Well, first up was a discussion about a possible thermohaline circulation shutdown. I have to admit that I find the idea of a future THC shutdown really interesting and it is an area that I have given a lot of thought to over the past few weeks, hence my rather long list of questions (I am guessing Millie quite enjoyed the fact that, for once, she wasn't the one my list of questions were aimed at!). As with the previous discussion groups we covered an awful lot so I am going to try and summarise the key points - feel free to ask if you want to know more! Again, much of the discussion revolved around the idea of hysteresis (which refers to the dependence of the state of a system on the history of its state, with the lag in a variable property of a system with respect to the effect producing it as this effect varies - thats kind of the idea of it all, I think). Honestly all of this takes a lot of time to get used to..... trust me I have been trying since we first started our EPQ's and I am, perhaps, only just starting to make some real progress!
|Sorry I know its not the best drawing of the graph |
but I was trying to make it as simple as possible! Basically, MOC
strength (in Sv) is up the y-axis and volume of freshwater
along the x-axis
Perhaps the greatest challenge facing researchers at the moment is applying this graph to real life - something which is only made more difficult by the complications generated by feedbacks and difficulties in validating model projections due to a lack of observations of the MOC strength. The feedbacks are also an area that intrigues me (there are loads involved in the earth system and I think we briefly touch on a few at A2) although, again, they can get a bit complicated..... There are primarily two feedbacks that I am most concerned about, in the oceans, with reference to my EPQ theme and that is the advective feedback and the convective feedback and it is the existence of these two positive feedbacks that generates the non-linear behaviour of the ocean circulation and determines the stability of the THC. What excatly do these feedbacks involve?
- Advective feedback:- the THC advects salty water northward in the Atlantic, which enhances salinity and density in the north which, in turn, keeps the THC circulating.
- Convective feedback:- convective vertical mixing continually removes freshwater from the surface in regions of net precipitation; thus prevents the formation of a fresh buoyant surface layer which is capable of inhibiting convection.
After this discussion, we talked about the impacts that a future MOC shutdown would have on the world, although primarily the UK and I should probably warn you that they do not look to great! In terms of temperature, after a complete cessation, models have projected anything up to a 8 degree drop for the UK accompanied by 4-5 months of snow cover, higher wind speeds, less rainfall, quite a high local sea-level rise and an increased frequency and severity in extreme weather events. After the problems caused by snowfall on recent years I think I will focus on how society would have to adapt if we were to annually experience 4-5 months of snow cover. The short period of snowfall experienced over past years caused societal collapse in this country as we simply did not have the means or the knowledge to adapt and so the thought of longer periods of snow throughout winter every year, as a result of a NADW cessation, should be of concern to politicians whom received much criticism for an apparent inability to deal with the situation. Agricultural patterns are likely to be the first to be affected; with growing seasons altered, a seasonal shift back to more labour intensive time consuming work, both quantity and quality of winter vegetables reduced with farmers unable to lift them during frosts and greater expense involved in keeping livestock. As with transport on a whole, food distribution would be restricted with particular issues in relation to fresh food. In January 2010, one milk producing company, who supply 80% of the organic milk consumed in Britain, had to throw away 100,000 litres of milk as tankers could not collect it and farms do not have the storage capacity. Combining the above would incur raised food prices and, in the long run, possibly even provoke farmers to move away from commercial-sized farms to reduce expenses. People’s mobility would be inhibited with roads unsafe to navigate and public transport not running, leaving those in remote rural locations and the elderly isolated with limited access to amenities, an issue that would only increase year on year if the UK’s population continued to age. Alternatively, this could promote a switch to people living more self-sufficient lifestyles with vegetable patches in gardens etc. so that they would not be as reliant on supermarkets as a food source. Further strain on already stretched health care would be generated, with accidents more likely in icy conditions, increase in time taken to reach those injured and issues with both patients and staff reaching hospitals. Education would be affected (yes, I realise that we all love it when college is closed due to snow but it would soon become a great convinence if it was to happen over a long period every year!), with snow capable of causing significant disruption during exam periods and to those who presently travel further afield to gain an education in the best possible institutions. One of the largest impacts would be on the economy, with the 2010 snowfall reducing economic growth by 0.5% in the UK, as consumer spending is often at its greatest approaching Christmas and so better contingency plans would have to be generated to minimise the impact on the economy. Increased snow cover could provoke a shift in employment sectors as construction work could become seasonal, therefore affect unemployment levels and perhaps increase the percentage of the population with office based work that, during winter months, could be completed at home. This could, in turn, reduce the number of TNC’s attracted to the country whose arrival often prompts cumulative causation. Finally, if this prolonged snow cover is going to occur frequently, then more will be expected of local councils to deal with the situation, keep roads gritted and services and schools open. That is just a few of the impacts that increased snowfall would have on the UK, and there are many other impacts associated with the other factors. So, how do you think society would cope with the impacts of a future MOC shutdown?
To finish off what was, again, a really great day, we attended a seminar on 'The Effect of the QBO on Lateral Mixing and Transport in the Stratosphere' and, to be honest, I only managed to follow like the first 5 minutes (which was still a struggle depsite the fact I tried to do a bit of reading up on QBO the day before). The other seminars I attended previously were hard enough to follow but this one very very quickly went way way way over my head! I will try and cover the real basics of QBO but I am still very unsure of it all so you will have to bare with me.....
|QBO observations - note alternating westerly and |
|Zonal mean wind as a function of time and latitude at 10mb|
- oscillation in mean zonal winds of the Equatorial stratosphere between easterlies and westerlies
- period of 28 months
- westerly shear zone propogates downwards more regularly and rapidly that easterly ones
-phase of the QBO affects the location of the extratropical surf zone by moving the zero wind line
- tropical mixing extent is dependent on the QBO phase
strong mixing extends to low latitudes of QBO pahse in stratosphere
- large-scale middle atmospheric circulation
- responsible for long-term persistent transport of air and chemical nutrients from the troposphere to the stratosphere
- driven by Rossby wave breaking in the tropical stratosphere
- Tropical Pipe model of stratopsheric transport
- Tropical region bounded by subtropical edges of the wintertime surf zone which is isolated from the vigorous mixing of the extratropic surf zones
- edges of the Tropical Pipe barriers are moving
The seminar discussed the "influence of the stratospheric potential vorticity distribution in lateral mixing and transport into and out of the tropical pipe, th elow latitude ascending branch of the Brewer-Dobson circulation" and then presented the clear pattern that apparently exists between the above and the phase of QBO. I think the idea was that the phase of QBO dictates the amount of mixing that occurs as the phase of the QBO affects PV (potential vorticity) structure in the stratosphere. So, during the westerly phase, a strong PV is expected at the Equator which would isolate the Southerm Hemisphere from mixing and allow for greater 'in' mixing. During the easterly phase, stronger PV gradients at the subtropics limits Northern Hemisphere mixing and so particles remain in the tropics, where little mixing into the tropics occurs. I not quite sure if this is making any sense at all as I think the more I write about it the more confused I get about it all - therefore it is probably best that I just leave it at that! Sorry, I realise that my write-ups about the seminars have been really bad but honestly, they have all very quickly gone way past my level of understanding so, writing about the basics that they have been based on is about as much as I am currently able to manage.
Anyway, it was another great day and I hope Millie enjoyed it too!