Posted by Co2sceptic on Apr 9th 2013
views 767,999
Here is part of a great blog from Paul Homewood concerning the Met Office’s Private Briefing Document For The Environment Agency....below is his section on the location of the Jet Stream, regular readers of Piers Corbyn would know the answer to this issue....

Following the wet summer in the UK last year, the Met Office provided the Environment Agency with a briefing document, giving an overview of the weather. This was discussed at the September Board Meeting of the Environment Agency, which Met Office officials attended.

As far as I know, this document, which I obtained through FOI, has never entered the public domain. It is brutally honest in admitting how little the Met’s scientists understand about what affects our climate, and, in particular, what caused the unusual weather last year. This is in stark contrast to many of the hyped up claims, made in public statements in the recent past by, among others, the Met Office themselves.

The full document is reproduced below, but there are four particular areas I wish to focus on.

....2) Jet Stream Changes

It is now well known that that last year’s wet weather, (and the drought that preceded it), was the result of changes in the position of the jet stream. The Briefing Document has this to say:-

What is causing this summer’s wet weather?

The jet stream has been displaced southwards compared to its climatological summertime position. The jetstream is the fast-moving ‘river’ of air at altitudes of around 30,000ft which forms in the mid-latitudes at the boundary between the cold air surrounding the poles and the much warmer air in the tropics. It usually runs from west to east, and acts to develop and steer the low pressure systems which are responsible for much of the UK’s rain. On average, these systems pass to the northwest of the UK, and hence northwestern parts of the UK – particularly higher ground such as in Western Scotland and Cumbria – receive the most rain.

However, when the jetstream dips to the south of the UK, the distribution of rainfall is skewed away from the climatological average, and southern areas can see periods of significantly above average rainfall and associated higher risk of river and surface water flooding. Not only do the low pressure systems steer across southern areas, but the following factors act to increase the risk of heavy rain and flooding:

· different prevailing wind direction means that different windward slopes will be subject to enhanced rainfall

· the frequent southerly to easterly component to the airflow means that warm, thundery air from the near Continent may be drawn towards the UK, increasing the potential for heavy rainfall

· fronts are more likely to become slow-moving, giving persistent rain in some areas

· between the low pressure systems themselves, the dominant low-pressure (‘cyclonic’) environment is conducive to formation of heavy showers during summer. Again, these may be slow-moving, with an increased risk of intense downpours and surface water flooding.

Low pressure systems of this nature are unusual in summer and because the atmosphere is warmer it can hold more water than in other seasons resulting in significant amounts of rainfall.

The $64000 question, of course, is why has it moved. The Met Office are admirably frank. They admit they do not have a clue. This is what they say:-

The jet stream, like our weather, is subject to natural variability – that is the random nature of our weather which means it is different from one week, month or year to the next. We expect it to move around and it has moved to the south of the UK in summertime many times before in the past. It has, however, been particularly persistent in holding that position this year – hence the prolonged unsettled weather.

This could be due to natural variability – a bad run of coincidence, if you will – but scientific research is ongoing research to investigate whether other factors at play.

Factors which might contribute include:

· North Atlantic Sea Surface temperatures are warmer than normal. These can drive low pressure during summer over NW Europe, and have been a consistent feature of the last five summers (June, July August), all of which have been wetter than the climatological average for 1971-2000;

· It has been suggested that the decline of Arctic Sea Ice may drive low pressure over the UK, although this remains very uncertain at present. Record loss of summer Arctic sea ice cover has also been a consistent feature of the last five summers;

· Recent summers have been under the influence of La Nina-type forcing from the tropical Pacific. Although the tropical East Pacific has warmed in recent months and there are indications of a transition to El Nino conditions, the recent weather patterns in the tropical Pacific are still representative of La Nina conditions, with very disturbed weather over Indonesia and the West Pacific. La Nina drives an increased risk of low pressure over the UK and predisposes the jetstream to shift southwards.

· There is evidence that the circulation changes over the UK are part of a pattern of changes which circumnavigates the whole of the northern hemisphere mid-latitudes.

So, while they are researching various factors, they actually have no evidence on any of them, and certainly none which can link jet stream changes to “climate change”.

But none of this appears to have stopped Julia Slingo telling the Telegraph “The trend towards more extreme rainfall events is one we are seeing around the world, in countries such as India and China, and now potentially here in the UK. “

Or head of the Environment Agency, Lord Smith, informing us “We are experiencing a new kind of rain. Instead of rain sweeping in a curtain across the country, we are getting convective rain, which sits in one place and just dumps itself in a deluge over a long period of time.”

Or DEFRA warning us that “The climate is changing. This means we are likely to experience more flooding”.

Click source to read FULL report