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Welcome to Did You Know'. A daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks.

Tonight we tackle a third necessity for a Tropical Cyclone - A pre-existing disturbance

Good old Cat 5 Marcia in 2015 formed from an ER Wave, we remember her well

Did You Know:

"Can you explain what is meant by a pre-existing disturbance when it comes to a Tropical Cyclone forming?"


All Tropical Cyclones can only form if something has disturbed the low level atmosphere to begin the uplift process.

Want to know more? well read on




Around 70-90 Tropical Cyclones develop every year globally, about 10-11 in the Australian Area Of Responsibility. About 70-75% of those near Australia develop along the Monsoon Trough (ITCZ). The large majority of the remainder primarily form due to Equatorial Rossby Waves that move from East to West across the planet. There are other smaller formation influences like the remnants of a frontal boundary/outflow of previous large mesoscale convective storm systems ( we will discuss how cold air spreading out from storms create new storms around them next week), Kelvin waves that move from west to east across the globe, The South Pacific Convergence Zone, some complex interactions between upper level tropical troughs and the surface and finally if there's a large enough cluster of storms around . But combined those other four influences account for less than 10% of Tropical Cyclone formation in any one year for us (that's about 1 clone a year in the Australian region). By far the biggest contributor is the ITCZ followed by Equatorial Rossby Waves.

The atmosphere across the oceans may be moist but the uniformity of the surface temperatures of the ocean means that for a majority of the time the atmosphere remains relatively stable above it so air will often flow across the oceans but not vertically like it does on land unless something starts the vertical lift. One of the things I was always fascinated in my younger days doing work experience at the BoM was a place off Cairns called Willis Island, see it took me a long time to understand as a kid why Willis Island being a tropical island out in the beautiful warm Coral Sea could only manage about 1000mm of rain a year (less than my home city of 'Br'Townsville), yet Cairns at roughly the same latitude could double that and more. In the end it comes down to the atmosphere's ability to lift the moist air. Cairns has mountain ranges, Cairns is on land which heats up differently to water. Therefore Cairns has two major things going for it that Willis island doesn't. Long story short, that's why we need a boundary line, a convergence zone, something that gets the air over the ocean clashing/crashing/smashing and then moving upwards along with something that kicks off the spin for a Tropical Cyclone. That's where our two big guns come in. The ITCZ and the ER waves.

We know the ITCZ as the Monsoon Trough and you can read more about it here otherwise this 'Did You Know' will go on forever. An important sub-section of the monsoon is the MJO a westerly wind burst that travels towards the east in a wave like pattern every 30-60 days. When it travels over the Australian region it 'activates' the clash between westerly winds and easterly winds near the surface and this becomes the driver of large scale uplift of air both over land and MOST IMPORTANTLY the ocean. Rotation is imparted on the clashing air masses as well due to the Coriolis Effect (you can check out this 'Did You Know' about what that is). So we need a monsoon trough, that is actively causing the clashing of air-masses over the water. But.... the MJO only sticks around us for a couple of weeks every month or two so the monsoon trough isn't active all the time and when it's inactive the role of cyclone birthing turns to the Equatorial Rossby Wave.

To understand what an Equatorial Rossby Wave is we need to think of the atmosphere as a giant ocean. An Equatorial Rossby Wave is a wave that is trapped by the equator due to the earth's spin. Just like waves on the water ocean, the ER wave can cause the ocean of air that it travels through to rise and fall (rising air destabilises the atmosphere, sinking air stabilises it). The wave moves westwards very quickly if it has no weather associated with it, but moves much more slowly if it begins to produce convection. An ER wave also extends southwards and northwards from the equator however, like a ripple in a pond, the further north or south from the equator the wave gets the weaker it gets. So basically an ER wave does its best work closer to the equator and becomes less effective the further away we get away from it. BUT when it comes to helping Tropical Cyclones form it needs to also create some spin and it does its best work there between 500kms and 1500kms away from the equator because remember large scale spin at the equator or even near the equator isn't possible. At that distance away from the equator there is enough spin 'force' and there's still just enough ooomph in the wave's strength to get that air excited and moving vertically. Rossby Waves can't create Tropical Cyclones on their own, all they can do is provide the rotation and the vertical lift to initialise the process (most of the time that is all they will do and nothing will happen). We still need to have lots of moisture through a deep layer of the atmosphere to be able to tap into that lift, if the Rossby Wave comes along and just lifts dry air, well it's about as useful in birthing a cyclone as a bull is with breasts. We gotta get the air that lifts to also be full of moisture and that moisture needs to extend deep into our atmosphere, not many ER waves do that, most will lift the air which has lots of shallow surface moisture, help to create a few storms as they move westwards and that's where their usefulness will end. But If we can get an ER wave to come across at a time where we have lots of DEEP moisture around and weak vertical wind shear, that's when we get excited and you guys and girls should be getting a little nervous. The most exciting time in the Cyclone Season for us is when we get the MJO wave pushing East at the same time as an ER wave pushes west and they happen to do that over the waters near Australia, that's our version of Santa Claus and Christmas. The entire region explodes with some wicked weather and we almost always see the combination of these two waves work together to provide us with AT LEAST one Tropical Cyclone and more often than not we see a number of clones produced.

Both of these features (ITCZ with the active MJO wave and the ER waves) combined result in approximately 90% of the births of Australian Tropical Cyclones and you need to be alert where they are and when they are expected to move closer to us. The Bureau Of Meteorology has this great page that tracks these waves for you. You can find that page here. You can also become an OCC subscriber and I'll let you know when one of these waves is likely to make things interesting and exciting and when you can ignore these waves. Becoming a subscriber helps us document Tropical Cyclones and gives you the most comprehensive cyclone intel out there during the season. Head to for details



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