Tim O’Hare
observations, thoughts and useful stuff…Archive for Oceanography
Freak waves explained?
Years ago when I was a lowly postgraduate student in North Wales there was a mysterious case of a small fishing vessel that sank in relatively calm conditions in Cardigan Bay. Concerns were expressed by some that the boat had been sunk by a navy submarine and after the relevant court cases etc. the local BBC station decided to make a short documentary about the sinking and came up to the lab I worked in to interview an expert and do some filming. I was setting up some waves in a laboratory wave channel for them to film so I got to listen in on the whole process from start to finish (I also had my index finger filmed as I used it to press the ‘on’ button for the wave tank – if ever there was an impressive claim to fame surely that has to be it!).
At the start of the filming process the interviewer briefed the expert (I’ll not name them) about the line of questioning the interview would take and absolutely promised that they would not ask the expert whether the wave that swamped the boat had been caused by a submarine. You can probably guess what happened – the interviewer proceeded to ask about each possible natural cause of sinking and each was ruled out in turn. With no natural causes left the interview then dropped in the killer question – so could the sinking have been caused by a submarine? To which the expert was left with no answer other than an open mouth and an uncomfortable pauase before a hesitant “I suppose that is possible” type of answer.
Since then I’ve come across several reports about freak waves, sinking and damaging ships and there was a BBC science documentary about freak waves a few years ago. Now, there has been some new theoretical research which suggests that certain configurations of sand banks can cause freak waves, up to three times the typical wave size, to occur much more frequently than would otherwise be expected. The authors of the work, which was reported on the BBC Website [09 August 2009] are at pains to point out that their work is theoretical, but should be possible to test their work with measurements made in particular locations. If their work turns out to be correct and to have wide applicability then the world’s shipping companies will be beating a path to their door in no time at all.
Time for Plan B – Geoengineering
There has been a huge amount of coverage of the need to cut Greenhouse Gas emissions as the primary route to slow down, halt and eventually reverse the current global warming trend and rightly so. However, in the background there have been a number of suggestions for actions that mankind could take to directly counter-act global warming. Such measures are collectively known as geoengineering and include such things as the direct removal of carbon dioxide from the atmosphere (e.g. by planting trees or fertilising the oceans) and reflecting incoming solar radiation away from the Earth (e.g. by using mirrors in space or changing the land surface to make it more reflective). These measures have not recevied much public attention, partly because they are all really, really expensive, partly because no-one knows how effective they would be and partly because by discussing the ideas in public we might distract attention from the goal of reducing Greenhouse Gas emissions.
Now, the tide has turned a little. A recent report produced by the Royal Society has highlighted the need to urgently begin considering geoengineering as a Plan B to reducing emissions. The report works through various geoengineering ideas examining their affordability and effectiveness and suggests that there should be a major shift of funding into geoengineering research. The report was widely publicised in the media at the beginning of September and the geoengineering debate is nicely summarised in New Scientist, Issue 2724 [05 September 2009].
Marine life mixes oceans
One of my colleagues carries out research examining the small scale mixing processes that go on in the oceans. He uses a complicated camera system with lasers and holograms and maps out the swirling motion of the water by tracking particles in the water [see here]. In the past he has ended up with some interesting pictures of little (microscopic) creatures in the water and he has begun to think about how these creatures stir up the oceans as they move around.
So, it was interesting when news of some research conducted in California broke recently. The work has measured the effect that jellyfish swimming in weater have on the mixing of the water water itself. Jellyfish were used because they are relatively simple and can be simulated in models quite well but the principle of ocean mixing by organisms is being considered more widely. In fact, the idea was suggested by Charles Darwin’s grandson some time ago. It turns out that the new research suggests that the mixing could be significant although the extrpolation to all ocean-going organisms, in particular the really small ones (of which there are huge numbers) is a rather uncertain process. It has been suggested that this organismal mixing could be as big as that produced by the other key mixing processes – wind and ocean tides. The result won’t change the results of ocean models because these work by adding in as much mixing as is necessary to get the “right” results but it may point a way to understanding global ocean mixing more thoroughly and it suggests that my colleague’s potential to view the water motion around smaller organisms might be a really fruitful direction to go in.
The research is reported in New Scientist, Issue 2719 [01 August 2009] and also on the BBC Website [29 July 2009].
Spare a thought for sea grass
It is common knowledge that coral reefs and coral reef ecosystems are under threat but it is less well known that sea grass meadows are also struggling. Sea grass is found in shallow coastal waters across the whole planet and are important as a refuge for crustaceans, young fish and larger creatures such as dugongs and turtles. A recent meta-analysis (pooling data from 215 regional studies from 1879 to 2006) has revealed that the total area of seagrass meadows has declined by 29% since 1879. It is thought that much of the damage is done by sediment dumping, pollution and nutrient run-off which decrease water quality, starving the sea grass of the sunlight it needs to grow. The research is highlighted in New Scientist, Issue 2716 [11 July 2009].
Sea level rise – it just goes on and on…
New Scientist, Issue 2715 [04 July 2009] contains a substantial article on sea level rise which sets out the latest findings on rates of sea level rise and puts these into the context of past changes in sea level. It seems that a rise of 80cm by 2100 is a pretty standard projection, a rise of ~2m by 2100 is within the realms of possibility and even bigger rises cannot be discounted. An important point to bear in mind is that when projections are given for 2100 it is important not to forget that this isn’t the end of the story and that sea levels will continue to rise after this point and also that although much greater changes have occurred earlier in the Earth’s history we need to remember that humans weren’t around then…
A tale of two gases (and the Southern Ocean)
It’s reasonably well known (!) that carbon dioxide levels in the atmosphere are increasing… it’s also reasonably well known that increased carbon dioxide levels in the atmosphere are leading to increased carbon dioxide levels in the oceans (leading to ocean acidification). However, new research reported in New Scientist, Issue 2715 [04 July 2009] suggests that in the Southern Ocean, the picture isn’t quite that simple. Measurements from the Southern Ocean show that carbon dioxide levels have flattened off in recent decades (having previously increased) and a new modelling study points the finger of “blame” for this at the hole in the ozone layer in this region. Lower levels of atmospheric ozone (at high levels) and increased levels of carbon dioxide (at lower levels) have changed the energy balance in the atmosphere, generating stronger westerly winds, enhancing ocean circulation and encouraging carbon-rich water to rise up from the deep (a process known as upwelling). The result is surface water that is less able to absorb carbon dioxide from the atmosphere. This is a great example of an unexpected feedback effect of one part of the Earth-atmosphere system with another and just goes to show how complex and inter-connected all of these processes really are.
From World War II to surf forecasts
My favourite reference in my PhD thesis (1992) was to a paper from the 1940s which was titled “On determining the gradient of enemy held beaches”. I liked the way that the title gives an explicit description of what the paper is about and I also liked the way that it seemed so far removed from the time and purpose of my own study.
In reality though, a great deal of the modern-day research done on waves and beach processes has its roots in the work of scientists in the 1940s associated with predicting wave and beach conditions for Allied Forces landings in north Africa and Normandy. One such piece of work that anyone who studies waves to a reasonable level will come across is the SMB-method for predicting waves. The abbreviation SMB represents the names of the three protaganists in the development of the method – Sverdrup, Munk and Bretschneider – and the method uses a knowledge of three parameters, wind speed, duration for which the wind has been blowing and the fetch (distance) over which the waves are being built up by the wind to predict the significant wave height and peak wave period. It is based on a special diagram which cleverly combines all of the variables and presents the outputs in a graphical form called a nonogram.
So, given my own use of research originating from the World War II era, it was interesting to read an historical piece in New Scientist, No 2714 [27 June 2009] about how Walter Munk (now aged 91) became involved in wave prediction in relation to Allied Forces landings and how this work subsequently spawned the surf prediction industry. It’s a good example of how widely applicable research can be triggered by a specific requirement and how it is not always possible to spot the full value of research (Munk missed out on making money from surf prediction).
A tale of two oceans – the seas in 2050
Last weekend The Times newspaper carried various reports publicising The Times Cheltenham Science Festival. One of these focussed on the major threats to the world’s oceans including CO2 emissions, warming and acidification and carried a plea for immediate action. The report was accompanied by a nice graphic showing two versions of the seas in 2050 side-by-side. In the first of these, the major problems had been tackled and the oceans and the ecosystems they contain had “revived” and in the second, problems had not been solved and the oceans and ecosystems are “collapsed”. For each version of the oceans there are 15 or 16 points of note, either positive in the case of the “revived” seas or negative in the case of the collapsed” ones. The article is available online (The Times [06 June 2009]) and pleasingly, so is the colour graphic (either via the link in the article or directly via this link).
Microseisms rumble on
Back in the early 1990s when I was finishing off my PhD I was given office space in the Unit for Coastal and Estuarine Studies (UCES) at the then University of Wales, Bangor (in Menai Bridge). This unit was a centre for applied (contract) research and was based on a small island (Ynys Faelog) in the Menai Straits that was reached by a causeway. Anyway, sharing the building was a retired professor, Jack Darbyshire, who used to beaver away on his own little projects and delighted in collaring me to talk about one of these (relating to the formation of beach cusps). The primary area that Jack was interested in was microseisms (tiny earthquakes) that are picked up in seismographs. I can’t actually remember what it was about microseisms that interested Jack so much (he wasn’t the easiest person to understand being something of the classic mad scientist with an added very strong north Wales accent) but I was interested to see microseisms pop up again in New Scientist, Issue 2710 [30 May 2009]. Apparently, when ocean waves break they cause the crust to “hum” and they generate microseisms the intensity of which is related to the size of the storm waves. There is now an idea to look back at the extensive seismometer records that go back for almost a century to see whether there are detectable changes in this wave “noise” that can be used to infer changes in storm patterns due to climate change. This is of interest because the seismometer records go back further in time than good wave measuring systems. Perhaps I should have listened to Jack Dervyshire more carefully all those years ago.
Sink or sink – there’s little choice in the Maldives
The Maldives are a set of low-lying islands in the Indian Ocean. 80% of the islands are less than 1 metres above sea level and the highest point is only 2.3 metres up. But sea level has risen 52 millimetres in the last 15 years, the Intergovernmental Panel on Climate Change has predicted a rise of up to 59 centimetres by 2100 (not including glacial melting) and the most extreme predictions put sea level at 25 metres above current levels by 2100. So, it doesn’t take a genius to work out the for the Maldives it is not a cae of sink or swim but rather a case of sink – the only uncertainty relates to how long it will take to go under. It could be 50-100 years (IPCC) but it could be much, much sooner. What is to be done? There are various plans afoot that range from building artificial sea walls or a raised island to house the population or puting buildings on raised platforms. An article in New Scientist, Issue 2707 [09 May 2009] discusses the problem facing the Maldives and reveals that the current government there is also contemplating what, to me, seems like the most logical approach, namely to divert a large amount of the islands’ income from tourism into a fund to buy land elsewhere in the world to which the Maldive islanders can relocate at an appropriate time int he future.
