Week 2 – Blocking has come…

16.10.2010

A very interesting period is on the way, at least from the meteorological point of view…while I am writing a blocking regime is about to build up over the Euro-Atlantic sector. The intention of this post is a real-time description of the evolution of blocking. However, before looking at the maps, it might be worth to specify what we mean by blocking.

Schematic of wave-breaking evolution. At the top is the temporal behaviour of a Rossby wave breaking. At the bottom the difference between anticyclonic and cyclonic wave-breaking is represented. The contours represent theta on PV2, whereas the colours distinguish between warm (red) and cold (blue) air masses. (Giacomo Masato)

At the top of the figure a common evolution of a Rossby wave-breaking is shown. The overturning occurs over time and by step 3 the two air extrusions have rotated by almost 90 degrees, with the warm air mass pushed towards the sub-polar regions and the cold air mass travelling towards the Tropics. Rossby wave-breaking has been often thought of as the leading mechanism for blocking occurrence (see for example Pelly and Hoskins, 2003), where blocking can be defined as Rex (1950) did, according to the following points:

  • The basic westerly current must split into two branches;
  • Each branch current must transport an appreciable mass;
  • The double-jet system must extend over at least 45 degrees of longitude;
  • A sharp transition in the westerlies from a zonal type flow upstream to a meridional type downstream must be observed across the current split;
  • The pattern must persist with recognizable continuity for at least ten days.

In time these criteria have slightly been modified, but they perfectly set the scene for the following discussion. At the bottom of the figure a further characteristic of wave-breaking is added. It is shown that the meridional overturning can be either anticyclonic (left hand side) or cyclonic (right hand side). The former usually occurs downstream and on the equatorward side of the jet stream, whereas the latter occurs upstream and on the poleward side of the jet.

The second case in particular has been associated with the occurrence of the negative phase of the North Atlantic Oscillation index (NAO – see Woollings et al., 2008, for reference), which is the leading pattern of variability over the Atlantic sector. A negative NAO phase corresponds to high pressure values over Greenland and a southward-shifted jet-stream, that can be located over Southern Europe or even North Africa. Moreover, the enhanced meridional direction of the flow (as opposed to a strong westerly jet) that occurs during blocked-type regimes is very likely to cause temperatures higher than normal at higher latitudes and, conversely, lower than normal at lower latitudes. To verify how this actually happens, let us follow real-time the evolution, day-by-day, of the occurring cyclonic blocking.

The image below shows the streamlines at 200 hPa, taken from this web-site, that hosts updated maps from the most important NWP centres around the world.

It illustrates the situation at the tropopause for today, 16 October 2010. In particular we can observe two ridges, one over the East Atlantic (off the Irish coasts), the other over the western side of the basin. The axis along which the latter is building up has been underlined with a black solid line. This wave’s amplification is the one we are interested in, the one that will initiate blocking.

18.10.2010

Two days after, on the 18th of October, the wave has travelled from the Western to the Central Atlantic. It has amplified dramatically and extended up to Greenland and Northern Canada. The cyclonic curvature of the breaking is now visible (underlined by the red solid line), with a ridge over the southern tip of Greenland and the low over North America. Blocking has now started. It is interesting to note how the jet-stream has split into two branches (first point according to Rex’ classification of blocking). The first is associated with the strong winds occurring downstream, with an enhanced north-south direction, responsible for the very cold temperatures that we are going to experience during the next days over the UK. The second is the south-shifted one, typical of a NAO- situation, that is present over North Africa.

20.10.2010

We are on Wednesday the 20th. Now blocking is in its mature phase, the wave has travelled further east, although its downstream movement has decisively decreased. The blocking dipole is still visible between Greenland and North America, exhibiting its cyclonic signature. The two jet’s branches have moved accordingly. The northern one has now extended into France and Northern Italy, bringing progressively colder and unsettled conditions over those regions. The southern branch has moved downstream, with its maximum now placed between Libya and Turkey. At the moment, while I am writing (23:28), the department meteorological station reads 1 degree Celcius…

21.10.2010

The last night the minimum temperature has reached -1.4 degrees. The Rossby wave responsible for blocking has now weakened a lot, although the blocking dipole is still present between Greenland and North America. Upstream strong westerlies are about to replace the blocked regime…maybe. Tomorrow this post will be published and there won’t be more time to follow the actual blocking in real time. Therefore, let’s have a look at the forecasts.

The map shows the situation  on Monday at OOUTC. A new wave, centred on Central Atlantic, has amplified and it has reinforced the blocking structure, that keeps maintaining its cyclonic signature. Downstream, once again, a strong north-south wind’s component is well visible, although this time the core of the jet is marginally affecting the UK. Nevertheless, the blocked regime established since the beginning of the current week is predicted to last until the beginning of the next one, alongside the anomalously cold temperatures. The last point on Rex’ list stated: ‘the pattern must persist with recognizable continuity for at least ten days’. It seems this blocking event will be a proper one!

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