PLOT OF THE MONTH

Primary M2 Tide for New Zealand
Auckland, New Zealand

Contributed by: Derek Goring National Institute of Water and Atmospheric Research Ltd.

Derek Goring is the Principal Scientist of the Coastal Hydrodynamics Group, based in Christchurch, on the South Island of New Zealand. The Coastal Hydrodynamics Group is part of the National Institute of Water and Atmospheric Research LTD (NIWA). It is a government-owned, but fully autonomous company.

Derek is involved in research of coastal, river and eco-hydraulics - the mechanics of flowing water. The goal of this research is to understand how sea level varies around New Zealand's coast.

The plot of the month shows how the primary M2 tide for New Zealand propagates in a counterclockwise direction around the country (a Kelvin wave), completing the cycle every 12.4 hours. Red represents high tide, blue represents low tide, and green represents zero elevation. It shows that at any time there is a high tide somewhere on the New Zealand coast. This is very unusual and occurs only one other place - Iceland.

The stretch of water between the North Island and the South Island is Cook Strait. At times there is high tide (red) on one side and low tide on the other. This means there is a two meter difference in sea level across a relatively small distance. This causes a very strong tidal flow, as seen in the tidal currents animation below.

These results were computed from a finite-element hydrodynamic model of New Zealand's Exclusive Economic Zone which extends from 24 to 65 ºS and 156 to 210ºE. The model was driven on the open boundaries by tide data derived from the TOPEX/Poseidon oceanographic satellite. The results were verified by accurate measurements of sea level at 20 locations around the coast.

The model produces amplitude and phase for water surface displacement, as well as eastward and northward velocity for each tide. This is accomplished using about 30,000 nodes in an irregular grid, as shown in Figure 3. A FORTRAN program is used to synthesize the data at around 40 intervals over a tide cycle (12.42 hours). Each of these intervals is written as a zone using the Tecplot binary TecIO. The resulting .plt file is loaded into Tecplot, a contour plot is specified, and then animated over the 40 intervals (or zones).

Figure 4 shows the way tidal data are usually portrayed with a cotidal chart. Draughtsmen plotted them by hand before NIWA began using Tecplot. Amplitude is shown as colored contours, and phase are displayed as lines. In New Zealand, where the tide is so complicated, it is difficult to absorb all the information in a cotidal chart. However, in a Tecplot animation information becomes abundantly clear.

"When we first animated the tides back in 1995 using a very early version of Tecplot, we shouted 'Eureka!' We finally understood how the tide worked! Subsequently, we used that animation to persuade politicians and stakeholders that we were doing good work that deserved generous, long-term funding. That strategy worked, because everyone can get an appreciation of how things work from an animation."

More details and plots are available on the Coastal Hydrodynamics Group's Web page.

Tecplot Tips:

• Writing binary Tecplot files from FORTRAN using the TecIO facilities makes loading very fast. Chapter 11 of the Tecplot Reference Manual explains how to do this in an easy to understand manner.
• Running animations from Raster Metafiles using Framer is more flexible than using AVI files, especially for presentations using PowerPointT, where the speed of the animation can be preset.
• A flooded contour plot on a black background makes a stunning animation for PowerPoint presentations.