How Tropical Cyclones Form

Hurricane from space

Weather tends to happen in seasons, and tropical cyclones (or hurricanes, or typhoons, they are all the same thing) have their own seasons, too. In Australia cyclone season is November to April. But tropical cyclones don’t form just any old time, in any old place, certain conditions need to come together for them to become a coherent spinning mass of moving air and water with destructive capabilities.

1. First of all, you need a large area of warm water over 26.5 degrees Celsius, the warmer then better. This places cyclone formation to tropical areas, where the Sun spends a lot of time overhead and provides a lot of heat to warm all that water down below.

2. Second, tropical cyclones generally don’t form within 6 degrees of latitude of the equator, as they need a little bit of a nudge from the Coriolis effect to start their spinning motion. (The pedantic among you can probably find at least one exception to this, but lets just say “in general” to keep it simple for the moment.)

3. Third, you need an absence of shear. In weather terms shear is wind moving in different directions at different heights. Thunderstorms need a bit of shear to form, but cyclones prefer there to be none.

4. And lastly, you need a convective wave or pulse to kick-start a big clump of cloud formation. Across the top of Australia this comes from what is known as the Madden-Julian Oscillation, or MJO for short.

The MJO can be imagined as a conveyor belt of atmospheric waves rolling across the tropics from the east coast of Africa to the western Pacific, always from west to east. It’s complicated, doesn’t run smooth, and we’re still learning about it.  The Bureau of Meteorology has some information on it here, including what phase it is in at the moment. But by monitoring what phase it is in is how meteorologists have an expectation of when a cyclone might form.

Looking at the map below, which shows the tracks of all the tropical cyclones between 1985 and 2005, there is a noticeable gap along the equator, where the Coriolis effect is not large enough to get things spinning. There is also a two other big empty spaces, in the South Atlantic and the south-eastern Pacific, up the west coast of South America. In fact, South America seems a pretty good place to go if you want to avoid cyclones altogether! There is good reason for this; running up both sides of South America, and the west coat of Africa, are cold ocean currents. While these cold currents make for great fisheries, they dampen chances of cyclone formation.

Actually, don’t move to Brazil – do you notice the one, lone track on the map? It used to be a curious anomalie.  South Atlantic tropical cyclones are starting to become a more regular occurrence. Four more have been recorded since then.


Where cyclones occur

The accumulated tracks of tropical cyclones from 1985 – 2005    Source: Wikimedia Commons

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