Tropical cyclones: Extra-tropical transition

Each year, an average of about nine tropical cyclones form in the South Pacific between November and April. Of these, three or four move south out of the tropics, and most of them transform into a different kind of weather system before they reach New Zealand.

This process is called “extra-tropical transition”. After it happens, the system might still be called either Cyclone So-and-so (just without the word tropical) or Ex-Tropical Cyclone So-and-so. Even though its name changes, it can still bring severe weather, as Cyclone Bola did in March 1988.

True Colour satellite animation of Cyclone Tam approaching New Zealand on 17 April 2025 (satellite images courtesy of the Japanese Meteorological Agency, 2025)

So, what causes this transformation?

At its strongest, a tropical cyclone has a clear eye at its centre surrounded by a ring of intense thunderstorms. It’s a tall, well-organised system, like a towering column of storms standing upright through the atmosphere.

But as these tropical cyclones travel south, they move over cooler seas, which no longer provide the warm energy they need. They also move into the mid-latitudes 0 the part of the world where the westerly winds grow stronger the higher into the atmosphere you go. This pattern of increasing winds with height is called vertical wind shear.

This wind shear breaks up the tropical cyclone’s neat structure. It’s a bit like a woodcutter chopping the top off a coconut tree; the upper part is swept away, leaving only the lower trunk behind. (In reality, it’s a much slower and more subtle process!)

Together, these changes reshape the system, breaking down the processes that made it a tropical cyclone in the first place. The remaining extratropical (“outside” the tropics) cyclone may get carried off in the westerly winds or undergo further development if conditions are right.

By the time the extra-tropical cyclone reaches New Zealand, it has changed, turning from a tightly wound tropical cyclone into a broader mid-latitude system. While it may be different in structure, it is often still able to bring severe wind, rain, and waves to Aotearoa New Zealand.