More important than tropical cyclones: Scientists finally track the hidden weather systems behind most tropical rainfall

Scientists have finally identified and tracked the weather systems that contribute more significantly to rainfall in the tropics than tropical cyclones. This groundbreaking research, published in the journal Weather and Climate Dynamics, advances a new method for studying how tropical rain may change as Earth warms up. In the tropics, weather patterns are typically understood through familiar systems like cold fronts, high-pressure, and low-pressure systems.

However, tropical regions have lacked a comparable framework to identify and categorize the numerous systems responsible for organizing day-to-day clouds and rain. Previous studies primarily focused on individual thunderstorms or wide-scale atmospheric waves that influence multiple weather systems simultaneously. Emerging theories have suggested the existence of moisture modes, slow-moving systems capable of organizing tropical thunderstorms and rainfall over several days.

Other research has explored large-scale atmospheric gravity waves, which influence vast regions of rising and sinking air, and move clouds and rainfall over extensive distances. Unlike tropical cyclones, which receive significant attention due to their major impacts, moisture modes and gravity waves have received minimal attention outside specialized atmospheric science circles.

Researchers from Monash University in Australia analyzed over three decades of satellite observations to pinpoint moisture modes as trackable weather systems and assess their contribution to tropical rainfall. The findings reveal that moisture modes, previously unrecognized but now categorized as distinct objects, are crucial for understanding tropical weather patterns and rainfall.

The study employed satellite data from 1984 to 2016 to identify extensive areas of high, cold clouds associated with organized tropical convection, treating these cloud areas as individual objects and tracking their movement, duration, and speed. Moisture modes moved slowly, while gravity-wave systems traveled much faster, with systems falling somewhere in between classified as mixed.

The research showed that moisture modes are particularly significant near the northern and southern edges of the humid tropics, including regions around tropical Australia. In some areas, they account for between 45% and 75% of extreme three-hourly rainfall. This discovery is particularly relevant for Australia, as the continent spans from tropical to extratropical regions.

While meteorological dynamics in the extratropical parts of Australia are well-understood, tropical Australia has largely focused on tropical cyclones, with moisture modes receiving less attention. The study's authors emphasize the need to recognize moisture modes as important weather systems in tropical Australia. By converting theoretical concepts into identifiable, trackable weather systems, the researchers have developed a new approach to examine the atmosphere and the rainfall it generates.

The next step involves understanding how these moisture modes and other weather systems will behave as the climate warms. As the atmosphere holds more moisture due to global warming, the authors predict that moisture modes may amplify or give rise to additional weather systems. Understanding these processes better could enhance confidence in projecting the Australian monsoon and precipitation extremes in a warming world.