The continental shelf is the relatively shallow region, only a few hundred metres deep, surrounding the land. New Zealand has a very large continental shelf. Current flow along these shelf regions is strongly affected by the rotation of the earth.
The Coriolis force
The Coriolis force, named after the 19th-century French engineer–mathematician Gustave Gaspard Coriolis, is caused by the earth's rotation. If the earth did not rotate, a parcel of water set in motion (by a gust of wind, for example) would travel in a straight line. However, rotation causes the water to move in a curve – to the left in the southern hemisphere, and to the right in the northern hemisphere. The Coriolis force depends on the speed of the water – the faster the water goes, the stronger the force. It also depends on the latitude – the force is zero at the equator and strongest at the poles.
The Coriolis force is the reason that hurricanes rotate anticlockwise in the northern hemisphere and clockwise in the southern hemisphere (where they are called cyclones). The force is weak in New Zealand (about 4,000 times weaker than gravity), so it only affects systems that are many kilometres in size, such as storms and large oceanic eddies.
Because of the earth's rotation, southern hemisphere winds that blow along the coast with the shore to the right generate currents that drag surface water offshore. The near-shore water is replaced with deeper, cooler, nutrient-rich waters in a process called coastal upwelling. Once near the surface, the nutrient-rich waters cause an increase in ocean productivity (the growth of microscopic marine plants). In New Zealand, upwelling occurs along the eastern shores of Northland, and in places along the South Island’s West Coast. Extreme upwelling has contributed to the disappearance of kelp beds and changes in crayfish populations.
The ocean waves we can see are created by the difference in density between the air and sea water. Density variations beneath the water surface also allows for underwater waves that are hidden from view, known as internal waves. The difference between light surface waters and the heavier underlying water is less pronounced, which means that waves, especially when forced by the tides, can sometimes be enormous. The shelf seas around New Zealand’s east coast, between North Cape and the Firth of Thames, support such waves. They can be more than 80 metres high and can be seen from space.
When ecologists speak of connectivity, they are referring to the processes that allow populations to spread over a broad area. For many organisms, particularly those that are not very mobile, connectivity is related to coastal currents – they are swept along by the water. A number of Australian organisms have colonised New Zealand waters, having drifted across the Tasman Sea on the East Australian Current.
Although arrows are drawn on maps to indicate coastal and ocean currents, there is usually a degree of variability that graphical representation cannot show. Within currents there are eddies, mixing waters, stratification and other processes at work. Variability influences the survival rate of organisms that drift wherever currents take them.