Situazione anomala ma non del tutto eccezionale

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È anomala la presenza contemporanea di due anticicloni: l’anticiclone subtropicale Atlantico (caldo) che si è allungato a nord fino nell’alto Atlantico e l’anticiclone siberiano (freddo) che si è allargato ad ovest verso l’Europa

L’ondata di freddo non è eccezionale, né sono state raggiunte temperature fredde da record. È eccezionale l’espansione contemporanea di due anticicloni: l’anticiclone subtropicale Atlantico (caldo) che si è allungato a nord fino nell’alto Atlantico e l’anticiclone siberiano (freddo) che si è allargato ad ovest verso l’Europa. Alle isole Falkland (oltre il circolo polare artico) fa più caldo che a Milano o ad Istanbul. Lo dice la Wmo (l’Organizzazione meteorologica mondiale).

Secondo la Wmo l’attuale ondata di freddo polare sull’Europa è certamente anomala, ma non è, dal punto di vista meteorologico, un evento eccezionale. L’anticiclone freddo siberiano che si allarga a dismisura e blocca le correnti atmosferiche occidentali non è un fenomeno raro: era già successo varie volte in quest’ultimo decennio e, di recente, nell’inverno 2009-2010 (quando ci fu il clamoroso fallimento della Conferenza sul clima di Copenhagen). Anche se le temperature minime registrate in Europa variano dai -37°C della Russia fino ai -15°C dell’Europa centrale, non sono temperature record. L’aria fredda artica proveniente dalla Russia, passando attraverso i Balcani, si è poi scontrata con l’aria più mite ed umida del Mediterraneo provocando copiose precipitazioni nevose in Italia, Grecia, Turchia e perfino Algeria e altri paesi del nord Africa.

Sull’Atlantico, intanto, l’anticiclone subtropicale (noto in estate come anticiclone delle Azzorre) si è talmente allungato lungo i meridiani da raggiungere le aree subpolari del nord Atlantico, col risultato che alle isole Svalbard, che sono a 80°nord, cioè ben oltre il circolo polare artico, ci sono in questi giorni temperature che sono di circa 5°C superiori alle temperature di Milano o di Istanbul. La espansione verso il polo Nord dell’anticiclone atlantico subtropicale (anticiclone dinamico caldo) in concomitanza con l’espansione verso est dell’anticiclone siberiano (termico freddo) rappresenta, invece, una combinazione piuttosto rara.

In conclusione, l’eccezionalità, non sta nell’irruzione di aria fredda siberiana da nord est sull’Europa, ma sta nella concomitante combinazione di espansione di due anticicloni con due caratteristiche molto diverse che hanno bloccato i movimenti dell’atmosfera ed hanno accentuato la situazione di irruzione di aria fredda siberiana la quale non può considerarsi un evento eccezionale. (V. F.)

Cold spell in Europe in late winter 2011/2012

WMO: 7 February 2012

After unusually mild weather in December 2011 and early January 2012 almost all over Europe, the weather situation changed abruptly in the middle of January. An incursion of cold polar air, coming from northern Russia at the south flank of an extensive high pressure area brought extremely low temperatures over large parts of Europe and also some considerable snowfall especially over various parts of the continent.

This Siberian high pressure system is preventing milder temperatures and maritime storms from moving from the Atlantic Ocean eastwards over Europe. This high pressure area was very stable causing a continuous flow of cold air to Europe over many days . This “blocking system” is extremely large in its extent but it is not an unusual phenomenon in the Northern hemisphere winter.

A similar high pressure “blocking” system was also responsible for the more significant cold winter of 2009/2010, when cold conditions started in mid December and continued over most of January and
February period.

In the last few days of January and at the beginning of February, further extremely cold continental air from Russia arrived and brought ongoing frost to eastern, southeastern, central and large parts of western Europe. Minimum temperatures in Moscow went down to -25°C until the beginning of February. Some east European countries (Latvia, Belarus, northeastern Poland, Ukraine) experienced minimum

temperatures of around -30°C, northern Russia down to -37°C. In eastern Germany, minimum temperatures below -20°C were measured in many places, in wester n central Europe around -10°C to -15°C or below (e. g. Luxembourg -13°C on 3 February, Strasbourg in Franc e at the Rhine river -15°C on 5 February, Basel in Switzerland -17°C on 6 February).

However, all these minimum temperatures were not new records. The long duration of the cold period, its relatively late onset and the extent of the cold area are noteworthy but not exceptional. The continental cold air extended even over the Balkan peninsula; slight ongoing frost was recorded even in northern Greece.

The cold air coming from the north was fed with a strong moisture flux from the central Mediterranean Sea. This caused heavy snowfall over parts of south eastern Europe such as the Balkans, Romania, Bulgaria, Italy, and Algeria and other countries in Northern Africa.

In contrast, mild air moved over the North Atlantic northwards over Iceland up to the Arctic region. The temperature in Svalbard, far north in the Arctic, reached repeatedly up to 5°C in recent days, more than in
Milano (Italy) or in Istanbul (Turkey). Much of North America has also been unusually warm.

Part of the explanation is the the so-called Arctic Oscillation which is the difference in pressure between Polar areas and mid-latitude areas (where most of the population in Europe lives). At the moment there is a negative Arctic Oscillation, which favors cold conditions in Europe and relatively warmer conditions in the Arctic.

Similar cold spells with similar weather conditions occurred several times during the past decades. Similarly low temperatures in central Europe and even far higher snow depths were recorded as recently as February 2010.

Temperatures have been also been extremely low from the northern part of East Asia to Central Asia (in and around Mongolia and Kazakhstan) since mid-January. The influence of cold air has extended to Central to Western Europe as well as to all over Central Asia, such as Uzbekistan and Tajikistan, since the beginning of February.

This update is based on the information provided by WMO Member states in the region and the WMO Regional Climate Centres in Germany for Europe and Tokyo and Beijing for Asia