I spent my third day at the Met Office talking to various researchers about the work they are doing, how it relates to my EPQ topic and asking them lots and lots of questions (I did have to apologise to one person as I literally spent an hour quizzing them about the thermohaline circulation!). I am unsure as to how much knowledge some of you will have surrounding ocean circulation so some of this may not make a lot of sense - if you hunt around on the blog though you will find various posts on the basics of ocean circulation.
So, first up I had a very long discussion with someone about the thermohaline circulation. For those of you who don't know much about the thermohaline circulation it is the driving force behind the deep currents of the ocean that are collectively referred to as the Global Conveyor Belt. This deep circulation is caused by density changes in oceanic water resulting from changes in temperature and salinity, hence its name ‘thermohaline circulation’, which are caused by cold winds cooling surface waters, the input of freshwater from either precipitation or melting ice, the cooling and freezing of seawater into sea ice or the evaporation of sea water. The basic thermohaline circulation is initiated when denser water (predominately the cooler, saltier water) sinks below the more buoyant water (warm, with a low salt content). Convection penetrates to a level where the density of the sinking water matches that of the surrounding water. When this maximum penetration level has been reached, it will gradually spread into the rest of the ocean. Once the dense water masses have spread into the full extent of the ocean, they will slowly upwell to supply the slow return flow to the sinking regions and replace the surface waters lost - thus driving and sustaining this circulatory movement. Anyway, the discussion I had was focused on the likelihood of it weakening or shutting down and the resulting impacts such an event would have on us. In terms of the UK, if the MOC (Meridional Overturning Circulation) was to significantly weaken or shutdown then the UK would get a lot colder as a slower circulation means a slower heat transfer from the Equator to the Poles and thus a warmer tropics but colder high latitudes. Changes in wind patterns and precipitation would also occur although, in terms of precipitation, the biggest changes would be experienced by those who currently have monsoon climates as a change in the ocean circulation would impact on the movement of the ITCZ. Due to the colder poles, more sea ice is likely to form and this is something that would impact the path taken by the Gulf Stream. The warm moist air brought to us by the North Atlantic Current ( a branch of the Gulf Stream) is what allows us to have a temperate climate, one far milder than countries of the same latitude. However, if the MOC stopped then the path of the Gulf Stream would be altered and it would split at a much lower latitude in the North Atlantic and so we would not get the warm water and moist air that dictates the climate we currently experience. Therefore this would amplify the effects of an MOC shutdown and further cool our climate. When you then factor in the predicted changes due to global climate change, it becomes slightly more complicated. To some extent it is believed that global climate change could, to a certain extent, offset the effects of an MOC shutdown and so they could act to cancel each other out in most of the world. In the Northern Hemisphere though, due to the reduction in speed of the meridional heat transfer, the UK and areas above it would stay cold or perhaps even get colder! So, what would be needed to cause a shutdown or weakening? Well, something that will upset the delicate density differences and so the most likely thing to do this would be an input of freshwater. They are a few possible sources of freshwater that could do this. First is the collapse of ice sheets/shelves in Greenland due to rising sea and air temperatures as not only would you get the obvious freshwater input from that but after ice shelf collapse glacial flow is also increased. Another possibility is the melt of pack ice in the Arctic and the finally possibility is an increase in precipitation as a result of global warming. It is debatable which poses the greatest threat to the MOC as little is known about the stability of ice sheets and shelves in Greenland etc (lots of research is being done at present) and so many think that the most likely threat could come from an increase in precipitation - something that I realised would have an impact but was surprised to know was believed to have the biggest impact in the near future. A question I was quite interested to know the answer to was how much freshwater would be needed to shutdown the MOC and on what time scale would a shutdown be likely to occur. However, no one really knows and research into this is currently being conducted around the world. One thing that was said for certain was that a shutdown is unlikely in the near future and that one would not occur on the time scale portrayed in The Day After Tomorrow (film review on its way soon!)!!! To try and answer these questions lots of models are being used to try and calculate what state the MOC is currently in.
This graph is rather simple but does help when trying to explain some of the theory behind the response of the MOC to freshwater and how it may recover from an input - another area that really intrigues me. So basically along the x-axis is the volume of freshwater and along the y-axis the strength of the MOC/THC. The idea that many have is that the MOC, despite an input of freshwater, will maintain a constant strength until a certain point is reached. At this point, a rapid decline in its strength will be witnessed and then the MOC will fall off its 'track' (kind of onto the bottom line). Therefore, for it to re-initiate, a decline in freshwater would need to occur, which I suppose is like a reversal of the current pattern, for it to get back on 'track'. The idea of this graph originates from some of the thinking behind how the oceans would respond to an input of freshwater and the contrasting ideas produced by models.
The first comes from more complex models which seem to suggest a more gradually weakening in response to a freshwater input. The simple models, upon which the first graph I showed you is based on, suggest something different.
The rapid decline in strength after a certain point has been reached is something that many believe would have in reality.
The same graph as the first one, but based on the response results produced by the complex models, is slightly different.
This graph suggests that a gradually response would occur to an input and that, therefore, a gradual return to normal would occur as freshwater was removed and the density differences restored. Excatly which model is correct has yet to be decided but validating one of thiese models is crucial if we are to understand how the THC/MOC will respond and how it is likely to be re-started. Understanding this is also likely to help with calculating just how much freshwater the MOC can 'cope' with before weakening/shutting down.
Another area that is being researched is what feedback system the oceans are currently in. All of these ideas tie in together and understanding the mechanisms is vital if we are to gain a better understanding of how the ocean responds to different factors.
The negative feedback loop represented, rather simply, above is the state that the models used suggest that the oceans are in; which is the idea that freshwater goes in to the North Atlantic, sinks, and then salt water emerges in the South. However, scientists actually believe that the oceans are in a state of positive feedback instead......
Work into which one the oceans is in is currently being done as by understanding this, it is hoped that more reliable predictions for the future will be able to be produced. Just as kind of a side note, scientists think that the oceans may be able to switch between the two feedbacks mechanisms with this switch being provoked by changes caused to the basic circulation pattern of the oceans, in repsonse to freshwater input.
I realise that this is a bit all over the place but I hope by now you have got the idea that a freshwater input could cause a shutdown or weakening of the MOC and this would have a large impact on the climate of the UK, amongst other countries. Hopefully you we also be able to understand how the alterations of our climate would effect other factors that affect us and other aspects of our lives like, for example, agriculture and food production. Something else though that needs to be taken into account when talking about a possible shutdown is the location of the freshater input. An exact location which would have the largest impact is yet known, but in general, it is believed that any input into the North Atlantic could have a major impact, especially in comparison to the Pacific or Indian Ocean. Again, understanding some of the things above will help aid the discovery of where is the key point in terms of the MOC and this just demonstrates how interlinked all of these things are. During this discussion we covered loads more, hopefully some of which will appear in my EPQ, but I have probably talked about this all a bit too much already! There is some other really interesting stuff that I might right about in a couple days that we also covered - in reflection I do feel a bit sorry for the woman who kindly gave up a lot of her time to talk to me about all of this stuff as I literally bombared her with hundreds of questions!!!
Once last thing I need to mention, which is related to the above, may be of general interest. I don't know if any of you have ever read anything about the thermohaline circulation or meridional overturning circulation but whilst doing research for my EPQ it has confused me how some people switch frequently between THC and MOC and was excatly the difference is. So, I thought who better to ask then the very intelligent scientists at the Met Office! Well, here is the answer............. the MOC is used as to represent the integral across an ocean basin of the meridional flow (south to north movement hence why it is often used to describe the THC in the Atlantic). Given a flow field, the MOC can be precisely defined as a function of depth, or potential density, and latitude. On the other hand, the THC is a less precise concept and is used to describe the whole Global Conveyor Belt circulation. The THC is generally, broadly, taken to mean the part of the circulation that is directly and solely driven by density differences rather than wind stress. However, it is important to note that there are thereotical difficulties in disentangling which forcings are responsbile for a specific part of the circulation and in many cases the MOC is not purely a result of the THC. In many cases, though, the conceptual difficulties may be less severe when considering the changes to the circulation rather than determining the causes of the circulation. Therefore, the reason for some papers alternating between the two is that whilst there are some differences, in some circumstances, both terms are appropriate for use - I hope that explains the small difference!
Then, to finish off what has been an amazing three days, I had a chat with two scientists who are researching the impact of climate change on the ocean ecosystem. These guys were in the Biogeochemistry working group meeting I talked about earlier in the week and so they do a lot of work into CO2 uptake etc. and the idea that phytoplankton can control temperature. Our discussion included many things such as the way in which they model ocean ecosystems and how the models have developed over the years, some of the basics of the cycles involved in the ocean ecosystems and how excatly climate change is likely to alter the delicate ecosystems. This meant that the discussion moved on to the issue of ocean acidification and how studies are being conducted into the impacts this has on ecosystems, especially fish and corals. This is a rather large topic and so, again, I think I will write a seperate post on this over the next few days. At some point in conversation (can't really remember when!) we moved on to a discussion about Daisyworld and the
Gaia hypothesis/books. Daisyworld is one of the ways in which James Lovelock tried to represent the ability of the Earth to self-regulate and for a quick idea of what Daisyworld is all about follow the link -
Daisyworld animation - its a bit simple but portrays the general idea. I have read the first one and have nearly finished the second one (thanks Millie for getting me to read them as it enabled be to fully participate in the discussion!) and it was quite interesting to get the view of two scientists on the book which is written for the non-scientists about some of the work they are researching.
Anyway, so thats what I got up to during my 3 days at the Met Office (sorry that the posts have been all over the place and written a bit late). I had an incredible time and met some really lovely and highly intelligent people who were more than willing to answer my questions, talk about the work they are doing and offer lots of general advice. There are so many people I need to thank for helping me get into the Met Office, preparing me for the priviledge, allowing me in in the first place, organising many things for me to participate in, for giving up their time to talk to me, allowing me to bombard them with questions and for just being so welcoming whilst I was their. I learnt so much from the experience and its given me a lot to think about in terms of what I want/need to do in the future - I have a lot of thinking to do! It has also reinforced the fact that I know so little about the world of Geography and so I have a lot of reading to do to try and cover as many of the things I didn't quite understand as possible and just expand me knowledge (I will say sorry in advance as a lot of writing is likely to appear on here over the next few weeks!) - I relish the challenge!!! So, all that is left for me to say, apart from another thanks to everyone who has helped make this possible, is that I can't wait to hopefully go back at the end of August!
First up was a very interesting discussion with a researcher about ice sheets. There is currently a lot of uncertainity about ice sheets and their stability, with much of this uncertainity stemming from a lack of understanding of the mechanisms for ice sheet melt. Currently, it is believed that Antarctica is the most likely to experience significant melt and this is because much of it is under the sea. If the proportion that lies below sea-level was to melt, which scientists think could occur over the next few hundred years (although this may sound quite slow, it is considered to be rather rapid!), it would raise global sea-level by 6 metres!!! Greenland melting is an area that often recieves much attention, perhaps becuase we are unsure just how much freshwater could be released or how quicly it would occur, but for now alteast, many feel that a large freshwater input is unlikely. Before this discussion, I had never really given much thought to the influence that the angle of the bedrock, upon which the ice sheet lies, on melting but it does, in fact, seem to play quite a large role. The bedrock that the Greenland ice sheet lies upon slopes away from the sea, whilst that in Antarctica slopes towards the sea and thus making runaway melting possible. When the Laurentide Ice Sheet existed it was sat upon 'flat' bedrock, something that many considered essential for the occurence of Heinrich events. Due to this, depsite the fact that models currently cannot replicate Heinrich events, it is believed that this current climatic state cannot provoke Heinrich events. The fact that surprised me the most was that increased surface air or sea surface temperatures, as a result of global climate change, are not the biggest threat to the stability of ice sheets and would not be responsible for instigating the greatest volume of melt. Instead it is increased wind speeds.....but why? 
Well, increased wind speeds would raise the height of local sea-level and increase Ekman Pumping. Increased Ekman Pumping would provoke old warm waters from the deep ocean to be dragged closer to the surface and over the terminal morraine, which marks the end of glaciers or ice sheets, and towards the base of the ice sheet, thus causing melting to occur. This is believed to be responsible for more melting than raised surface air/sea temperatures. One thing that scientists are unsure of though is what happens to the warm water once it passes the terminal morraine. Does it hit the base of the ice sheet and then continually circle, gradually melting away the base or does it bounce back off and return to the rest of the ocean? Understanding this is, again, crucial if predictions of ice sheet melt are to become more cetain..... 
