‘Super tides’, the weather and coastal flood risk

UPDATED 27/02/2015 – this blog has been updated under the section ‘So what are ‘super tides” with the help of the National Oceanography Centre.

In this joint blog from the Environment Agency and the Met Office, we look at the issue of so-called ‘super tides’.

There has been a lot of media coverage about the potential impact of so-called ‘super tides’ which are due from today (Friday, 20 February) through to Monday.

So what are ‘super tides’?

Tides are governed by the gravitational pull of the moon and, to a lesser extent, the sun. Because the sun and moon go through different alignment, this affects the size of the tides.

When the gravitational pull of the sun and moon combine, we see larger than average tides – known as spring tides. When the gravitational pulls offset each other, we get smaller tides known as neap tides. We see two periods of spring and neap tides roughly every month.

Yet some spring tides are higher than others. This is because tidal forces are stengthened if the moon is closest to Earth in its elliptical orbit (astronomers call this perigee). Tide forces are also enhanced when the sun and the moon are directly over the equater. For ths Sun this happens on or around 21 March or September (the equinoxes). Spring tides are always higher at this time of year. The moon’s orbit also takes it above and below the equator over a period of 27.2 days. Just as with the Sun, the tide generating forces are at their greatest when the moon is directly overhead at the equator.

Very large spring tides occur when these astronomical factors coincide. Approximately every 4.5 years the moon is closest to the Earth, and is also overhead at the equator, at either the March or September equinox.

In some places, these extreme tidal conditions can cause water levels to be 0.5m higher than a normal spring tide, but the weather can have a greater impact than even these largest of tides

What is the role of the weather in sea levels?

It’s important to realise that just because we are expecting big astronomical tides over the next few days, these won’t cause the highest sea levels we’ve seen – even in the last few years. That’s because the weather can have a much bigger impact on sea level than the 18-year tidal cycle.

Strong winds can pile up water on coastlines, and low pressure systems can also cause a localised rise in sea level. Typically the difference in water level caused by the weather can be between 20 and 30cm, but it can be much bigger.

On the 5th December 2013, for example, the weather created a storm surge that increased the water level by up to 2 metres. Although an estimated 2,800 properties flooded, more than 800,000 properties were protected from flooding thanks to more than 2,800 kilometres of flood schemes. The Environment Agency also provided 160,000 warnings to homes and businesses to give people vital time to prepare.

This highlights the importance of the Met Office and the Environment Agency working together to look at the combined impact of astronomical tides, wind, low pressure and waves on flood schemes to assess the potential impacts for communities around our coast.

Will we see coastal flooding this weekend?

Given the height of the tides there may be some localised flooding. Weather isn’t playing a large part in water levels over the next few days, although strong winds on Monday are likely to generate some large waves and push up sea levels slightly. This is nothing unusual for winter. You can see more about what weather to expect with the Met Office’s forecasts and severe weather warnings.

The Environment Agency and the Met Office are working together to closely monitor the situation, and the Environment Agency will issue flood alerts and warnings as required.

In the Humber Estuary, for example, we are expecting total water levels of between 4.20-4.39 metres – well below record levels of 5.22m.

John Curtin, Environment Agency’s Director of Incident Management and Resilience, said:  “We are monitoring the situation closely with the Met Office and will issue flood alerts and warnings as required.

“It’s possible we could see some large waves and spray and urge people to take care near coastal paths and promenades and not to drive through flood water.

“However, we can only get a warning to you if you’ve signed up to our free service. People can also see their flood risk and keep up to date with the latest situation on the GOV.UK website at https://www.gov.uk/check-if-youre-at-risk-of-flooding or follow @EnvAgency and #floodaware on Twitter for the latest flood updates.”

For those in Scotland, you can see flood updates for your area on the SEPA website here.

For those in Wales, you can see flood updates in English and Welsh on the Natural Resources Wales website here.

You can also see John explaining the Environment Agency’s flood warnings here:

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5 Responses to ‘Super tides’, the weather and coastal flood risk

  1. nuwurld says:

    Sorry Met, but I’m afraid I’m not in complete agreement here. From a description of spring and neap tides you have omitted a period of striking resemblance to the noted 18years.

    You have said,

    “However there is a longer cycle at work too, associated with the gravitational pull of the planets in the solar system. This means we can see additional, albeit relatively small, increases and decreases in the size of spring and neap tides over long periods of time.

    We are currently at the height of those increases, so the astronomical tide is at an 18-year peak – although this is only a few centimetres bigger than a more average spring tide.”

    There is a period of 18.5996years and is in fact, not due to the planets but due to the precession of the lunar nodes.

    The lunar orbit is aligned at 5deg to the plane of the ecliptic. The ecliptic is the plane of the apparent path of the Sun on the celestial sphere, and is coplanar with both the orbit of the Earth around the Sun and the apparent orbit of the Sun around the Earth.

    The nutation period at 18.6 years when the moon and sun can align directly with superimposed gravitational vector components directly aligned results in greatest tidal effect at perigean spring. The Earth is closest to the Sun around Jan 3rd so we are still fairly close at this time. This coupled with an alignment with a perigee moon during nodal alignment produces the largest astronomical tide (acknowledged that would include a full planetary opposition).

    The largest planetary gravitational effect is from Venus and there is a transit this Saturday, however at 0.000113 times the solar effect which is 0.46 of the lunar effect this effect is very small. There is a lunar nodal transit of the ecliptic plane on Saturday as well.

    How do you view the effects of the planets compared with the direct alignment (zero vector cancellation in two planes, ie no eclipse) of gravity from the Sun and Moon at nodal alignment? Can you quantify?

    Regards, Geoff.

    • I omitted to make clear in my post that in a month or two Mars will of course not be in opposition (along the imaginary line I mentioned) but at solar conjunction ie completely on the other side of the Sun to ‘our’ side – thus the line I referred to would need to be extended beyond the Sun in the other direction (I gather that Mars and Venus are quite close to each other in the evening sky at present as viewed from the UK).

    • nuwurld says:

      Dear MetO.

      This is a ‘copy and paste’ like my last quote,

      “UPDATED 27/02/2015 – this blog has been updated under the section ‘So what are ‘super tides” with the help of the National Oceanography Centre.”

      Thanks for (not) responding to notification of errors in your original wording.

      If the original data was correct, that these recent high tides are the highest for eighteen years as in your original wording then these latest high tides are the result of lunar nodal precession, with a nutation period of 18.6 years.

      If you are now saying that astronomical tides were equally high 4.5 years ago, then good luck with that.

      According to my local tide table the last HAT was 02/03/2006, still classed as the HAT for recent times at the point of print. That’s 9 years ago not 4.5.

      However, at 4.5, 9, 18, are you not breaking a fundamental frequency into its harmonics?

      The moon feels the position of the Sun as it rotates around the Earth/moon barycentre, like the solids and fluids of Earth. Its periodic changes in closest approach become synodic with the system rotation with respect to the overall system periodicity, given sufficient time. The rotation of Mercury and Venus are locked to rotation periods around the Sun. The Earth/moon system is vastly more complicated but the moon acts as an extension of the system transferring momentum from the solar system into Earth’s atmosphere/ocean/solid mass as a constant, evolutionary process. The changing of the QBO being an effect of the first available harmonic of the lunar draconian month with the tropical year. Forces being exerted through the gravity differential experienced at points away from the barycentre.

      Solar inflation of the atmosphere into space producing greatest momentum transfer to the Earth lithosphere (with respective lag) modulating fundamental winds.

      If all mass were ‘points’ in space, rather than ‘extended’, there would be no gravitational differential acting upon different parts of such bodies, thus no production of momentum exchanging differential rotation/tidal distortion.

      Regards, Geoff.

  2. revjimbob says:

    ‘However there is a longer cycle at work too, associated with the gravitational pull of the planets in the solar system.’
    What kind of nonsense is this from the Met Office? Shameful stuff from a scientific govt agency. Where can I complain about this?

  3. I looked into this online. A lunar standstill coinciding with last week’s new moon and the risk of high tides exacerbating any weather-caused coastal flooding around 22 and 23 February (there’s a 18.6 year cycle for the lunar standstill – a cycle that is unrelated to the separate approximately 18 year Saros solar and lunar eclipse cycles).

    I recall a ‘major’ lunar standstill from 2006 – see this informative blog (I remember this thing about the moon and the Isle of Lewis from 2006):

    This time it’s been a ‘minor’ lunar standstill. Which involves the moon being closer to Earth than normal (with minimum declination in the sky compared to maximum back in 2006). And at new moon Earth, the moon and the Sun are aligned (syzygy). In addition I understand that both Venus and Mars are currently in positions close to an imaginary straight line drawn outwards from the Sun towards Earth and beyond (Venus being on ‘our’ side of the Sun at present rather than the far side ie inferior conjunction rather than superior).

    The last para here is interesting:
    “However the tides are predicted to be even higher during the spring equinox, March 21 [no it’s 20 March this year], when the Earth, Moon and Sun all line up exactly, in what is known as ‘the syzygy effect,’ which increases the gravitational pull. It will also coincide with an almost complete eclipse of the Sun”.

    So – although nothing to do with the Saros cycle – at and just after new moon on 20 March there will again be spring (as opposed to neap) tides, because around the vernal or autumnal equinoxes the Sun, Earth and moon tend to be more closely aligned than usual – and this alignment is also bringing about the total solar eclipse to the north of the UK earlier in the day (partial eclipse over the UK). So there could be a greater chance of a solar eclipse when a new moon occurs on or close to the date of either equinox, due to the Sun-Earth-moon alignments at such times.

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