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Cyclone years are predictable

Ken Ring | View Archive February 13, 2013, 1:51 pm

In the late nineteenth century cyclones were named after annoying politicians. Following objections they were then variously named after the politicians' wives, as most women were assumed to be temperamental and unpredictable. In 1977 after protests by various women's rights groups the American meteorological organisation NOAA allowed a name selection that alternated genders. 1979 was the first year cyclones became subject to a fairer naming list.

To qualify for a name a a tropical depression must sustain winds strengthening to between 39-73 mph (62-117kmh), when it becomes a tropical storm, receiving a formal name and becoming a named storm. This degree of wind turbulence is best produced when the Moon is close (perigee) and rising over the equator.

Another factor is the king-tide in the air that occurs at the same time as the king-tide in the sea, caused by the Moon, and which allows for a lowered height of the atmosphere when the Moon is out of the sky. Because the air insulates the earth like a blanket, preventing too much heat from either entering or leaving too quickly, any lowered air height can expose the earth to extra radiation from the Sun.

The period from full moon to last quarter in the summer months is the time when tropical cyclones most develop. With the Moon and increased air height are absent from the hot afternoon sky (because the full moon does not rise until susnset) the Sun can beat down more strongly and the seas can reach the 26-28degC required for the massive evaporation that cyclone formation needs.

This usually occurs when the Sun is over a particular hemisphere and close to or past its solstice position of that tropic, which will allow sufficient time for the Sun’s rays to heat the sea to that critical temperature.

The reason there are no cyclones off Brazil is probably due to the very large volume of cold fresh water from the Amazon that keeps the temperature of the adjacent oceans below that required for a cyclone to form. Where conditions are warm enough for cyclonic development the heating process will continue after the Moon has crossed the equator and into the other hemisphere.

The build-up of heat will be speeded to that level during a full moon phase and into the Last Quarter because that is the time for the low atmospheric tide, both from the Moon being a night moon and because the Moon is over the opposite hemisphere.

Also, if the Moon is at perigee, the atmospheric tide will be even lower because of the greater gravitation effect from the Moon in that period. Occasionally the new moon will also cause a TC to form, due in part to the increased gravitational pull occurring, the timing of another king-tide and especially if the Moon is at or near to crossing the equator at this time.

The season always runs from November to April and about every 9 years a destructive cyclone revisits with the Moon in a similar position in the sky. To observe the pattern it only depends on how far back we wish to look, and we can see cyclonic weather appearing in historical records going back to 1820.

Sydney recorded 140mm over 3 days on 30 April 1864 that was probably cyclonic. Auckland records taken from Albert Park only continuously go back to 1869. These show what would have been cyclonic falls at each of the 9-yr dates after that, for example 21 Feb, 1873 (90mm in 3 days), 2 March 1882 (100mm over 3 days with 60mm on one day), 4 April 1891(37mm), 24-25 March, 1908 (80mm), 3 Feb, 1917 (141mm), 19/20 May 1926 (160mm) and the 120mm on 15 Feb 1935 two days before full moon.

The pattern is very regular. 50mm fell in Albert Park on 20 Feb 1944, 70mm on 15 Mar in 1944, in April's first week in 1953 Albert Park recorded 100mm, then on 23 Feb 1961 (within two days of full moon) came 53mm. On 11 April 1979, 100mm fell over 4 days, as full moon crossed the equator heading south.

After that came 7-8 March, 1988 within a week of a close perigee. Auckland at that time received 36mm and Gisborne suffered 70mm with gale-force winds. Over 7 days Auckland received nearly 200mm. That was Cyclone Bola. 9 years later in 1997 we endured another called Cyclone Dreena.

Some of the heaviest cyclone years are when the perigee-moon days in the cyclone season occur when the Moon is rising over the equator, because these are years when perigee is averagely closer. Some these years have been 1979/80, 1984, 1988/89, 1993, 1997, 2001/2, 2006/7, 2010/11, and 2015.

The last heavy cyclone period was 2009-10, with the perigeal moon averagely rising above or slightly aside the equatorial band. Lately perigees have been drifting south, reaching southern declination in July 2012. The whole cycle takes 8.85 years to return to a starting position.

In 2013 perigees are currently around southern declination, quite out of range of cyclones that would be strong enough to be destructive. They usually form north of Fiji but in ‘fizzer’ years such as this, peter out by the time they reach NZ. We have between 1 to 2 years of relative reprieve.

By 2014 most perigees occur between southern declination and the equator. Then in 2015 and 2016 perigees will be at or near the equator and in 2017 between the equator and the northern declination. This makes 2015-17 the next phase of heightened cyclonic activity.

We may as well say it is the same cyclonic pattern revisiting every 9 years but under different aliases. It means that in two years time a Bola/Dreena system can be expected to return to our shores, potentially affecting similar areas, and wearing a new name tag.


Ken Ring of www.predictweather.com is the author of the Predict Weather Almanacs for NZ and Australia for 2013, publisher Random House.

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