Tim O’Hare
observations, thoughts and useful stuff…Archive for oceanography in the news
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.
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].
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.
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.
Down down, deeper and down
There’s a report on the BBC website about a new robotic submarine that is currently undergoing final preparations ahead of an attempted dive to the deepest part of the world’s oceans, The Challenger Deep in the Mariana Trench (~11,000 metres down). This depth is deeper than Mount Everest is high (incidentally, did you know that George Everest’s name was actually pronounced Eve-Rest rather than the Ever-Est that we now use to describe the mountain that was named after him?). The robotic submarine has been developed by scientists and engineers at the Woods Hole Oceanographic Institution in the USA and is named Nereus (after the son of Pontus [the sea] and Gaia [the Earth] in Greek mythology). Challenger Deep has previously been visited only twice before, both times by human-operated vehicles, so there is plenty of potential for Nereus to turn up some interesting information. The Challenger Deep is part of a major subduction zone in the western Pacific in which oceanic crust that forms the base of the Pacific Ocean is forced down and underneath the oceanic crust that neighbours the Asian landmass and for this reason it is a major earthquake region. At this kind of depth the pressure experienced due to the weight of water supported is over 1000 times greater than the pressure we experience at sea level (due to the weight of the overlying air in the atmosphere).
The other CO2 problem
Most people have been aware of the problem of global warming linked to increased levels of Greenhouse Gases in the atmosphere, but there is another problem created by the increased levels of carbon dioxide (CO2) which is only recently coming into the public realm. This is the problem of ocean acidification. The problem is nicely set out in a short film produced by children at a local secondary school (Ridgeway) in conjunction with scientists from the Plymouth Marine Laboratory. There is a piece about the film in the local newspaper from where it is also possible to view the film (or view it directly with this link).
Well done to everyone concerned.
Sea level rising faster – or is it?
New Scientist, Issue 2699 (14 March 2009) carries a rather confusing news story that reports that recent measurements show that sea level has been rising by 3 millimetres per year since 1993 which is higher than the 2007 forecast of the Intergovernmental Panel on Climate Change (IPCC). It seems that the differences comes from melting from the Greenland and Antarctic ice sheets which was not included in the IPCC forecast because of uncertainty over the models used to predict this. The IPCC forecast was for a sea level rise of 18-59 centimetres by 2100 but apparently if the current trend continues a rise of 1 metre or more by 2100 is likely. However, if you take the 3mm per year figure from the current measurements and extrapolate this out to 2100 you get a rise of about 27 cm (that’s 3 mm/year x 90 years = 270 mm = 27 cm) which is slap-bang in the lower-middle part of the 2007 IPCC forecast range. So, where’s the story gone…?
When the water level rises…
New research is suggesting that as sea levels around the world rise the actual rise that occurs in different locations may be very different. Apparently if the West Antarctic ice sheet melts, the meltwater will not spread evenly around the world’s oceans but, instead, will be focussed towards the east coast of North America (with sea levels there being 1-2 metres higher than elsewhere). According to climate models, this effect would occur for a number of reasons. First, as the ice sheet gets smaller it will exert a weaker gravitational pull on the surrounding oceans allowing it to move northwards more easily. Secondly, a lighter ice sheet will allow underlying land to rise displacing water elsewhere. Finally, and perhaps most importantly, the redistribution of this large mass of water would alter the Earth’s spin shifting the locations of existing bulges of water that occur between the equator and the poles. The research is described briefly in New Scientist, Issue 2695 (14 February 2009).
And that’s not all… a completely separate study has calculated that sea levels on the east coast of the USA could rise by around 0.5 metres by 2100 due to a combination of the slow-down of the Atlantic Meridional Overturning Circulation (AMOC) caused by global warming and thermal expansion of ocean waters due to rising temperatures. This research is described briefly in New Scientist, Issue 2700 (21 March 2009) .
All in all, the future doesn’t look bright for low lying parts of the east of the USA.
