THE two rainstorms that caused serious earth damage to the Gisborne district in the past month are not particularly unusual, say experts — and they are warning that we can expect more in the future because of climate change.
MetService spokesman Dan Corbett says last week’s event was like someone taking a water-loaded sponge and squashing it against a brick wall.
“We had a flow of warm, moist air dragged down from the subtropics and then rammed up against the hills of the East Coast,” Mr Corbett said.
“The hills block the airflow and in what is known as orographic effect, the terrain forces the air to rise, cool and condense. This is just like shoving that dripping sponge against a brick wall,” he said.
Deep valleys running in from the coast also add a further concentrating or squeezing effect.
In this month’s event the rain swept in from the southeast, from a deep low 200 kilometres off the coast.
The prolonged southeasterly flow and rain resulted from the low being blocked from moving away by a large high pressure system further to the east.
Mr Corbett said this blocking was a bit like a weather traffic jam and resulted in the East Coast getting a soaking.
The orographic effect also concentrates the rain in particular spots. He noted the large amount dumped on Te Puia and the Whareratas.
“Dense rain cells form as a result of the channelling effect of the terrain, and the land below can suffer severe flooding, soil erosion, washouts and slips.”
This has happened in the district many times before. One of the most notable was the rainstorm which devastated the Ngatapa area in 1985. The same effects also happened during Cyclone Bola.
In relation to the recent event, Mr Corbett wryly observed it was a bit mean for Gisborne to cop it while the katabatic effect stripped the air of its moisture over our hills, and gave Bay of Plenty, Rotorua and Waikato warm and sunny weather.
Gisborne District Council hydrologic data officer Greg Hall said neither of the March or April events was extraordinary for the district, although some spots had exceptional rainfall.
The two deluges arrived only a couple of weeks apart and it could be the frequency, not just the events themselves, that caused the damage.
Some locations had large dumps of rain this month — over the 5-6 days the Pyke’s Weir raingauge on the Te Arai recorded 353mm; the Monowai Bridge gauge on the Waimata River recorded 263.5mm; Pakarae had 351mm; Mangapoike 419.5mm; and Te Puia topped the lot with 430mm, of which 267mm fell in just one day.
Mr Hall says isohyetal charts showing the concentration of rain across the district clearly pinpointed those locations which got hammered. (see charts)
The New Zealand Climate Change Programme — which combines data from across the country’s scientific sector and reports to the government — says we can expect more frequent extreme weather events. This is in line with major reports from the world’s scientific community concerning climate change.
The rise in extreme events is linked to rising air and ocean water temperatures, with weather systems also thermally driven.
The Ministry for the Environment says although it expects Gisborne’s climate to be warmer and drier overall in the next 70-100 years, heavy rainfall and flooding events could be up to four times more frequent by 2070.
The Ministry says the severe weather events will continue to come from the east and southeast, and if they do become more frequent, there will be a significant rise in associated costs to farming and damage to infrastructure.
The National Institute for Water and Atmospheric Research (Niwa) says the outlook through to the end of June is for normal to above normal rainfall, but with temperatures near average.
Sea temperatures are expected to stay cooler than normal.
With more than half of the month still to go, Gisborne has had just over 200mm — about twice its normal April rainfall.
The biggest April rainfall total for Gisborne since records started in 1937 is 373.2mm in 2006.