How bad is our weather?

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Charles Heaphy, after whom the Heaphy Track in North-West Nelson is named, came to New Zealand in 1839 as a draughtsman for the New Zealand Company. After seeing and painting much of the country, he pub­lished a short book in London describing his experiences. In it he wrote that summer in New Zealand lasted five months, and winter only two. He also reassured Europeans that although the Maori had acquired firearms, they used them only to shoot in the air at parties. There was a dark irony in this comment: 25 years later, Heaphy would win the Victoria Cross for bravery under fire in the Waikato land wars.

His generous descrip­tion of New Zealand’s climate was also soon under fire as disillusioned settlers, their expectations raised after reading Heaphy’s propaganda, wrote home complaining of incessant wind and changeable weather. Their grumblings prompted the Colonial Administration to collect and publish, in 1856, the first weather statistics for New Zealand, to prove that the climate was not as grim as discon­tented settlers were painting it. Observatories were set up soon after­wards, and in 1861 the Auditor General, Charles Knight, was made first Director of Meteorological Services.

Whether or not we get more than our fair share of bad weather continues to be a controversial subject. However, a new study by Mark Sinclair of the National Institute of Water and Atmospheric Research has found that, in winter, the Tasman Sea is one of the most important areas of the Southern Hemisphere for the formation and intensification of vigorous depressions. Other areas include the higher lati­tudes of the Indian Ocean and the South Atlantic east of Argentina.

Sinclair’s finding, based on an examination of seven years’ data, is in sharp contrast with earlier studies which suggested the region between 60° south and Antarctica is the most important site for depression formation.

Sinclair found far more depressions in mid-latitudes than previous studies had, because he searched for points of maximum cyclonic circula­tion in the wind flow rather than points of lowest atmospheric pressure—the usual indicator of depressions. Many active depressions in New Zealand latitudes do not have closed isobars with a pressure minimum in the centre, even though they are associated with the hook-shaped rain-producing cloud bands seen on satellite pictures. This is because such depressions are embedded in fast-moving westerly flows, and don’t show up on weather maps as typical “lows” (see illustration).

There are several reasons why the Tasman Sea is a favoured region for depression formation and intensification in winter. First, there is a warm ocean current running down the east coast of  Australia. Cool air moving over the ocean from the Australian landmass experiences rapid heating and an increase of mois­ture as sea water evapo­rates into it. Both of these factors destabilise the air, enhancing the formation of depressions.

Second, the Tasman Sea lies just on the poleward side of the Subtropical Jet Stream, which is a belt of very strong westerly winds near 30° south at an altitude of 10 kilometres. This jet stream is at maximum strength in winter because of high level outflow of air from the monsoon over the Asian landmass in the northern hemisphere summer. The jet tends to slow down as it moves over the Tasman Sea, and this favours depression development on its poleward side.

In addition, the Tasmar. Sea lies downwind of the Great Australian Dividing Range. The passage of westerly winds over these hills can lead to a process known as lee cyclogenesis which can trigger the formation of a major depression. Most of the depressions that form ove the Tasman Sea track east or southeast, passing over or near to New Zealand and bringing rain and strong winds to many par of the country.

Although winter is a more active time for depression formation ove the Tasman Sea, they can and do form in summer. A recent example was the depression which crossed the country on February 20-23. The northerlies ahead of the depression had very high water vapour content, and brought rain to most of central New Zealand. Behind the main cloud band, however, a line of thunderstorms formed rapidly around midnight on the 22nd. These thun­derstorms brought very heavy rain to parts of Nelson and Marlborough with one-hour falls of between 40 and 55 mm of rain recorded at Nelson and Woodbourne airports as well as in the Richmond Ranges. Such high inten­sity rainfall caused wide­spread flash flooding, and water entered many houses and shops in Blenheim and Nelson. Repairs to state highways affected by scouring, washouts, and slips over the northern half of the South Island were estimated by Transit New Zealand to cost around half a million dollars.

In the Richmond Ranges the east branch of the Motueka River rose rapidly by almost five metres and washed away a but where two Department of Conser­vation goat hunters were staying. The body of one was found four kilometres downstream, while the other was still missing at the time of writing, presumed dead. The but floor was found 10 kilome­tres downstream, and the pack of one of the men was sighted more than four metres up a tree.

Luckier was Stephen Ivory, who was rescued from the flooded Waimea River. He had been sleep­ing in his housetruck next to the river, and woke to find himself surrounded by water. The water contin­ued to rise, and eventually the truck floated away. He escaped from the cab and was swept downriver holding on to a seat squab and, later, a log. People threw ropes towards him, but could not reach him. In a rough patch below Appleby bridge he was bashed by a couple of logs and went under, expecting not to come up again.

However, he resurfaced, and made it to a sandbank in midstream. When asked how he would prefer to be rescued, chopper or boat, he told police, “I’ll go for the chopper.”

The hydrologist with the Tasman District Council estimates the return period for the Upper Motueka flood as over 200 years; for the Waimea, as little as seven years.

Stephen Ivory no doubt hopes he’ll be spared that kind of weather for some time to come.

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