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Multiple drivers create challenges for forecasting weather direction

How times change. This time last year, we were in the early days of a month which went on to become the wettest February for the UK in a series stretching back to 1862.

Storms CiaraDennis and Jorge, which swept in west from the Atlantic, brought a large proportion of the month’s total rainfall on moisture-laden and relatively warm winds

So will February 2021 follow a similar pattern? Met Office weather presenter Aidan McGivern recently said:

“What we have started with this month is a different pattern, especially in the north, with more of an easterly influence than one from the west, and there are several reasons for this, but the major factor is the strength of the westerly flow, which has and is being influenced by opposing climate drivers.”

A key driver of winter weather in the UK and Western Europe is the North Atlantic Oscillation (NAO) – a measure of average atmospheric pressure difference between an area of high pressure near the Azores and the semi-resident low pressure between Iceland and Greenland. Aidan McGivern recently said:

“This sounds complicated, but what you have to remember is that when the pressure difference is greater than average you have a so-called positive NAO which steers low-pressure systems toward the east across the Atlantic; a negative NAO slows down the flow, and can even reverse it completely by bringing the flow from the east. That is the prospect, a much colder prospect, that we see in the forecast for at least a few days from the weekend.”

So as the NAO is a principal determinant of the UK’s weather in winter, what drivers can affect the NAO? Aidan McGivern continued:

“When meteorologists are trying to gain an insight into longer-term forecasts it is like trying to work out who is going to win a marathon. You can study the form of individual athletes but on the day any number of factors can determine the outcome: and, of course, the favourite doesn’t always win because of the ‘chaos’ inherent within a sporting event, or the atmosphere.”

In meteorological terms there are a number of influences that can affect the positive or current negative phase of the NAO and they can all play out at different timescales.

Atmospheric events even as far away as the tropical Pacific can exert an influence on the strength, shape and position of the jet stream and subsequently the NAO.

As this winter approached, La Niña – the cool phase of the El Niño Southern Oscillation – was taking place. La Niña has historically been linked to colder conditions during late autumn and early winter followed by increased rainfall and milder conditions during the second half of winter.

However, as winter progressed, it soon became apparent that other drivers would provide an opposing influence to La Niña and at shorter timescales.

Often a big driver for affecting the phase of the NAO is a Sudden Stratospheric Warming, where a breakdown of the Stratospheric Polar Vortex (SPV) – a ring of westerly winds high up in the stratosphere – causes a disturbance and a potential reversal of direction which can influence the jet stream below, causing it kink and buckle. An SSW event in early January helped to disrupt the jet stream and contribute towards a south-shifted Atlantic storm track during recent weeks. The SPV has not yet recovered and so can still exert an influence.

The Madden-Julian Oscillation (MJO) is an eastward-moving phase of enhanced tropical rainfall that primarily affects the Indian and Pacific Oceans and occurs on weekly to monthly timescales. When rainfall is enhanced in the West Pacific, it has been shown to lead to weather patterns close to the UK that induce a greater chance of cold weather during winter. At the start of this week, the MJO moved into an active phase over the West Pacific. A strengthening high pressure block to the north of the UK during the next few days, helping to push the Atlantic storm track even further south and resulting in easterly winds, is consistent with what might be expected from this phase of the MJO.

Aidan concluded:

“In recent years, meteorologists have become more aware of the many different influences on the UK’s winter weather – from the stratospheric polar vortex to rainfall patterns in the tropics. Some influences, such as La Niña, can be identified well ahead of the start of winter. Others, such as the MJO, operate on much shorter timescales. Add into the mix the inherent chaos of the weather and you’ve got all the ingredients for a particularly challenging forecast.”

Paul Davies is the Met Office’s Chief Meteorologist and he has the ultimate responsibility for production of the forecast. He recently said:

“Our team of meteorologists have the complex task of producing a forecast taking into account not only the global drivers, but also looking at effects closer to home.

“Some of the differences in the forecast can be quite marginal depending on exactly the area in focus. For example, in Scotland you may have coastal communities lashed by heavy rain, whereas their inland and uphill neighbours may notice a marked change from heavy rain to snow. But all of these weather conditions present hazards, but communities rely on the Met Office to cope with the ever-changing and multi-hazard nature of winter weather and produce a forecast which helps to keep people safe.”

 

Channel website: http://www.metoffice.gov.uk

Original article link: https://blog.metoffice.gov.uk/

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