Ocean composition, layers and circulation

What is in seawater

• Seawater averages about 35 g of dissolved salts in every kilogram (a salinity of 35), mostly chloride and sodium ions, with sulfate, magnesium, calcium and potassium.
• The ions come from rock weathering carried in by rivers and from hydrothermal vents and volcanic gases; the proportions of the main ions are almost the same everywhere, only the total changes.
• Surface salinity rises where water is removed (evaporation, sea ice forming) and falls where fresh water is added (rain, rivers, melting ice).
• Seawater also holds dissolved gases: oxygen and carbon dioxide dissolve best in cold water, which matters for the carbon cycle and for life.

Temperature, salinity, pressure and density

• Seawater density is usually 1020 to 1030 kg m⁻³. Colder water is denser because its particles move less and pack closer; saltier water is denser because more dissolved mass is packed into the same volume.
• Unlike fresh water, which is densest at 4 °C, seawater of normal salinity keeps getting denser as it cools, right down to its freezing point of about −1.9 °C, so the coldest seawater sinks.
• Pressure increases by about 1 atmosphere (101 kPa) for every 10 m of depth, from the weight of water above: P = ρgh plus the air pressure at the surface.
• Water arranges itself by density: the least dense water floats at the surface and the densest fills the bottom of the ocean basins, so the deep ocean is cold, dark and stable.

Try it: pressure and depth

Three layers, and the gradients between them

• The surface mixed layer is heated by the Sun and stirred by wind and waves, so its temperature and salinity are nearly the same from top to bottom.
• Below it, the transition layer has the steepest changes: a thermocline (temperature falls fast with depth), often with a halocline (salinity changes fast), together forming a pycnocline (density rises fast).
• The deep layer, below about 1000 m, is cold (about 0 to 4 °C), dense and barely changes; it holds most of the ocean's water.
• A strong pycnocline acts as a barrier: it takes a lot of energy to mix light surface water down through it, so nutrients from the deep stay below and heat and gases from the surface stay above.

Why the thermocline changes with the seasons near New Zealand

• In the tropics the surface is warm all year, so there is a permanent strong thermocline; in polar seas the surface is as cold as the deep water, so there is almost none.
• At New Zealand's mid-latitudes, summer brings strong sunshine and generally lighter winds: the surface warms, a thin warm mixed layer forms, and a sharp, shallow thermocline develops.
• In winter, less solar energy and stronger, stormier westerlies cool the surface and stir it deeper, so the mixed layer thickens and the thermocline becomes weaker and deeper.
• Because density depends mostly on temperature here, the pycnocline follows: strong and shallow in summer, weak in winter, when deep mixing brings nutrients up and spring sunshine then triggers a phytoplankton bloom.

Wind-driven surface currents

• Wind drags on the sea surface, setting the top layer moving; that layer drags the one below, and so on, each a little slower and turned a little further by the Coriolis effect: the Ekman spiral.
• Averaged over the top 100 m or so, the net water movement (Ekman transport) is at right angles to the wind: to the left of the wind in the Southern Hemisphere, to the right in the Northern.
• The trade winds and westerlies, turned by the Coriolis effect and blocked by continents, drive huge circular gyres; the South Pacific gyre turns anticlockwise.
• Around New Zealand, warm subtropical water arrives from the north-west via the East Auckland Current, and the cold Antarctic Circumpolar Current flows eastwards to the south; they meet along the Subtropical Front, which runs along the Chatham Rise.

The Coriolis effect

• The Earth spins eastwards, and a point on the equator moves at about 1670 km h⁻¹ while a point near a pole barely moves.
• Anything moving freely across the surface keeps its own motion while the ground turns beneath it, so seen from the Earth its path curves: to the left in the Southern Hemisphere and to the right in the Northern.
• The effect is zero at the equator and strongest at the poles, and it only changes direction, not speed; it matters for large, long-lived flows such as ocean currents and winds, not for draining sinks.
• That leftward turn in the Southern Hemisphere sets the anticlockwise South Pacific gyre and the direction of Ekman transport, which decides where upwelling happens.

Thermohaline circulation

• Thermohaline circulation is the slow, deep part of ocean circulation, driven by differences in density from temperature (thermo) and salinity (haline).
• In the Weddell and Ross Seas, winter winds blowing off Antarctica freeze the sea surface; sea ice holds very little salt, so the brine left behind makes the water below saltier as well as near freezing.
• This very cold, salty, dense water sinks down the continental slope and spreads north along the sea floor as Antarctic Bottom Water, the densest water in the ocean; dense North Atlantic Deep Water forms in a similar way near Greenland.
• The deep water slowly warms and mixes upwards, and the Antarctic Circumpolar Current links all the ocean basins, so a parcel of water can take around a thousand years to complete the loop; east of New Zealand a deep current carries this bottom water north.

Try it: density-driven circulation

What Achieved, Merit and Excellence look like

• Achieved describes a process correctly; Merit explains how and why, linking two processes; Excellence discusses how several processes connect into one system for the context given.