Scientists from Monash University have, for the first time, identified and tracked “moisture modes” and related systems as discrete, trackable weather objects, showing that these less dramatic systems collectively contribute more to tropical rainfall than tropical cyclones.
The study, published 6 November 2026 in Weather and Climate Dynamics (Respati, Singh & Jakob), uses more than three decades of satellite observations (1984- 2016). Researchers identified large areas of high, cold cloud linked to organised tropical convection and tracked them as individual objects, classifying them mainly by propagation speed:
- Moisture modes: relatively slow- moving systems that organise thunderstorms and rainfall over several days; strongly coupled to moisture perturbations.
- Inertio- gravity (IG) waves and gravity- wave systems: faster- moving systems with stronger temperature perturbations.
- Mixed systems: intermediate speeds and characteristics.
This turns moisture modes from largely theoretical concepts into trackable objects analogous to tropical cyclones.
Key findings include:
- Moisture modes are especially important near the northern and southern margins of the humid tropics (including areas near tropical Australia), where they are linked to 45- 75% of extreme three- hourly rainfall events in some regions.
- Overall, the defined systems (including TCs) account for a large share of total tropical rainfall and an even larger share of heavy rainfall.
- Mixed systems tend to matter more in equatorial regions such as the Pacific ITCZ; IG waves and TCs each contribute less due to lower frequency and spatial coverage.
Professor Christian Jakob noted: “Tropical cyclones are the divas of the tropics… But less dramatic systems occur much more frequently, and collectively they are very important for rainfall.”
Lead author Muhamad Reyhan Respati emphasised that defining moisture modes as objects allows them to be tracked like tropical cyclones. The approach helps fill a longstanding gap: mid- latitude weather is routinely described via discrete systems (fronts, highs, lows), while the tropics previously lacked a comparable framework between individual thunderstorms and planetary- scale waves.
Source press release: Monash University, 11 November 2026.
Paper: “The moisture mode-to-gravity wave spectrum as a framework to define tropical weather systems,” Weather Clim. Dynam., 7, 2171- 2192, 2026.
More important than tropical cyclones: Scientists finally track the hidden weather systems behind most tropical rainfall – Monash University
The moisture mode- to- gravity wave spectrum as a framework to define tropical weather systems
The paper introduces a practical framework for defining and tracking synoptic-scale tropical weather systems (other than tropical cyclones).
It bridges two previous approaches:
- Object tracking of mesoscale convective systems (MCSs)
- Spectral analysis of planetary- scale waves (e.g., MJO, equatorial waves)
Using theory on the moisture mode- to- gravity wave spectrum, the authors classify convectively coupled systems by zonal phase speed:
| Category | Phase speed (approx.) | Dominant physics |
|---|---|---|
| Moisture modes | Slow (≲ 9 m s⁻¹) | Strong moisture anomalies; weak temperature gradients |
| Mixed systems | Intermediate (~9- 27 m s⁻¹) | Hybrid moisture and buoyancy characteristics |
| Inertio- gravity (IG) waves | Fast (≳ 27 m s⁻¹) | Stronger temperature and buoyancy anomalies |
Key findings
- Consistent with theory, the ratio of low- level temperature to moisture anomalies increases with phase speed (moisture variations remain non-negligible even for IG waves).
- Moisture modes and mixed systems show distinct dynamical structures near vs. off the equator; IG waves are less clearly differentiated.
- Characteristics are broadly consistent across the tropics, with some regional variations.
- Rainfall importance:
- Moisture modes account for up to 75% of extreme rainfall events along climatological moist margins (including regions near tropical Australia).
- Mixed systems dominate rainfall contributions in the Intertropical Convergence Zone (ITCZ).
- Together with tropical cyclones, these systems explain a large fraction of total and especially heavy tropical rainfall.
Methods (high- level)
- Satellite observations (primarily outgoing longwave radiation / high cold cloud areas associated with organised convection, 1984- 2016).
- Lagrangian object tracking of cloud systems combined with spectral filtering by phase speed.
- Composites of thermodynamic and dynamic fields from reanalysis (ERA5) and precipitation data (GPM- IMERG).
Significance
This framework treats moisture modes as discrete, trackable “weather objects” analogous to mid- latitude cyclones or fronts. It opens new possibilities for:
- Improved understanding of day- to- day tropical rainfall organisation
- Conditional forecasting of extremes
- Attribution of rainfall changes under global warming (frequency vs. intensity of different system types)
Direct links:
- Full paper (PDF): https://wcd.copernicus.org/articles/7/2171/2026/wcd-7-2171-2026.pdf
- Supplement: https://wcd.copernicus.org/articles/7/2171/2026/wcd-7-2171-2026-supplement.pdf
- Code and data: Linked on the article page (Zenodo for analysis code; ERA5 for atmospheric fields).
This is the scientific paper behind the recent Monash University press release about systems “more important than tropical cyclones” for tropical rainfall.
Paper: The moisture mode- to- gravity wave spectrum as a framework to define tropical weather systems
Journal: Weather and Climate Dynamics, 7, 2171- 2192, 2026
Provided: Monash University
Published: 6 November 2026
Open access: Full PDF and supplement available on the Copernicus site.
Authors: Muhamad Reyhan Respati,
Martin S. Singh,
and Christian Jakob
Abstract
While mid-latitude weather is commonly described in terms of discrete “weather systems”, such as cyclones and fronts, an equivalent framework is less established in the tropics due to the wide range of relevant scales. Instead, tropical weather is often analysed either in Lagrangian terms, through the tracking of mesoscale convective systems, or via spectral decomposition into planetary-scale wave-like disturbances. Here, we use both spectral analysis and Lagrangian object tracking to develop a novel framework for analysing tropical weather systems. We apply recent theoretical advances to classify convectively coupled tropical motions into three types based on their zonal phase speed: moisture modes, inertio-gravity waves, and mixed systems. We show that, consistent with theory, the observed ratio of low-level temperature to moisture anomalies increases with the system’s phase speed, although moisture variations may not be negligible for inertio-gravity waves. Moisture modes and mixed systems show distinct dynamical structures near and off the equator, while inertio-gravity waves are less clearly differentiated. These characteristics are broadly consistent across the tropics, with some regional variations. We find that moisture modes account for up to 75 % of extreme rainfall events along climatological moist margins, whereas mixed systems dominate in the intertropical convergence zone. Our tropical weather systems framework offers a new way to link large-scale tropical dynamics with precipitation.
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