Understanding rainfall variability in the tropics
The tropics, located near the Equator, are known for their high temperatures and abundant rainfall. However, not all places in the tropics are equal, and one key reason for this is the distribution of water vapour, or humidity, across the globe, writes Monash University PhD student Corey Robinson.
In general, air near the surface flows from the ‘dry subtropics’, where rainfall is rare, to the ‘deep tropics’ near the Equator, where heavy rainfall and thunderstorms commonly occur. The boundary between these two regions is called the ‘moist margin’, and it contains almost all rainfall in the global tropics.
While the moist margin can be useful for understanding rainfall patterns, a complicating factor is that it constantly moves around. In other words, it is affected by the weather!
Our research examines how different types of weather systems are related to movement in the moist margin, such as low-pressure systems, tropical waves, and extratropical waves. Extratropical waves are like undulations in the upper-level jet stream. Tropical waves, meanwhile, are collections of disturbances in the atmosphere that spread out eastward and westward along the tropics, bringing rainfall.
We find that all these weather systems are important, but not all to the same extent. For example, while tropical waves are common, they only have a weak effect on the moist margin. Meanwhile, low-pressure systems are uncommon, but often cause strong disturbances in the moist margin when they do occur.
A particularly interesting case occurs when extratropical waves interact with the moist margin. These events commonly result in long-lived and widespread rainfall in regions that are typically dry. Our research is investigating these interactions between extratropical waves and the moist margin to better understand the processes leading to heavy rainfall.
This work was published as ‘Weather systems associated with synoptic variability in the moist margin‘ in the journal Weather & Climate Dynamics.