Imaggeo on Mondays: Atmospheric gravity waves

Imaggeo on Mondays: Atmospheric gravity waves

From the tiny vibrations which travel through air, allowing us to hear music, to the mighty waves which traverse oceans and the powerful oscillations which shake the ground back and forth during an earthquake, waves are an intrinsic part of the world around us.

As particles vibrate repeatedly, they create an oscillation, which when accompanied by the transfer of energy, creates a wave.  The way in which waves travel varies hugely. Take for instance a ripple in a pond: vibrations there are perpendicular to the direction in which the wave is travelling – transverse waves. When a slinky moves (or sound waves), on the other hand, vibrations happen in the same direction in which the wave travels – longitudinal waves. Ocean waves are more complex. The motion there combines surface waves, created by the friction between wind and surface water, and the energy passing through the water causes it to move in a circular motion. With a little imagination, it’s not so difficult to visualise these different phenomena.

But not all waves on Earth are so intuitive.

Unlike the waves we’ve discussed up until now, internal gravity waves oscillate within a fluid medium, rather than on its surface. In the Earth’s atmosphere, internal gravity waves transfer energy from the troposphere (the layer closest to the Earth’s surface) to the stratosphere (where the ozone layer is found) to the very cold mesosphere, which starts some 50 km away from the planet’s surface. They are usually created at weather fronts: the boundary where two pockets of air at different temperatures and humidity meet. Air flowing over mountains can also generate them.

Because they propagate across layered fluids (the different layers of the atmosphere, for example), internal gravity waves can be responsible for transferring considerable amounts of energy over large distances, which is one of the main reasons why they are important in atmospheric and ocean dynamics.

But only with improved satellite and remote sensing technologies have scientists been able to observe them clearly. Today’s featured image is a great example of one such wave.  It was acquired by the European Space Agency’s Envisat satellite (which aimed to carry out the largest civilian Earth observation mission to date – launched in 2002), on September 16th 2004.

A short animation showing just how impressive these waves are when travelling across the Mozambique Channel – using data from the Meteosat 5 satellite. (Credit: Jorge Magalhaes and Jose da Silva)

The image covers an area of about 580 by 660 km and was acquired as the satellite flew over the Mozambique Channel. The two-dimensional horizontal structure of a very large-scale atmospheric internal wave can be seen in the center of the image travelling southwest. The crest length of the leading wave, in this case, extends for more than 500 km and its crest-to-crest spatial scale is approximately 10 km on average.

It is interesting to note that several (but not all) of these individual waves are made visible by characteristic cloud bands, which form as the vertical oscillations find the necessary conditions (high moisture in the atmosphere) for condensation to occur.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at

Geosciences Column: Do coastlines have memories?

do coastlines have memories

Did you know that the shape of coastlines is determined by the angle at which waves crash against the shoreline. It has long been thought that fluctuations in the wave incidence angle are rapidly felt by coastlines, which change the shapes of their shores quickly in response to shifting wave patterns.

Or do they?

Researchers at the British Geological Survey, Duke University (USA) and Woods Hole Oceanographic Institution in Massachusetts, have performed experiments which show that spits and capes hold ‘a memory’ of their former shapes and past wave climates, influencing their present geomorphology. The findings have recently been published in the EGU’s open access journal Earth Surface Dynamics.

Gradients in sediment distribution within wave-driven currents and shoreface depth play an important role in shaping coastlines. But the angle between an offshore wave crest and the shoreline is chief among the parameters which shape coasts worldwide.

Low-angle waves – those with approach the coast at an angle of 45° or less – have a smoothing effect on the coastline and keep its shape relatively steady. On the other hand, high-angle waves – those with slam against the shore at an angle of 45° or more – introduce instability and perturbations which shape the coast.

The figure shows the experimental set-up used in the study. It also nicely illustrates how coastlines are shaped by the angle of the incoming wave. The arrows indicatenet flux direction under waves incoming from the left; arrow lengths qualitatively indicate the flux. Sand is not transported through cells which are in shadow for a particular wave. From C. W. Thomas et al., 2016.

Alterations to the patterns of shorelines are caused by enhanced erosion and/or deposition, driven by changes in wave climate. Ultimately, coastline geomorphology evolves depending on the relative degree of high and low-angle waves in the wave climate, as well as the degree of irregularity in the wave angle distribution.

Climate change will alter the wave climate, particularly during storm events, so we can expect shorelines to shift globally. Predicting how coastlines will adapt to changing climatic conditions is hard, but more so if coastlines retain a memory of their past shapes when responding to changing wave regimes.

Flying spits (finger-like landforms which project out towards sea from relatively straight shoreline) and cuspate capes (a triangular shaped accumulation of sand and shingle which grows out towards sea) are particularly susceptible to climate change. They form when high angle waves approach the shore at a slant. Animal communities living within fragile marine and estuarine ecosystems largely depend on the protection they offer. They are also of socio-economic importance as many shelter coastal infrastructures. Understanding how they will be affected by a changing climate is vital to develop well-informed coastal management policies.

To understand how changing wave climates affect the evolution of flying spits and cuspate capes (from now on referred to as spits and capes), the team of researchers devised experiments which ran on a computer simulation.

They generated an initially straight shoreline and set the wave conditions for the next 250 years (which is the length of time it takes in nature) to allow the formation of spits and capes.

To test whether pre-existing coastal morphologies played a role in shaping coastlines under changing wave climates, over a period of 100 years (which is loosely the rate at which climate change is thought to be occurring under anthropogenic influences), the scientists gradually changed the angle at which waves approached the coast.  After the 100 year period the simulation was left to run a further 650 years under the new wave conditions.

The investigation revealed that when subjected to gradual changes in the angle at which waves approach the shoreline, capes take about 100 years to start displaying a new morphology. The tips of the capes are eroded away and so they slowly start to shrink.

Spits adjust to change much more slowly. Even after 750 years the experimental coastlines retain significant undulations, suggesting that sandy spits retain a long-term memory of their former shape.

Snapshots of simulated coastline morphologies evolved under changing wave climate. U is the fraction of waves which are approaching the shoreline at 45 degress or higher. Coastlines evolved for 250 years under initial conditions. (aii, bii)> The U values of the changed wave climate show the coastline morphologies evolved 200 and 500 years after the wave climate is changed at 250 years, and the morphologies evolved over 1000 years under static wave climates with the same U. From C. W. Thomas et al., 2016. See paper for full image caption. Click to enlarge.

The implications of the results are far reaching.

Be it implicitly or explicitly, many studies of coastal geomorphology assume that present coastal shape is exclusively a result of present wave climate. The new study shows that even with steady wave climate conditions at present, coastline shapes could still be responding to a past change in wave climate.

Reconstructions of ancient coastal geographies and paleo-wave climates might also be approached differently from now on. The researchers found that as spits adjust to changing wave climates they can leave behind a complex array of lagoons linked by beach bridges. Though there are a number of process which can lead to the formation of these coastal features, researchers must also consider alterations of coastlines as a response to changing wave climate from now on.

The findings of the study can also be applied to the management of sandy coastlines.

Currently, forecasts of future shoreline erosion and sediment deposition are made based on observations of how coasts have changed in recent decades. The new study highlights these short observation timescales may not be enough to fully appreciate how our beaches and coasts might be reshaped in the future.

This is especially true when it comes to climate change mitigation. Decisions on how to best protect the world’s shores based on their environmental and socio-economic importance will greatly benefit from long-term monitoring of coastal geomorphology.

But more work is needed too. The experiments performed by the team only consider two types of coastline morphology  (spits and capes) and only two types of wave climate. While the experiments provide a time-scale over which spits and capes might be expected to change, other factors not considered in the study (wave height, shoreface depth, etc…) will alter the predicted timescales. The time-scales given by the study should be used only as a guideline and highlight the need for more research in this area.


By Laura Roberts Artal, EGU Communications Officer



Thomas, C. W., Murray, A. B., Ashton, A. D., Hurst, M. D., Barkwith, A. K. A. P., and Ellis, M. A.: Complex coastlines responding to climate change: do shoreline shapes reflect present forcing or “remember” the distant past?, Earth Surf. Dynam., 4, 871-884, doi:10.5194/esurf-4-871-2016, 2016.

Imaggeo on Mondays: Coastal erosion

Imaggeo on Mondays: Coastal erosion

Coastlines take a battering from stormy seas, gales, windy conditions and every-day wave action. The combined effect of these processes shapes coastal landscapes across the globe.

In calm weather, constructive waves deposit materials eroded elsewhere and transported along the coast line via longshore-drift, onto beaches, thus building them up. Terrestrial material, brought to beaches by rivers and the wind, also contribute.  In stormy weather, waves become destructive, eroding material away from beaches and sea cliffs.

In some areas, the removal of material far exceeds the quantity of sediments being supplied to sandy stretches, leading to coastal erosion. It is a dynamic process, with the consequences depending largely on the geomorphology of the coast.

Striking images of receding coastlines, where households once far away from a cliff edge, tumble into the sea after a storm surge, are an all too familiar consequence of the power of coastal erosion.

In sandy beaches where dunes are common, coastal erosion can be managed by the addition of vegetation. In these settings, it is not only the force of the sea which drives erosion, but also wind, as the fine, loose sand grains are easily picked-up by the breeze, especially in blustery weather.

Grasses, such as the ones pictured in this week’s featured imaggeo image, work by slowing down wind speeds across the face of the dunes and trapping and stabilising wind-blown sands. The grasses don’t directly prevent erosion, but they do allow greater accumulation of sands over short periods of time, when compared to vegetation-free dunes.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at


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