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Cyclones in New Zealand and its effects on Marine reefs

By Koa Nelson

Currently we are having a pretty crap summer, being hit by tropical storms that make their way from the equator down to New Zealand. With the advent of global warming, we can expect to see increases in these more extreme weather events, and so we decided to write a bit about a special weather system we are experiencing.

A tropical cyclone (taupoki) is a revolving storm that begins in the region between the tropics of cancer and capricorn - near the equator. They start as a cluster of thunderstorms in an area of low pressure which begins to rotate. If the conditions are just right, it will grow to around 500km across - drawing energy from heat that is released when warm ocean water vapour condenses into cloud. The rotating thunderstorms form spiral rain-bands around the centre (eye) of the cyclone where the strongest winds and heaviest rain are found (eye wall), transporting heat 15 km or higher into the atmosphere.

I know you might be thinking “NZ isn´t a tropical place! We are just above Antarctica, so how can we be getting these cyclones?” The answer is that New Zealand's climate is complex and varies from warm subtropical in the far north to cool temperate climates in the far south and a bit of severe alpine conditions in our mountainous regions. This means that from December to April, the warmer ocean temperatures that power the cyclone are also occurring around New Zealand and this allows the cyclones to range further than they normally would. There is a Maori expression relating to this cyclone-prone season that goes; ´Ngā uaua o te whitu rāua ko the ono´, literally translating to: ´the strenuous times of the sixth and seventh months´ (in the traditional calendar).

We would be remiss as a dive shop to not mention the effects cyclones can have on our marine environments. Across all kinds of marine environments, there is a reduction of biodiversity after a cyclone event, this is not good because biodiversity increases the efficiency and resilience of the ecosystem in question. There is also a broad reduction in productivity as the energy of the ecosystem is used more in healing and repairing damage than in forms such as reproduction or growth.

Specifically pertaining to coral reefs, there is some interesting research that shows not all the effects of cyclones are negative, the deposition of sediment can aid growth by providing nutrients within easy reach and the influx of cooler waters in the wake of a cyclone can allow coral to recover from bleaching. That said, these positives also have a downside; the sediment is only beneficial to coral reefs if it is nutrient rich - if it is not it can even inhibit growth, the cooler water is often a result of heavy rainfall which affects the pH balance and reduces calcification in coral reef systems - resulting in less resilience.

Cyclones also have a major effect on the inhabitants of our coastal waters. Most marine animals will head into deeper water when a storm comes because they will be less affected by the rough surface, however this is not always a possibility for our smaller coastal critters. Many of them will find places to hide in rocks and caves where they can find some shelter from the swell. If they shelter too close to the sea floor they can be in danger of their gills clogging up with sand and perishing. Kelp is a resilient and fast-growing sea plant which creates a home for many of our fish in the reserve. Normally it can recover quickly from predation or storm damage but when these are combined with less-than-perfect growing conditions such as; changes in ocean chemistry/temperature, it can have catastrophic results. If there is a weakness in the plants, the cyclones will do many times more damage than usual.


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