare, there were fewer months of data to analyze for that specific category compared to weaker events. This meant that while the patterns were clear, the statistical certainty for the strongest La Niña phase was slightly lower, causing the map of significant changes to look slightly different than for the more common moderate events. Ultimately, this work provides a clearer, more detailed picture of how the Indonesian archipelago breathes in response to the world's most powerful climate cycle, showing that while rain and moisture follow a simple rule of "more or less," the energy that drives the storms plays a complex game of shifting locations.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.