Ocean heat, tides, waves and El Niño

The ocean as a heat store

• Seawater has a specific heat capacity of about 3990 J kg⁻¹ K⁻¹, four times that of air (about 1005 J kg⁻¹ K⁻¹), and the ocean has about 270 times the mass of the atmosphere.
• So the ocean warms and cools slowly and stores enormous amounts of energy: it has taken up about 90% of the extra heat trapped by greenhouse gases since the 1970s.
• Water also absorbs a lot of energy when it evaporates (latent heat), which is released into the atmosphere when the vapour condenses into cloud, often far away.

Try it: land and sea heat differently

Moving heat towards the poles

• The tropics receive more solar energy than they radiate away and the poles less, so the ocean and atmosphere carry heat polewards; the ocean carries most of it in the tropics and the atmosphere most of it at higher latitudes.
• Warm western boundary currents, such as the East Australian Current, carry tropical water south; part of it crosses the Tasman Sea and flows past northern New Zealand as the East Auckland Current.
• Deep, cold water returns towards the equator in the thermohaline circulation, completing the loop.
• Because the sea warms and cools slowly, coastal places have smaller temperature ranges than inland ones: Hokitika's summers and winters are much closer in temperature than Alexandra's.
• When ocean heat builds up, marine heatwaves follow: in the summers of 2017 to 2018 and 2021 to 2022, seas around New Zealand ran several degrees above normal, stressing kelp, fish and shellfish.

Waves carry energy, not water

• Wind transfers energy to the sea surface. The size of the waves depends on wind speed, how long it blows and the fetch (the distance of open water it blows over), which is why the Southern Ocean's westerlies make big swells.
• As a wave passes, each water particle moves in a near-circle and returns almost to where it started; the energy travels on, not the water. The circles shrink with depth and almost vanish below half a wavelength.
• In deep water a wave's speed depends on its period: v = gT ÷ 2π, so long-period swell outruns short wind chop and arrives first at the coast.
• In water shallower than about a twentieth of a wavelength, speed depends only on depth: v = √(gd). As a wave reaches the shore it slows, its wavelength shortens, its height grows, and it breaks.
• A wave's energy is proportional to its height squared, so doubling the height means four times the energy.

Try it: waves carry energy

Tides: energy from the Moon and Sun

• The Moon's gravity pulls hardest on the side of the Earth facing it and least on the far side, stretching the ocean into two bulges; as the Earth turns, most coasts get two high tides about every 24 h 50 min.
• The Sun does the same, about half as strongly. When Sun, Moon and Earth line up (new and full Moon) the effects add: spring tides with the biggest range; at the quarter Moons they partly cancel: neap tides.
• Around New Zealand the main tide travels as a wave anticlockwise round the country, taking about 12 hours to go round, so high tide comes at different times along the coast; across Cook Strait the two ends are close to opposite in the cycle, which drives the strait's strong tidal currents.
• Tidal currents rub against the sea floor, turning the energy into heat; harbours with a large range and narrow entrances, such as the Kaipara, have been studied for tidal power.

Try it: bulges, springs and neaps

El Niño and La Niña

• Normally the south-east trade winds push warm surface water west across the equatorial Pacific, piling it up near Indonesia and Australia, where air rises and rain falls; the air returns east aloft and sinks over the cool eastern Pacific. This loop is the Walker circulation.
• With warm water pushed west, the thermocline is deep in the west and shallow in the east, so cold, nutrient-rich water upwells off Peru and Ecuador and supports huge fisheries.
• In an El Niño the trades weaken or reverse; warm water slides back east, the eastern thermocline deepens, upwelling stops bringing up nutrients, and the rising air and heavy rain move to the central and eastern Pacific.
• In a La Niña the trades are stronger than normal: more warm water piles up in the west, the east is colder than usual, and the western Pacific is wetter.
• The atmospheric half is the Southern Oscillation, measured by the Southern Oscillation Index (SOI) from air pressure at Tahiti minus Darwin: strongly negative in El Niño, positive in La Niña. Ocean and air feed back on each other, so the whole pattern is called ENSO.

What ENSO does to New Zealand

• El Niño: more south-westerly winds in summer and southerlies in winter, so New Zealand tends to be cooler; the West Coast of the South Island is wetter, while the north and east of both islands are drier, raising the risk of drought in places like Hawke's Bay, Canterbury and Marlborough.
• La Niña: more north-easterly winds, warmer seas and air around the country, and more rain in the north and east of the North Island, often from ex-tropical cyclones; the south and south-west of the South Island can be drier.
• ENSO shifts the odds rather than deciding the weather: NIWA estimates it explains less than a quarter of the year-to-year changes in New Zealand's seasonal rainfall and temperature, alongside the Southern Annular Mode and local weather.
• Fishers notice it too: warm La Niña seas bring subtropical fish south, while El Niño's cooler, windier conditions change where fish feed.

What Achieved, Merit and Excellence look like

• Excellence answers follow the chain of cause and effect all the way from the wind to the outcome, and say why for every link.