Fantastisch verhaal van de Met Office. Ik vind het zo goed dat ze hun kennis met ons delen. Zo neem je het publiek mee in de wonderlijke en fascinerende wereld van het weer. Chapeau!
What does El Niño imply for the UK this winter?
Unlike some parts of the world, the effect of El Niño on Europe is relatively subtle. In El Niño years there is a tendency for early winter to be warmer and wetter than usual and late winter to be colder and drier.
Despite this, it is just one of the factors that influence our winters, so other influences can overwhelm this signal – it is relatively straightforward, for example, to find years where these general trends were not followed.
Grappig en opvallend dat MetOffice hier een andere mening heeft dan het KNMI. Om eerlijk te zijn denk ik dat het KNMI fantastisch werk doet, maar door alle bezuinigingen en relatieve kleine omvang niet mee kan in de kennis van MetOffice. Uiteraard ben ik geen kenner op dit vlak, maar ik heb sterk dat idee.
Closer to home, sea surface temperatures to the west of the UK have been notably lower-than-average in recent months. While it is true the westerly winds that we typically get in winter would have to pass over this region, it is unlikely that this will directly have a strong bearing on expected temperatures. This is because temperatures at this time of year are strongly affected by the direction of the wind. Eastern Europe and Scandinavia are 10-20°C colder than the Atlantic Ocean in winter, so our weather will depend much more on how often winds blow in from the north and east than whether the Atlantic is 1-2°C cooler than usual.
More broadly within the North Atlantic Ocean, sub-tropical temperatures to the south of this cool region are widely above average. This combination results in an increased north-south temperature gradient, which is expected to provide greater impetus for Atlantic depressions. For the UK, this would favour relatively mild, unsettled weather conditions.
-> Het zeewaterpatroon is dus gunstig voor een positieve NAO. Oei!
Our weather is also affected by changes in the stratosphere
European winters are also sensitive to what is happening in the stratosphere, the layer of the atmosphere between 10 and 50 km up that lies above the weather. The equatorial stratosphere is home to the Quasi-Biennial Oscillation (QBO), a cycle that sees winds switch from easterly to westerly and back roughly every 27 months. First noted by Met Office scientists over 40 years ago, the link with European winter weather has stood the test of time. This year, the QBO is in a westerly phase, which implies an increased chance of a mild and wet winter at the surface.
Stratosfeer nemen ze ook mee. Westelijke QBO vergroot kans op een milde en natte winter.
A considerable part of the year-to-year differences between UK winters is related to the occurrence of sudden stratospheric warmings (SSWs). In these events, the polar stratospheric vortex – the fast moving circulation of stratospheric air that whirls around the North Pole in winter – abruptly breaks down. They occur one winter in two on average, and events are most common in January or February. In the majority of cases SSWs lead to the establishment of cold easterly flow at the surface across Europe and the UK. The last SSW was in January 2013, and this event contributed to the cold late winter and early spring in that year.
Whether we get an SSW or not depends on a number of influences, such as El Niño and the QBO. Currently our models suggest an increased likelihood of an SSW from January onwards.
If this were to happen, its effects would not be felt much before the end of our November to January outlook period. At the moment, therefore, this is still a long way off, and we consider this suggestion to be tentative.
Kans op een SSW neemt later in de winter toe.
So what can we expect in the UK this winter?
Most of the global drivers discussed above tend to increase the chances of westerly weather patterns during our November to January outlook period. Our numerical prediction model, being sensitive to these drivers, also predicts a higher-than-normal chance of westerly conditions. This results in an outlook for an increased chance of milder- and wetter-than-usual conditions, and a decreased chance of colder and drier conditions, for the UK. Our outlook also indicates an increase in the risk of windy or even stormy weather.
Meeste indicatoren wijzen op een vergrote kans op westelijke winden. Natter en zachter dan gewoonlijk. Kans op storm is groter dan gebruikelijk. Let wel, hier in de tekst hebben ze het niet over de verlaagde zeetemperatuur. Dat kan natuurlijk wel schelen.
It should be noted that these shifts in probability do not rule out the less favoured types of weather completely. Also, a general tendency for one type of weather over the three months as a whole does not preclude shorter spells of other types of weather.
Finally, there are hints that the outlook might be rather different in the late winter, with an increased risk of cold weather developing. Nevertheless, it is currently too early to be confident about this signal.
Er zijn signalen dat de tweede helft van de winter duidelijk ander weer geeft, met toename kans op kouder weer.
Zoals altijd hou ik jullie op de hoogte van de ontwikkelingen!