Dr Neil Cherry – 20/8/99
Our weather in New Zealand is very variable because of our position. We are halfway between the equator and the south pole. This results in the dynamic movement of contrasting weather systems (anticyclones and depressions) which our prevailing westerlies carry onto the country, sometimes in a regular sequence and sometimes in a chaotic fashion. Our backbone of mountains creates a wetter west coast and drier east coast, and adds to the variability of the weather by making it critical from which direction the airflows actually approach the country, from the west or from the east of the mountains. A small difference in the direction of approach makes a big difference to our weather. A persistent westerly bias can bring floods to the west coast and drought to the east coast as happened last year. During last year we swung from El Nino to La Nina. El Nino and La Nina are extreme phases of a massive ‘see saw’ involving the ocean currents and the atmosphere in the equatorial Pacific Ocean. The normal situation has giant ocean currents flowing anticlockwise around the South Pacific Ocean. The currents sweep eastwards around the Antarctic in the Great Southern Ocean, up the South American coast, westwards along the equator and then curve southwards to the east of New Zealand and into the
Coral and Tasman Seas.
A large anticyclone lies to the west of South America and a persistent depression lies over northern Australia/New Guinea/Indonesia. Southeasterly Trade Winds flow from the east Pacific High to the New Guinea Low, dragging massive quantities of warming tropical water towards the western seas to the north of Australia. Strong convection, big thunderstorm clouds and a mass of rising air pushes against the stable air of the stratosphere, sending two sideways branches, one looping northwards towards the Philippines and one looping over Australia. High level branches also head into the Indian Ocean and another back across the Pacific.
The North/South loops are called the Hadley circulation. The Australian loop, where it descends over South Australia, produces the Australian anticyclone, which in turn spawns the ridges and anticyclones which flow from Australia onto New Zealand. The latitude of these high pressure systems is critical, as it determines the angle of approach of air masses onto southern New Zealand, and hence the predominant weather on the West
Coast and in Canterbury.
When the tropical system is much stronger than average, with strong trade winds, higher pressure in the east and lower pressure over northern Australia, we call it La Nina. It adds a northerly component to our prevailing westerlies: i.e. more nor’westers, bringing drought to the east and floods to the west. At the peak of the La Nina some drought relief can occur when warm moist tropical lows are brought onto the country. The reverse swing of this system sees a weakening of the trade winds; the accumulated tropical hot water pool travels across the tropical pacific towards South America, creating wet stormy weather in central and South America and northwards along the California coast. In New Zealand the anticyclones travel along a path which is displaced further northwards, allowing more frequent southerlies and southwesterlies to dominate the South Island. In the past this would have meant colder, wetter conditions. However, with global warming the high pressure systems are stronger and southerlies find it harder and harder to penetrate over the South Island. Hence we are getting drier and drier, with the occasional very wet period. Part of the response of the climate system to a warmer world is more extreme El Ninos and La Ninas.
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