The Australian Monsoon is here... Tomorrow Jan 16 2025 - A Scientific Guide
Author: Chris Nitsopoulos
Posted on: Wednesday, January 15th, 2025, 9:40:28 AM
The Australian monsoon is a fascinating weather phenomenon that marks a significant change in atmospheric dynamics over northern Australia. In this article, we’ll delve into what a monsoon is, how it differs from a regular trough, and the critical components that define its presence. By examining a real-life example of a short-lived monsoon burst, we’ll highlight the unique dynamics at play.
What is a Monsoon?
The term "monsoon" refers to a seasonal reversal of winds accompanied by changes in atmospheric pressure and precipitation. Unlike a typical trough, which is a region of low pressure without strict wind-flow criteria, a monsoon involves specific wind patterns, particularly a cross-equatorial flow that defines its structure.
For an Australian monsoon to be classified as such, the following conditions must be met:
Key Components of an Australian Monsoon
- Cross-Equatorial Surface Wind Flow with Easterlies to the South
At the surface level, winds flow across the equator from the Northern Hemisphere to the Southern Hemisphere. These cross-equatorial winds interact with easterly trade winds in the southern regions, forming the foundational structure of the monsoon. We can check that criteria off as early as Thursday
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Strengthening Cross-Equatorial Winds at 850 hPa
As we ascend to around 850 hPa (approximately 1,500 meters above sea level), the cross-equatorial wind flow intensifies. This is crucial for enhancing moisture transport and increasing the likelihood of convective activity. So the wind flow here should be stronger than at the surface. We can definitely check this criteria off too. Notice the winds in the image below at this level blow stronger than at the surface.
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Cross-Equatorial Winds Reaching the 500 hPa Level
While the wind speeds decrease above the 850 hPa level, the cross-equatorial flow must extend up to the mid-level atmosphere (500 hPa, or approximately 5,500 meters). This deep-layer wind structure is a hallmark of a well-established monsoon. Here's where while we might be on shakier ground - the criteria is in fact met.
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Upper-Level Divergence at 300 hPa
At higher altitudes (300 hPa, or around 9,000 meters), an area of divergence must exist. Divergence at this level allows the "exhaust" of convective systems, facilitating the rise of warm, moist air from below and encouraging new convection to develop. We see a weak airflow aloft at the 300hPa level on Thursday to Saturday satisfying this criteria as well.
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Cross-Equatorial Return Flow at 200 hPa
At the uppermost levels of the troposphere (200 hPa, or approximately 12,000 meters), winds must flow back across the equator toward the Northern Hemisphere. This upper-level return flow, moving in the opposite direction to surface winds, completes the circulation loop and ensures the monsoon's integrity. We can as you guessed check this off our criteria list as well
When the Monsoon Criteria Are Met
For the monsoon trough to officially form, all of the above criteria must be present. The good news is that, over the northwest parts of Australia, these conditions are predicted to align on Thursday and Friday, leading to a monsoon burst.
However, the bad news is the forecasted brevity of this event. This particular monsoon burst is expected to last only 24 to 48 hours, potentially making it one of the shortest bursts on record.
What Causes the Monsoon to Break Down?
As early as Saturday, a low-pressure system is expected to form off the coast of Broome. This development will disrupt the monsoonal flow, redirecting it westward. Simultaneously, upper-atmospheric conditions will deteriorate, leading to the collapse of the monsoon trough. The next anticipated return of the monsoon is in February, marking a significant pause in monsoonal activity.
Why will this monsoon help to create a Tropical Cyclone?
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Enhanced Moisture Supply: Monsoons transport large volumes of warm, moist air from the ocean, providing the necessary fuel for intense convection, a key ingredient for tropical cyclone development.
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Low-Level Convergence: The monsoon trough acts as a zone of low-level convergence where winds from different directions meet, forcing air upward and creating conditions for cyclonic rotation.
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Upper-Level Divergence: Divergence at higher altitudes (e.g., 300 hPa) during monsoonal phases allows rising air to evacuate efficiently, promoting sustained convection and pressure drops.
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Vertical Wind Shear Reduction: Monsoonal environments typically exhibit reduced vertical wind shear, which is critical for allowing a developing cyclone to maintain its structure.
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Vorticity Generation: The monsoon trough enhances low-level vorticity (rotational energy), increasing the likelihood of tropical cyclone spin-up.
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Prolonged Atmospheric Instability: The monsoon sustains unstable atmospheric conditions, fostering continuous convection, which can organize into a cyclone if other factors align.
The image below Thanks to the ECMWF shows us the probability that this monsoon will spawn a Tropical Cyclone by Monday morning 20th January 2025. 
Conclusion: A Learning Opportunity for Weather Enthusiasts
The Australian monsoon provides an excellent case study for understanding complex atmospheric processes. By examining the key components and dynamics of this system, students and weather enthusiasts can better appreciate the intricate interplay of wind, pressure, and moisture that defines monsoonal activity. While this particular event may be short-lived, it underscores the importance of monitoring and analyzing atmospheric conditions to predict and understand such phenomena.
For aspiring meteorologists and weather nerds, tracking monsoons is not just about rainfall predictions—it’s a window into the fascinating mechanics of our atmosphere.