Scientists caught in shallows

By SIMON COLLINS

It was a normal sort of day on the east coast of the South Island; the weather was fine, there were no earthquakes, nothing untoward.

Then the tide went out, and out, and out.

Scientists at the National Institute of Water and Atmospheric Research (Niwa) are struggling to explain data from July which shows the sea level dropped inexplicably by about half a metre - not once but twice.

The scientists, who noticed it on their remote-controlled sea level recorders, had never seen or heard of anything like it before.

But then, two weeks after it first happened, it happened again. Both times it lasted for about a week and was recorded by Niwa instruments at Kaikoura, Sumner in Christchurch, Timaru and on the Chatham Islands.

Niwa's principal scientist for coastal hydrodynamics, Dr Derek Goring, said this week that storm-induced upward surges in sea level occurred 20 or 30 times a year around New Zealand, but these two events were the first known cases of "negative storm surges".

They could have been almost as disastrous as high seas.

"We didn't hear of any problems with ships going aground in these events but it could have happened if a ship was manoeuvring at low tide in shallow water, because the depth was half a metre lower than registered on the charts," he said.

He believes the phenomenon may have been a remarkable spinoff from a series of intense storms that were raging at the time over the wild Southern Ocean, hundreds of kilometres south of New Zealand.

In storms, or depressions, warm air from the north rises above cold air from the south, causing low air pressure, wind and rain. The lower air pressure allows the sea level to rise in the eye of the storm.

Dr Goring believes this rising sea level far to the south sucked in water from as far afield as New Zealand, causing the dramatic drop in sea level along the South Island coast.

"We haven't observed that before," he said. "But we may have missed it."

He said Niwa started recording sea levels only in 1994 for research on tides, storm surges and tsunamis.

Two of the four stations that recorded July's events - those at Timaru and in the Chathams - were installed only in the past year.

Although records have been kept for much longer in the United States and Australia, Dr Goring said the South Island was further south and more exposed to the vast Southern Ocean.

"Because we are isolated in the middle of the ocean, other people don't experience these things to the same extent that we do.

"The big systems that are occurring at sea hardly see New Zealand as a land area, so it's a really interesting problem and one that is significant for New Zealand and not for a lot of other places."

Niwa scientists are developing computer models to try to explain the effect before they report it officially.

Niwa has also noticed that the sea level around the country tends to drop during El Nino years, when huge anticyclones over Australia produce cool, dry southwesterlies over New Zealand.

Conversely, the sea level rises in La Nina years, when warm moist winds blow from the northeast.

But Dr Goring said the typical sea level variation in El Nino or La Nina years was only around 4cm, compared with 50cm in the July events.

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