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Whither our weather

MONDAY JANUARY 25, 2010

 

Those whose livelihoods depend on it know that NZ weather mostly comes from the direction of Australia.  There are two main systems.  Either it slides beneath the continent, through the Bight, across Tasmania and smacks us in the southwest, or it comes down from the NW or NE Pacific bouncing like a billiard ball off the east coast of Australia and hammers our far north.  

 

The first direction pattern contains westerly or SW winds which collect moisture that dumps first on Fiordland then spreads up to Taranaki, and often deflects south at the Tararuas into the lower NI.  The airflow is forced to rise and cool as it passes over the country's alpine backbone, warms again going north only to cool again when deflecting off the lower North Island ranges.  After releasing its load of water on the western slopes of the alps as rain or snow the air now drier descends to the east, warming as it descends, creating a warm dry wind over the eastern plains.  Persistent westerlies can eventually cause droughts in the east.  This westerly pattern follows approximately a 6-10 day cycle.  The W and SW airflows can also drag cold fronts onto the south of the country which is why the west coast and the far south generally get rain at roughly the same time.  Between these cold fronts there are ridges of high pressure called anti-cyclones which bring light winds and generally fine weather.  Mostly, a storm will only last for two to three days, followed by a couple of days of fine weather.

 

The second system comprising patterns from the north, generally NWs will bring rain to the West Coast, and often high winds in the mountains with rain sneaking over to the main divide for a few kilometers.  Rain-bearing NWs strike the South island first at around Nelson and may cause floods if the tides are high.  The further east of the divide you go, the drier and less clouded the skies become, and the winds will increase in intensity until the NW wind reaches the southern east coast where it is dreaded in summer for being so very warm and dry.  From the eastern plains you may see dark grey clouds obliterating the Southern Alps and ridges of tightly formed arches above the inland eastern plains, whilst skies on the eastern coast will be clear.

 

This country's snow storms come from the south.  A southerly storm progresses up the east coast, bringing a drop in temperatures and cool rain in summer or a snow storm in winter.  During winter it is rare for a southerly to last longer than three days.  After the southerly front has passed through, the clouds will be moved away by the cool southerly airflow and a period of fine weather will follow with increasing patches of blue skies on the southern horizon.  Anticyclones are generally deeper in the winter in the south and shallower during summer, whilst depressions are generally shallower during winter months.  Barometric pressures alter as the moon changes hemispheres.  Winter anticyclones will be cool but unbelievably fine.  As the fine spell progresses air temperatures will rise as the air flows move into the westerly quarter.

 

A light warm westerly airflow does bring some cloud cover on the western side of the divide.  In the early morning the West Coast is often clear of cloud and as the day develops, white fluffv cloud starts forming over the rainforests at about 1200m.  The cloud intensifies, then rises up the valley systems onto the snowfields.  Easterlies and NEs do not occur very often in the South island.  The former often brings light warm rain with very little wind to the east coast, and sometimes as far inland as the Alps, while fine weather is experienced on the West coast. Most east coast farmers are overjoyed with this rain. NEs are common in the north of the country and are the winds that bring most rain to Northland, Auckland, and the Bay of Plenty, especially during the cyclone season but are rarer in the south as they tend to peter out quickly.  In Fiordland and the Darrans it will most likely have stopped raining there.

 

Can we look to Australia to see what weather is coming to us?  To a certain extent we can, particularly in coastal districts like Tasmania, Brisbane and Sydney.  A foul cyclonic weather system coming from Tasmania or NSW will still be potent enough to affect NZ.  A departing anticyclone in the Tasman will deliver rain to Sydney and its leading edge may push a trough onto NZ.  When the moon is in the north in summer months, higher temperatures will be experienced in both Queensland and the north of NZ, and in winter when the moon is in the south both the lower South Island and the south of Australia are similarly affected.  The best indicators of our weather are an analysis of our own cycles based on gathered data, which unfortunately is not great.  Many NZ locations have had automatic stations only since the 1980s and without cycles data going back 200 years it is hard to say what is normal for any place.  The same may be said for our neighbour, although the Australian Bureau of Meteorology has been operating much longer than the NZ Metservice.

 

But one should not be swept up into the drought hysteria.  For example, not all of Australia is in drought.  Some coastal areas and some ranges get plenty of rain, some areas get regular dumpings from cyclones and many rivers get floods.  The Interior and Inland are dry because they are, like the African and N American deserts, some distance from evaporatable water.  Much of Australia is drought prone because the continent is in the sub-tropical latitude of the great deserts - if you look at a globe of the world you will find most of the biggest deserts are in the middle of continents about 20 to 35 degrees north and south of the equator.  The desert areas of the world are, going west, the W side of N America (Great Basin, Mojave, Sonoran and Chihuahuan), the W side of S America (Peruvian, Atacame and Patagonian), the top half of Africa (Sahara and Somali-Chalbi), SW Africa (Kalahar and Namib), the Arabian (E Mediterranean), Karakum, Lut and Thar in E Europe, Taklimakan in C Russia, and in Australia (Great Sandy and Simpson).  South America desert areas are not 'great' ones.

 

The shape of continents and the distribution of ocean currents also effects where the more arid regions are.  The wet equatorial region extends north and south of the equator through most of the tropical region where over the tropical oceans warm moist air rises into the upper atmosphere, and rainfall is high in many tropical land areas, especially nearer the coast.  Beyond this are the sub-tropical zones where drier air is descending and this produces the great inland desert conditions.  Moving further north and south into the temperate zones on continental areas we find it is a mixing zone between drier warm inland air and polar cold outbreaks that gives more rainfall.  So inland drought is more or less 'normal' due to Australia's unique geographic location, although in some years the drought is far more severe and some years are characterised by drought breaking floods.

 

What makes NZ wet and green is the proximity of most places to water of some description.  What drives the climatic situation is several shorter and longer term solar cycles and embedded within these the reacting lunar cycles, each with warmer and cooler phases that effect dominant large scale climatic patterns, making it wetter or drier depending on what location one is looking at, so for example when it is drier in eastern Australia (El Nino) it is wetter in western South America, and vice-versa (La Nina).

 

When looking at global climate, we can see the longer term effects of all these combinations of various length cycles in the fact that average global temperatures rise and fall over long time periods, so that around 1100AD the Earth was perhaps a degree or so C warmer than it is now and Greenland was called that because it was a thriving agricultural community.  Then came the 'mini ice age' through the middle of the millennium where average temperatures dropped below what they are now by perhaps a couple of degrees C and Greenland became an uninhabitable frozen wasteland.  The books from a couple of centuries ago talk about the Thames river in London being frozen over every year - there were several warmer and cooler cycles within this period.  Since the early to mid 1800's we entered another warming phase to get to where we are now which is another warm period, and we are due to change direction into a longer term cooling phase (midpoint of 179-186 year cycle) that may see a return to cooler conditions before the end of the 21st century.

 

© Ken Ring


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