Thursday, 15 September 2011

ENSO: a kind of synopsis

It looks like we are going to be doing a lot of purely Human Geography for a while and so, as well as writing a post about what I learnt about development this week I thought I would write a post on ENSO, a purely physical topic, that, if I am honest, really really fascinates me but is unfortunately no longer on the A-level syllabus. Just incase you don't manage to get to the bottom of this post, I am planning on writing up atleast one detailed case study that could be used in our essays (sorry to remind you all!) and so I am looking for some suggestions on which one to do - I have a few ideas but any suggestions would be greatly appreciated!

So, anyway, this one is for all you Physical Geographers out there!

What excatly is the El Nino Southern Oscillation (ENSO)?
The term 'El Nino' has been applied, in the past, to somewhat different oceanographic events in the tropical Pacific but, nowadays, is commonly used to describe the anomalous warming of the sea-surface temperatures (SST) that occurs every few years in that region, although typically focused in east-central equatorial Pacific. El Nino events last several months and are associated with widespread changes in the climate system (dominating climatic flucuations observed on interannual timescales). These climatic changes have huge socio-economic impacts on countries, affecting agriculture, infrastructure, health, energy and, of course, development.

What is La Nina and the ENSO cycle?
La Nina is the term used to describe episodes of cooler than normal SST in the tropical Pacific (easiest way to think of it is the opposite of El Nino). The Southern Oscillation bit of ENSO is the term used for the atmospheric changes that occur in the tropical Pacific and accompany El Nino and La Nina events. Both events involve strong interactions between the oceans and atmosphere and tend to alternate (although not always the case) and this whole process is referred to as ENSO cycle. The ENSO cycle is not regular with various sizes and durations in both events but ENSO is a natural phenomenon and proxy evidence (principally from coral growth rate measurements) suggests ENSO has existed for thousands of years.

What causes El Nino?
As simply as I can put it, the ENSO cycle is the consequence of slow acting feedbacks in the ocean-atmosphere system, helped by strong air-sea interactions in the tropics that permit long-lived long-range connections to operate in the system.

Normally, the equatorial Pacific ocean has a pool of relatively warm water in the upper ocean to the west and a shallower layer of relatively cool water in the east; easterly surface winds are intrinsic to the maintainance of this balance. The first few tens of metres of oceans are well-mixed and lie above a thin thermocline, with cold water below.

The exact trigger of El Nino events is still debated and, due to a lack of observations, not a great deal can really be said about them with much certainity.There are a few possbile triggers though - Raised SSTs in central/east Pacific can be caused either through the action of westerly windbursts (short-lived storm-like events in West Pacific), or the gradual development of the ocean waveguide which moves the thermocline. Increased SST influences the atmospheric winds which, in turn, influence the upper ocean and the thermocline such that the SST is further increased = positive feedback. Only when the conditions are favourable will this feedback generate an El Nino event  where you get increased SSTs, reduced easterly winds and a 'flatter' (for want of a better word) thermocline across east to west Pacific.
La Nina

El Nino events then also cause gradual changes throughout the tropical Pacific ocan which develop in such a way that the SST across the equator slowly returns to normal, thus ending this event. However, the system normally overshoots and the feedbacks act to amplify small cooler anomalies, initiating a La Nina - as said before, it is basically the opposite with strengthened easterly winds and a increase in thermocline gradient. As this process is self-limiting the cycle continues....

What are the meteorological consequences?
Global impacts of El Nino
If you have managed to stick with this to this point, this is probably the bit that will interest you the most - bringing in a bit of Human Geography you could even link this to the development essay as ENSO has, arguably, hindered development in countries across the world!

 During El Nino events changes in SST, alterations to atmospheric circulation, temperature and precipitation occur; with these alterations, via atmospheric dynamics, extending far beyond the tropical Pacific region. As the impacts vary with location, it is practically impossible to identify a general pattern but generally speaking the eastward shift of precipitation in western Pacific tends to provoke huge deficits in the Philippines, Indonesia, north and east Australia, whilst central Pacific experiences increased rainfall.

Global Impacts of La Nina
I was fornutate enough to get to quiz an ENSO researcher at the Met Office,  about everything related to ENSO and I saw lots and lots of similar maps to those opposite - one question that instantly came into my mind was why excatly do the maps suggest that ENSO has no impact whatsoever on Europe. Well, in short, the European region is only relatively weakly effects and there is substantial variability in the observed conditions. Therefore, only when you analysis a long historical record of events will you see the impacts. Such analysis suggests that there is a clear late winter response associated with moderate strength El Nino events; with the tendency for cold conditions in northern Europe and mild southern Europe, with an increased precipitation in a wide band across central Europe (accompanied by decreases elsewhere). Due to this though, it is hard to distinguish between the different factors, with El Nino just one, that are capable of generating such climatic anomalies.

What are El Nino 'flavours'?
I had never, ever heard of this idea of El Nino 'flavours' before and my knowledge is limited to what I am about to write (definetly requires a bit of further indepth research on my behalf!) but I thought I would mention it anyway as its quite an interesting idea. This topic has recieved quite a bit of attention in recent years, due to the slightly differing impacts. The 'normal' El Nino witnesses SST warming in eastern and central Pacific, while the 'dateline' or 'modoki' (apparently Japanese roughly translating to 'similar but different') type primarily experiences warming in the central equatorial Pacific. It has been suggested that the main difference is the influence in Atlantic tropical storms, with such storms typically fewer in 'normal' El Nino years, with 'modoki' events not reducing hurricane activity.
So, I think that is probably the basics of ENSO and I think, before I move on to talking about modelling it and how is it measured, I will probably have to write a post on the basics of climate modelling (it will be the real basics as all this modelling stuff is like a totally seperate science which I won't be able to tell anyone about in any great detail - trust me, its tricky stuff!) and, when I get my head around some scientific papers on ENSO the Met Office provided, I will also write a post on extrinsic forcing factors that effect ENSO. Just one last think to leave you with, follow the link to see up to date measurements taken in the Pacific as part of the Tropical Atmosphere Ocean project.

Taken yesterday (14/09/2011)

I am guessing that most are probably aware that we have just come out of a La Nina (cause of flooding earlier this year in Australia etc.) but at the moment scientists are divided over what will happen next as, although SSTs in Nino 3.4 started to rise again, they have since started to fall again - causing a split amongst researchers as to whether we will remain in the 'normal' state or go back into a La Nina (no one really seems to think we are going to go immediately into an El Nino). If you are interested and catch me around college, I have some plots, and ensemble forecasts which illustrate this.

I hope this made some sense (and was interesting), let me know if you didn't quite get everything as I realise that I am not great at explaining things but I am more than happy to try my best to explain it in a more understandable way if necessary. I am quite intrigued by the idea of El Nino 'flavours' and so if anyone knows anything more about it, please let me know!!! Especially things like what dictates whether an El Nino turns out to be of the 'normal' type or 'modiko' type?

There is some development stuff on the way, along with a few book reviews and a post primarily for all the new AS Geographers on 'Flooding, Farming and the Future'. Millie has emphasised the importance of knowing some case studies really well, with reference to development and colonialism, so I thought I would try and write a post on atleast one, sometime over the next few days, so let me know which one you would most like me to write about - its up to you!

1 comment:

  1. Excellent Blog Vicki!!

    I have learnt a lot through your numerous blogs and I hope you continue them in the same style!!