NIWA provides weather and sea conditions for the Rena oil spill


NIWA is using forecast models to show us the offshore conditions around the grounded Rena, and the likely path of the oil plume, as fuel oil washes up on Bay of Plenty beaches.

Dr Charlotte Severne, NIWA Chief Scientist Oceans, says, "Oil spreads quickly and its movement is determined by tides, currents, and wind direction. People need to know what the marine conditions will be, including the weather, waves, tides and currents in the area."

NIWA scientists are running models to predict these conditions, to assist with salvage operations and their timing.

NIWA's Dr Mike Revell is looking at rain, wind, and sea-state with EcoConnect, NIWA's environmental forecasting and information service. EcoConnect is running a forecast at around 1.00 p.m. each day.

The model indicates that winds over the Tauranga harbour entrance region should peak from the north -north-east at about 45 km/h gusting up to 60 km/h during Wednesday morning gradually easing and turning more easterly during the day. We are expecting onshore winds with frequent periods of rain. On Thursday and continuing into Friday winds are expected to turn to the westerly quarter – offshore – and reduce to 20 km/h gusting to 30 km/h with the rain clearing. We expect significant wave heights of 3 to 4 metres today, gradually reducing to 2 metres tomorrow.

In the next couple of days, NIWA scientists expect results from the River and Coastal Ocean Model (RICOM), they are applying to the Bay of Plenty. This will be able to forecast which way the plume will travel, and where the oil will hit the shore. It runs a 48-hour simulation using the best available wind data, generated every 12 hours, providing an animation of the oil's movement.

NIWA is also running an ocean model called ROMS (Regional Ocean Model System). ROMS is an ocean and coastal hydrodynamics model that looks at the bigger picture of ocean currents including the winds that drive them. It uses a 48-hour simulation using the wind data, generated every 12 hours, providing a map of the current pattern.

Tides at Astrolabe Reef

Besides weather and wave information, knowing what the timing and height of tides at Astrolabe Reef will be is critical to the salvage operation.

Predictions of tidal height for the reef have been provided to Maritime New Zealand by NIWA. Unfortunately, the Rena struck the reef soon after high tide around 1.8 m above the local lowest-tide datum (see plot). This means that there are more limited opportunities for re-floating the vessel than if the grounding happened at low tide.

NIWA's predictions of tides at the reef for the year show that 29-30 October will bring the highest high tides, known as "king tides" (2.2 metres above local lowest tide), and also lowest low tides for 2011. The king tides provide a tidal difference of an additional 0.4 metres in tide level compared with the tide level when the grounding occurred. This will also depend on barometric pressure and winds which can raise or lower the sea level relative to the predicted tide.

These king tides or perigean-spring tides peak every 7 months. They occur a few days after New or Full Moon coincides with the Moon being closest to the Earth (or perigee), in its monthly orbit around our planet. In this case a New Moon and the Moon's perigee occur together on 27 October, with the highest tide ranges two to three days later. This astronomical coincidence provides the year's best window of opportunity for re-floating the Rena.

You can see a feed of the EcoConnect data here.

Astrolabe Reef tides. Credit: NIWA Tides
RiCOM grid coloured according to depth. In deeper water larger triangles can be used. In shallower areas, especially those with complex coastline, the grid grades to smaller triangles for finer resolution. Solid colours in the figure indicate where the triangles are so fine that they blend together.
NIWA meteorologist Mike Revell checks the latest data from EcoConnect. [Dave Allen]
The coast at Mt Maunganui with Motiti Island in the background. (Rob Bell)