New Zealand lies in the path of eastward-flowing currents, which are driven by winds that blow across the South Pacific Ocean. These winds – the south-east trades to the north, and the roaring forties to the south – drive water along the equator, down Australia’s east coast in the East Australian Current, and across the Tasman Sea. The flow splits around the western side of New Zealand and joins up again near the Chatham Rise, east of the South Island. Bathed by relatively warm water from the subtropics, New Zealand has a temperate climate.
The East Auckland Current flows south-east along the north-east coast of the North Island. The current travels at speeds up to 50 centimetres per second. The origins of these waters are tropical; occasionally tropical-reef fish are found at the Poor Knights Islands. Typical surface temperatures at the current’s northern reaches are 20–22°C in summer, and 15–16°C in winter.
Part of the East Auckland Current continues south, where it becomes the East Cape Current. When this current encounters the Chatham Rise, it is forced offshore and flows eastwards along the rise. Inshore, the Wairarapa Coastal Current flows north-east along the Wairarapa coast, bringing relatively cool water to the region. As this water moves up the coast, some gets pulled into the East Cape Current, so that the Wairarapa Coastal Current probably does not extend north of Māhia Peninsula. Temperatures in the Wairarapa Coastal Current are 1–2°C cooler than in the East Cape Current.
Several permanent or nearly-permanent eddies (circular currents) have been identified around New Zealand, such as the North Cape and East Cape eddies and the Wairarapa Eddy. Permanent eddies tend to arise where the sea floor forces currents to make large changes in direction.
Currents to the west of New Zealand are weaker and more variable than those along the east. The West Auckland Current flows southwards along the west coast of the North Island from North Cape to Raglan, where it is met by north-flowing currents in the North Taranaki Bight. In the South Taranaki Bight, the D'Urville Current flows south-east and through Cook Strait.
The Southland Current is the main current along the east of the South Island. It flows north-eastwards past Stewart Island and along the Otago coast, reaching speeds of 25 centimetres per second, and extending 130 kilometres offshore. At the Chatham Rise, it veers east to become part of the subtropical gyre (giant circular current on the surface of the ocean).
On the South Island’s West Coast, the Westland Current flows north until it reaches the south Taranaki Bight, where it contributes to the D'Urville Current.
South of New Zealand, the Southern Ocean’s westerly winds drive the Antarctic Circumpolar Current, which flows continuously around the globe. This is the world's strongest ocean current, reaching down 4 kilometres to the ocean floor and transporting about 100 times the volume of water of all the world’s rivers. The current does not directly affect New Zealand’s main islands. However, the Campbell Plateau to the south deflects the current south and channels it north past the Antipodes Islands before the flow resumes its eastward course. Further south, cold, downward-moving winds, known as katabatic winds, flow off Antarctica. These winds drive a westward current and form a clockwise gyre in the Ross Sea.
New Zealand tides are moderate by world standards. The tidal range is 1–2 metres, and tidal currents are generally about 2 kilometres per hour (1 knot). The exception is Cook Strait, where tidal currents can be much stronger.
Tides are mainly driven by the gravitational pull of the moon and sun on the ocean. The moon has the most influence as it is the closest to earth.
The moon rotates around the earth in about one day. Its gravity attracts a bulge of water (high tide) that travels around the earth. The reason there are two high tides is the common centre of mass about which the moon–earth system rotates. The centre of rotation is about two-thirds out from the centre of the earth (rather than in the centre itself). The earth acts like a centrifuge, causing a second bulge in the ocean opposite to the moon.
When the sun, moon and earth line up, their gravities act together and cause especially high and low tides known as spring tides (which have nothing to do with the season). When the sun and moon are at right angles their gravities cancel each other out, causing small tides known as neap tides.
Tides are the result of complex natural influences, and are produced by as many as 62 constituents. The most important of these is the gravitational pull of the moon and sun. The moon does not revolve around the equator, but at an angle to it. So the two bulges of water that travel around the earth are above and below the equator. This means that some places have two high and low tides a day (semi-diurnal), while other places have daily (diurnal) tides.
A diurnal tide occurs in the Ross Sea around Antarctica. The tide returns once a day (every 24.84 hours), and its height reduces to almost zero every 13.66 days.
New Zealand has semi-diurnal tides. This twice-daily rise and fall of sea level is primarily caused by the main lunar tide (known as the M2 – the ‘M’ stands for moon and the ‘2’ for twice a day). The time between high tides varies from day to day because the orbits of the moon around the earth, and the earth around the sun, are not exactly circular. On average, the moon rotates around the earth once every 24.8 hours, so that the M2 occurs in half this time – 12.4 hours.
Because the orbits of the moon and earth are regular, it is relatively straightforward to predict the tides far in advance. All one needs is a month-long record of sea level at the site of interest. Tide gauges can be used to measure the height of the tide every few minutes. However, not all places have gauges, so computer models have been developed to compute tidal constituents for all coastal locations. A New Zealand-specific tidal computer model is run by the National Institute of Water and Atmospheric Research (NIWA) in Wellington.
French Pass is a narrow gap that separates D’Urville Island from the South Island. The tides flowing through it can create a maelstrom. When the tides change, the currents are sometimes strong enough to stun fish, which float to the surface. Māori living on the island could collect the fish and enjoy their easy meal.
Despite having a smaller tidal range (the height difference between high and low tide) than most places in New Zealand, Cook Strait has some of the strongest tidal currents in the world. The reason is that the main lunar tide is out of phase on either side of the country. High tide arrives on the Pacific Ocean side of the strait five hours before it arrives at the Tasman Sea side – when it is high tide on one side it is nearly low tide on the other. This difference in water level drives very fast tidal currents – up to 1.4 metres per second (3 knots) – through Cook Strait and into the Marlborough Sounds. Tory Channel and French Pass have currents that can reach 2 metres per second (4 knots).
When the wind blows over a calm sea, it produces small ripples that grow in size and wavelength (the distance between wave crests). The waves gather speed until they are moving at the same speed as the wind, when equilibrium is reached. However, the height and length of waves, and the period between them, rarely remain constant.
Seafarers have long differentiated between waves that are actively generated by a prevailing storm and those that arrive from a distant storm. A ship in a storm experiences a ‘wind sea’, whereas surfers ride ‘swell’. Waves in a sea show a large range of sizes and directions, while swell tends to be more uniform and regular.
Waves are described in terms of their height, length, period and direction. A wave’s height is measured from the bottom of its trough to the top of its crest. Its length is the distance between successive crests. The period is the time between successive wave crests as they pass by.
Waves in any given sea vary considerably, and oceanographers use the term ‘significant wave height’ as a measure of the characteristic wave size. This is the average of the highest one-third of waves in a sea.
Waves may have been partly to blame for the demise of New Zealand’s inter-island ferry, the Wahine, during the storm of 10 April 1968. Waves of 12–14 metres and winds blowing to 185 kilometres per hour (100 knots) in the same direction the Wahine was travelling meant that the rudder was useless. The ship veered sharply to port, struck Barrett Reef and subsequently sank.
New Zealand lies across two wind zones. To the north, south-east trade winds dominate, although an occasional cyclone will come down from the tropics. The rest of the country is in the path of a broad band of westerly winds that span the middle latitudes of the southern hemisphere – often called the roaring forties. Among the stormiest seas in the world, the roaring forties extend over most of the southern part of the Tasman Sea and the Southern Ocean. This area has some of the world’s largest waves – maximum wave heights regularly exceed 4 metres (by comparison, waves are less than 2 metres high in the subtropics).
Swell generated in the Southern Ocean arrives on the west and south-west of New Zealand. These coasts have the country’s most energetic waves. On the north-east, waves are much smaller because the North and South islands block waves from the south-west.
The computer models that measure the growth and development of waves are based on wind fields. This data is assembled from a variety of sources, including satellite observations and models of the atmosphere. These models become complex near the coast, when they also need to take into account the effects of bed friction (waves dragging on the sea floor).
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, 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.
Changes in climate are not caused only by conditions in the atmosphere. The top few metres of the ocean can store as much heat as the entire atmosphere, and relatively small changes in ocean circulation can move vast amounts of heat around the planet.
The ocean acts as a long-term reservoir for carbon dioxide, which is a primary greenhouse gas (a gas that absorbs radiation in the atmosphere). Microscopic marine plants (phytoplankton) in the surface waters take up dissolved carbon dioxide during photosynthesis, just as land plants do, and convert it to organic carbon. About 10–30% of the carbon taken up by phytoplankton sinks out of surface waters, eventually settling in the abyssal depths, where it is effectively removed from the ocean–atmosphere system. This process is known as the biological pump, and is thought to have a major role in mitigating climate change.
Cooling of surface waters near Antarctica, and in the Greenland and Labrador seas, creates cold, heavy water that sinks, setting up what is known as the great ocean conveyor belt. This brings warm surface water from the equator to the poles, and returns cool waters at depth. The cooler deep water mixes so that it warms and rises as it travels towards the equator. The conveyor belt slowly moves water from one ocean basin to another, redistributing heat, salt and nutrients. This recirculation smooths out the earth’s temperature – without it, the poles would be considerably colder and the tropics significantly hotter.
Water masses are oceanic waters that have different temperatures or salinity levels from each other. The main water masses around New Zealand are subtropical and subantarctic waters. Subtropical water has arrived from near the equator. Heated and evaporated by the sun, it can be as warm as 22°C and as salty as 35.6. Subantarctic water is relatively cool and fresh – rarely exceeding 10°C, with a salinity of 34.4.
The journey of water is estimated to take 1,200 years to come full circle. The conveyor belt moves past New Zealand at a depth of about 2–5 kilometres along the east of the Campbell Plateau, and around Chatham Rise and the Hikurangi Plateau.
ENSO is a fluctuation of the ocean–atmosphere system in the tropical Pacific Ocean that occurs every 3–7 years, and has worldwide effects.
During normal years, trade winds on either side of the equator blow water westward so that it piles up in the western Pacific, near New Guinea – like a fan blowing water across a bath. As it crosses the Pacific, the water is warmed by the sun. Waters near New Guinea are as hot as any on the planet, and are called the warm pool.
During El Niño years the trade winds weaken and the warm pool flows back to the east, leading to a warming of the sea in the eastern equatorial Pacific Ocean. The increase in temperature (and corresponding cooling in the western Pacific Ocean) changes the heat that moves from the ocean to the atmosphere, which subsequently alters the winds over the Pacific Ocean. Around New Zealand at these times, the winds tend to prevail more from the south-west than usual. In El Niño years the weather is wetter in the west and south, and drier in the east and north.
This simplest of ocean properties turns out to be very difficult to consistently define. Originally, ocean salinity was defined as the amount of all salts in the water measured in parts per thousand by weight. However, in modern practice salinity is measured electronically and is defined as a ratio that has no units. Thus, the old measure of ocean salinity of about 35‰ (parts per thousand) becomes 35.
These large-scale processes can have an impact on particular parts of the ocean. For example, during the 1992–93 El Niño weather pattern, the north-east coast of New Zealand experienced dense and extensive plankton blooms. These were so severe and long-lasting that nearly the entire kelp population from Whangārei to Great Barrier Island was killed.
La Niña is a weather pattern consisting of stronger trade winds and a larger warm pool. Changes in the atmosphere and ocean tend to be the opposite of those during El Niño. In La Niña years, New Zealand’s winds tend to be more from the north-east than normal. The weather is drier in the west and south, and wetter in the east and north.
New Zealand’s dynamic weather and geology combine to form some remarkable crossing points between land and sea, through which currents and tides flow.
An extreme example of ocean stratification is found in Fiordland. A remarkably high rainfall creates runoff from the steep sloping sides so that a brackish layer floats over the saltier and denser ocean water of the fiords. Fresh surface waters in Doubtful Sound can have as little as 20% of the salinity of oceanic waters. The top layer is fed by highly variable rainfall and moved around by tides and wind. Organisms living in this environment need to be well adapted to the varying salinity.
Swimmers and divers at New Zealand beaches notice layers of cold and warm water. Warm water is lighter than cold water and floats to the surface. The salt content also affects density. The combined result is a stratified ocean with distinct layers sometimes only a few metres thick, which mix only during strong storms.
In New Zealand huge quantities of sediment are carried downriver and form buffer zones between fresh and sea water in the form of muddy tidal estuaries. These estuaries, like Manukau Harbour with its 4-metre tidal range, change with the tide every six hours from coastal lakes to expansive mudflats crossed with river channels. This variability makes it very difficult to model and study their behaviour.
Tidal filling and emptying of the estuary basins can generate complex, dangerous wave patterns and strong currents that travel through relatively narrow passages. For example, Manukau Harbour entrance sustains currents travelling at speeds of around 1 metre per second, which swirl around the headlands and make boating difficult.
Heavy rainfall can bring about plumes of sediment at river mouths. Their evolution directly affects life in the nearby sea and shoreline. Plumes are a mixture of fresh water and the river’s sediment load, with some dilution caused by currents. The plume emerging from Pelorus River, in the Marlborough Sounds, moves out into Pelorus Sound at almost 1 metre per second.
On the open coast, plumes are pushed along by the wind and tides, and slowly drop suspended sediment. The sediment accumulates depending on the local waves which, if strong enough, will re-suspend the sediment and keeping it moving. Plumes are easily visible from the air anywhere along the New Zealand coast where rivers meet the sea.
In the winter of 1874, scientists on the Challenger expedition threw dredges over the side of the ship when it visited New Zealand waters, probing the unseen depths of an ocean they had little understanding of. They were constrained by a cursory knowledge of the major ocean currents, with only the most rudimentary instruments at their disposal.
Today’s oceanographers use satellites to map the ocean surface. Space-borne sensors can determine surface temperatures, the height of the ocean surface (which changes with the currents and temperature), the size and direction of waves, wind strength, and the amount of chlorophyll present.
But even the most sophisticated satellite technology cannot provide analyses of the ocean beyond a few centimetres deeper than the surface. Research voyages remain the best way to explore the depths. In New Zealand these trips typically last a month or more, during which time scientists lower sophisticated gear over the side of their vessel, sometimes immediately pulling it back on board, at other times leaving it moored for a year or more to record changes in ocean properties. Valuable data is archived and synthesised to build a picture of how ocean environments behave.
Ships are expensive and cannot be everywhere. An ambitious approach to mapping the ocean interior uses smart floats – floats with computers in them that adjust their buoyancy to rise and fall in the top 2 kilometres of the ocean and record position and water properties as they go. When a float is at the surface it transmits its data to a base on land. There are more than 1,000 floats drifting through the world’s oceans, including the Pacific Ocean and Tasman Sea.
Computer models can be used to predict ocean behaviour in places and times for which there is no data. For example, they can forecast how ocean currents might behave in future climates. Most models simulate the ocean as a grid of vertical and horizontal boxes, and determine how water moves from one box to the next.
Simulations can cover all the world's oceans (global models), or smaller areas (regional models). The most sophisticated global models also take into account the atmosphere and how it interacts with the sea (global coupled models).
This technology is in its infancy. Modelling is most effective when used in conjunction with observation. Mathematical or laboratory approximations of oceans work best when based on known ocean behaviour.
Acknowledgements to Mike Williams, Melissa Bowen, Philip Sutton, Brett Mullan, Rob Bell, Richard Gorman, Graham Rickard, Stephane Popinet and Mark Hadfield.
Harris, T. F. W. Greater Cook Strait: form and flow. Wellington: DSIR Marine and Freshwater, 1990.