The sea is anything but level

Did you know that if you swam from a point south of India to Indonesia you would be swimming 200 metres uphill?

Did you know that the Ecuadorean peak Chimborazo is two kilometres higher that Mount Everest if measured from the centre of the earth?

Those are not the only issues with sea level. When the Swiss and the Germans built a bridge at the border at Laufenberg, the Germans calibrated the height to a benchmark in Amsterdam, while the Swiss used one in Marseilles. They knew there was 27cm difference, but they added where they should have subtracted and met with a yawning gap of 54cm!

Thing is, the earth is not round, the surface isn’t even, freshwater is less dense the saltwater, water expands with temperature, and countries by and large have their own idea of zero. Sea level is gauged to 10 different locations in Denmark. At least the Germans and the Swedes take their cue from Amsterdam. The Brits have had three goes at straightening out their measurements. The second attempt started in 1912, and was completes in 1952. By that time I’m sure the whole country would have tilted.

This conundrum is addressed in an article in the New Scientist (paywalled).

This link explains that the earth’s rotational velocity (1,674.4 km/h) causes the planet to bulge at the equator. The Earth is what is known as an “oblate spheroid”. This is from the Earth2014 global relief model, with distances in distance from the geocentre denoted by color:

Earth has an equatorial diameter of 12,756 km, and a polar diameter of 12713.6 km, nearly 43km less. Mt Chiborazo is closer to the equator than Mt Everest. While it is 6,263.47 meters above the local sea level compared to Mt Everests’s 8,848 meters, depending on who you ask, Mt Chiborazo is the highest protrusion on the earth’s surface.

The difference between India and Indonesia is due to gravitational differences. Mountains and heavier rocks get pulled towards the earth’s centre.

Apparently around 100 different standards are used around the world, but help is at hand:

    In 2002, NASA and the German Aerospace Center launched the Gravity Recovery and Climate Experiment (GRACE) satellite, and seven years later, the European Space Agency launched its Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) mission. GOCE orbited until 2013, while GRACE is still in orbit, and the two now have enough data to make a geoid model accurate to within a few centimetres…Together, the two provide the millimetre accuracy required for, say, building bridges.

Within about five years everything will be standardised relative to a point at the centre of the earth. Thing is, on the surface everything changes all the time.

For perspective, have a think about this:

    In short, objects located along the equator are about 21 km further away from the center of the Earth (geocenter) than objects located at the poles. Naturally, there are some deviations in the local topography where objects located away from the equator are closer or father away from the center of the Earth than others in the same region.

    The most notable exceptions are the Mariana Trench – the deepest place on Earth, at 10,911 m (35,797 ft) below local sea level – and Mt. Everest, which is 8,848 meters (29,029 ft) above local sea level. However, these two geological features represent a very minor variation when compared to Earth’s overall shape – 0.17% and 0.14% respectively.

The mean diameter of the earth is given as 12,742km. I understand the average ocean depth is 3.6km, or about 0.056% of the radius. When you go back 15 million years or more to compare sea levels, the continents were in different places, and the shape of the ocean basins no doubt influenced how much land protruded above the waves.

19 thoughts on “The sea is anything but level”

  1. Interesting Brian. Added to all that is the plasticity of our mantle that rises and falls as pressures change and land masses shift. The The large Chinese Three Gorges dam triggered some 3,000 earthquakes as the stresses were redistributed.
    Australia is migrating roughly north at about 7 cm/year. It is quite a lot over one million years.

  2. Isaac Newton::

    Centripetal forces, gravitational force of Earth on itself, spinning Earth, so Earth shape oblate, seas mimic oblate Earth shape. He got sea tides, Moon’s orbit, planet and comet orbits right, as well as F = Ma.

    Got speed of sound wrong, but even a clever chap can’t be right about everything.

    (Unfortunately, he believed he was right about everything, but that’s another story.)

    The oblate shape is more pronounced for spinning Jupiter, and our spinning Sun, I think.

    School kids can measure Sun’s rotation by taking images of solar surface each day for a week or two and tracking rotation of sunspots on its surface. Solar equator rotates faster than mid-latitudesi I think.

    Reasonably uniform rotation of Sun, planetary orbits, and spin of planets and rotation of their moons suggest origin in a rotating, collapsing disc, shrinking due to Gi ravitational self-attraction. This was Laplace’s theory and has been pursued by Dr Andrew Prentice in his “Modern Laplacian Theory”. Mainstream astrophysics doesn’t accept his “supersonic turbulence” concept, but leans towards rotating discs as the forerunners of solar systems.

    Dr Prentice believes practically every star will have planets.
    Solar system AND moon systems of every planet that has moons, will follow a “Titius-Bode” relationship for orbital radii.

    It’s very interesting.

    BTW, thanks for your post, Brian!

  3. Sea not level?

    I thought it was the playing field we were supposed to be concerned about??!!

  4. Ambigulous, your comment put my mind in a spin.

    Geoff, you are spot on about the Three Gorges dam

    BTW, I believe the Andes mountain range is a late-comer which reached full height perhaps 14 million years ago.

  5. And the Himalayas are still rising as the Indian tectonic plate continues its northward drift at around 5cm/year. Limestone fossils can be found on the peaks of the Himalayas… great excitement for those who place much in the story of Noah and his flood.

  6. There is a related issue. It takes a lot of time for a localized change in level to get to the other side of the world. Relative levels can also be affected by water temperature.

  7. Thankyou Ambigulous.

    There is a related issue. It takes a lot of time for a localized change in level to get to the other side of the world.

    John, especially if the major ocean currents are interrupted.

  8. Yes, the wind is a bigger influence than people generally think, which leads to water piling up against land.

  9. In Queensland, isn’t tropical cyclone damage on coastlines particularly bad when a ‘king tide’ effect raises the sea level, with huge waves arriving regularly?

    Is a cyclone usually associated with strong winds blowing water inland? Or is that only during one of the phases of its visit??

  10. Trade winds can change water levels.
    In cyclones there is low pressure at the center of the cyclone and the winds are spiral ling in towards this center so you would expect levels in the center to be high. You would also expect levels to be high as the low passes over the coast on the side of the cyclone where the winds are blowing towards land. But no, I am not a cyclone expert and have only gone through two cyclones.

  11. Thanks John.

    Two more than me!!

    Your explanation makes a lot of sense. Spiralling due to Coriolis effect, eh?

    Over here in Victoria, Coriolis has this consequence.
    Summer, hot inland.
    High pressure system moves in.
    Clear skies, sun heats up land and forest fuel.
    Low humidity dries out fuel.
    Conditions for more intense bushfires.

    As H moves east, out over the Tasman Sea, winds turn northerly and strengthen. These are hot winds from the continental interior. Bushfire risks rise.

    Arsonists emerge from under their rocks.
    If a fire begins, strong northerlies help it spread rapidly along a north-south line, roughly. “Spotting” through ember transport by winds extends the fire front.

    Then a cold front arrives from the south-west, and if the fires are not already controlled, the N-S line now sweeps across as a very wide front, heading towards the north-east.

    As if the northerlies weren’t difficult enough….
    Cold front may be ‘dry’ with not much rain to quench fires, or may have lightning strikes to help start new ones.

    (But cyclones are devastating too, as are Brisbane floods.)

  12. When the water is warm enough, and when there is a lot of it (ie deep water) the water evaporates and rises, cools and forms thunderstorms. These can occur in clusters that can coagulate.
    As the air rises, there is the phase change “vapour into liquid water”. This releases massive amounts of heat – recall perhaps the amount of heat energy still required to convert water at 100C into steam, about 780 calories per gram I think. So the heat release is massive. This heat further energises the air in the system , and it becomes self forcing as the rising air reduces pressure and draws in more warm moist air from the surrounding sea. It will start to rotate because of the Coriolis effect, forming a vortex, reducing pressure even more,drawing in more moisture and literally feeding on the energy in that water – hence the need for a lot of water. And it explains why cyclone end pretty soon after crossing a coast.
    Cells form inside the walls and these distribute the heavy rains associated with cyclones.
    The low pressure will cause a dome effect in the centre areas, but as the cyclone approaches land the nature of the seabed has a profound effect on the water level. The wind-driven water is subject to the tide, the topography of the seabed, the characteristics of the land (how the sea might be funneled or focused) and other elements contribute to a storm surge.

    In Cairns right now the papers are reporting that we need a cyclone to cool down our water and stop the current round of coral bleaching. Those reports are pretty right in that cyclones do “use” up a lot of heat but also cause up-welling of the cooler deep waters.

  13. Crikey, Geoff!


    BTW, I must apologise to the shade of Monsieur Coriolis, if he thought I was attributing
    Hot inland
    Low humidity
    Presence of eucalyptus oil
    Cold fronts
    To his Effect. No, just those northerly winds, part of the ‘anticlockwise’ wind rotation around a High, where the slow outflow from H to L regions begins the rotation.

  14. Coriolis has a lot to answer for, and certainly for skewing winds one way or tuther. Probably responsible for all the ultra-sound effects attributed to wind turbines too.

    Interesting all the clamour about power shortages, gas, coal and renewables. Now Musk is saying he can fix SA’s problem with a bunch of Li-ion batteries in 100 days flat.
    I have an on-going interest in scaled up batteries and I am looking at them right now, ‘will be for a few months. It is revealing stuff.
    Whilst it is early days, I’m leaning to the flow battery as the best option for home/industrial applications where physical space is available. Li-ion is good where space is limited in say a home. But the Li battery has negative features that the redox (flow) batteries do not – e.g. relatively low life span, discharge sensitivity, heat disposal and some serious social and environmental issues. These latter problems are disturbing but I need to read more before taking a very harsh stance.

    In any event it seems there will be great uptake of home battery storage systems, especially as prices decline. I am sure the Utilities are wondering what to do as their networks are funded by a diminishing number of clients. It’s funny to see the coal lobby screeching about great price increases looming as some of the coal burning fleet is retired and they want the government to subsidise the so-called clean coal options. I suspect that people are adding the cost of “clean coal” to the reasons for getting off the grid. It seems to me that the LNP energy policy is failing very badly and in fact the policy has been largely captured by the voters who elect for solar power.

  15. On that last non-marine point, do you mean that the consumers are leading in that policy area by their purchasing rooftop solar with or without battery storage?

    Wouldn’t be the first time the autonomous consumer, homo economicus (and his lady wife, who has just as much say in power bills) in full flight, have led the charge. Jumpy will be pleased.

    In this country, we seem to be quick on the uptake of new gadgets, sometimes, regardless of the wishes of politicians.

    “Never stand between consumers and their self-interest!”

  16. Ambigulous, yes I did mean that. I think many roofies will take up the battery option after solar panels have demonstrated that they are worthy of further investment…and the price has dropped.
    I dunno about Jumpy – he has alternative logic (no offence Jumpy, I like creative stuff)

    Taking up gadgets – yes, certainly I do. But now we have The Internet of Things that seeks to bring gadgets under the control of the ether, hopefully controlled by the authorised person.
    There was a report lat week, associated with Assange, that a Samsung TV could be intercepted and used as a listening device. I asked a guy associated with the I o Things – he knew his stuff, was credible – and he said he had seen data being streamed from a Samsung. First hand account. I am concerned by this.

  17. about The Jump
    I meant, here’s a possible instance of individual autonomy as a purchaser, pursuing self-interest in collaboration with innovative private companies, and not much concerned with overarching govt policy.

    Then again, there are regulations about feeding- in, about domestic electrical safety, fire regulations, we’re all forced to use an AC grid at 240 Volts; bastards!! bastards!! When will they let the little bloke generate and use at 17V DC and do all her own wiring, and use grey water on the veges, and refuse to vaccinate the nippers, and home school using Google, and …. Bastards!!!!!

    Hi, Jumpy

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