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Cyclone energy dissipates rapidly in northern hemisphere

Ben Domensino, Wednesday September 19, 2018 - 13:56 AEST

After an incredibly active week of typhoons, hurricanes and tropical storms in the northern hemisphere, the atmosphere has settled down noticeably this week.

Last week, there were a record-matching seven named systems spinning simultaneously across the northern hemisphere. Four of these were positioned over the north Atlantic Ocean, which in itself is a feat not seen since 2008.

The two most destructive storms were Hurricane Florence, which caused record-rainfall and flooding in the southeast of the United States, and Typhoon Mangkhut, which left a path of destruction through the Philippines, Hong Kong and southern China.

Then, just days after these tempests reached their peak strength and displaced millions of people across the globe, everything went quiet. Monday marked the first time since July that there were no named tropical storms, hurricanes or typhoons anywhere in the world, according to Philip Klotzbach, a research scientist at Colorado State University.

#Manghkut has weakened to a tropical depression. For the first time since July 31, there are no tropical storms or #hurricanes/#typhoons anywhere around the globe. pic.twitter.com/8gxcjXvH6U

— Philip Klotzbach (@philklotzbach) September 17, 2018

So, how did we go from seven named systems, with two catastrophic landfalls, to nothing in the space of a week?

Typhoons, hurricanes and tropical cyclones, which are all different names for the same type of system, draw their energy from warm ocean surfaces. During their lifetime, these systems take large amounts of energy up from the ocean's surface and transfer it into the atmosphere, causing heavy rain, powerful winds and thunderstorms. After drawing energy from the ocean, they leave trail of cooler water in their wake.

The systems that were active across the northern hemisphere last week transferred a lot of energy out of the ocean and cooled large swathes of the Atlantic and eastern Pacific Oceans, particularly near the paths of Hurricane Florence and Typhoon Mangkhut.

Images: Sea surface temperature anomalies during early Septemer and mid September, showing the cooling effects of Typhoon Mangkhut in the western Pacific Ocean (top two images) and Hurricane Florence in the north Atlantic Ocean (bottom two images). Source: NOAA/NESDIS

As tropical cyclones are fuelled by warm oceans, it's not uncommon to see a lull in cyclonic activity in a given region following the passage of a powerful system.

Last week also marked the climatological peak of the Northern Atlantic Hurricane Season and the week certainly lived up to its reputation during 2018.

Australia's tropical cyclone season runs from November to April. This is when the monsoon becomes active near northern Australia and sea surface temperatures surrounding the country's tropics are warm enough to support tropical cyclones.

- Weatherzone

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