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Our changing weather patterns: a tale of abrupt transitions



Our changing weather patterns: a tale of abrupt transitions

Our weather is notoriously changeable – that is partly what makes it one of the nation’s talking points. But peering through the statistics a striking pattern emerges: one where the weather oscillates abruptly between different and contrasting phases lasting for a while before the next phase in the sequence begins to dominate.

Let’s look at January’s conditions: the average across the month isn’t headline grabbing. But the pattern back-and-forth between weather types is remarkable. 

The year began warm, but within the first five days it had transitioned to a notably cold and dry period. But then, after a further couple of weeks, it transitioned again to a mild and wet spell.

Tapping into warmth

Sea-surface temperature anomalies from the latest OSTIA analysis, 01 February 2024, compared to the European Space Agency Sea Surface Temperature Climate Change Initiative (ESA SST CCI) climatology data. Indicative of the source region for southern UK coming from the maritime heatwave west of NW France and Portugal.

February may turn out to be remarkable on many fronts. Exceptionally mild and wet across a broad swathe of the UK in the first week, with a major change from the end of this week and beyond as very cold air from Scandinavia floods south into the UK.

We can project these extremes onto large-scale weather patterns: with high pressure towards Greenland; low pressure over Scandinavia; and low pressure occasionally stretching across the UK and into continental Europe. This pattern is characteristically described as a block; when the atmosphere gets ‘stuck’ for long periods of time.

Paul Davies is the Met Office’s Principal Meteorologist. He said: “Against this backdrop, the tropical Atlantic is very warm. Sea temperatures off the north-west African coast are currently comparable to values more typical for July.

Winds will continue to tap into this area of warmth, briefly drawing modified air north into southern and western parts of UK. This tropical-sourced air is being forced to rise as it comes up against the blocked pattern and over western hills bringing persistent, locally heavy rain here, and as the air descends to the lee side of these hills it has been introducing very mild and gusty conditions; because of the foehn effect.” This effect – which occurs where air crossing mountains is warmer and drier on the lee side – was also responsible for the record-breaking January temperature in NW Scotland. The weather station at Achfary recorded 19.9°C on 28 January: a new UK daily maximum temperature for January; and a new winter record for Scotland.

Colder conditions ahead

Paul Davies continued: “Then comes the expected change in weather, with winds switching north and drawing very cold air from the Arctic, initially arriving across Scotland, Northern Ireland and Northern England later this week and then heading further south into southern England. This may mean a significant contrast between the recent weekend, and next weekend with spring-like conditions one weekend and the potential for wintry hazards, especially in those regions susceptible to east and northeasterly winds across northern parts of the UK next weekend.”

Temperature anomalies from the latest GloSea forecasts for the week commencing 12 February. It shows cold air over the UK and deep cold residing over Scandinavia. GloSea is the seasonal prediction system developed and run operationally at the Met Office.

The forecasters’ challenges will be to determine:

  • the exact arrival time of the cold conditions;
  • and the associated battleground between the cold air in the north, and the mild in the south. This will create a zone of disruptive weather, including heavy rain, snow and ice.

At this stage it’s too early to predict with any confidence the evolving story. But if the boundary pivots and stalls, it could result in snow and blizzards for some. If the cold air digs south more quickly, frost and ice may be the hazards to watch out for; while if the cold air is restricted to the north, the hazards may be rain and strong winds in the south, with snow confined to Scotland, Northern Ireland and Northern England.

Phil Stocker is the chief executive of the National Sheep Association. He said: “The late winter and early spring period can be an extremely concerning time for livestock farmers, particularly those working in the UK’s more exposed and upland areas. Any cold snap, especially if accompanied by significant snowfall, can bring notable hardship to livestock, and of course hill farmers. 

“I find it helpful that the Met office is doing more to warn of the risk of bad weather conditions – it gives farmers the chance to assess the risks and do what they can to prepare.”

Paul Davies added: “It’s a close call with only minor wobbles in the weather patterns changing the course of the evolution and associated impact to the UK.”

Through to early March, there is increasing chance of sleet or snow, especially in the north and east of the UK with greater than normal potential for disruptive snow. The specific locations wintry hazards will become clearer as the lead time decreases.

Our advice therefore is to keep abreast with the latest forecasts and warnings. 

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NEMO: a numerical ocean model



A numerical ocean model is a computer programme representing the equations of motion (momentum, conservation of mass and thermodynamics) for the ocean. The model stores each of the physical properties of the ocean (temperatures, salinities and currents) on a three-dimensional grid, writes Ana Aguiar.

Ocean models store physical  properties such as salinity, temperature and currents on a three-dimensional grid. Picture: Adobe Stock

Smaller ocean features can be resolved by using a finer grid with more points, but this requires more computational power. The model evolves these physical properties forward in time using its equations of motion. Models of sea ice and biogeochemistry work using similar principles.

Why do we need a numerical ocean model?

We need these models to predict the state of the ocean within short and long timescales for a variety of purposes, ranging from support to operations at sea (for example, search and rescue) to understanding the role of the ocean in the Earth’s climate system. As the ocean sits beneath the atmosphere, sea-surface temperature patterns have widespread impact on the weather over land. Largely because two-thirds of the Earth is covered by ocean and the heat capacity of water considerably outweighs that of the air, the ocean acts as a regulator of the atmosphere.

In polar regions temperatures become cold enough for seawater to freeze and sea ice forms on the surface of the ocean. Sea ice plays an important role in the climate system because it insulates the ocean from the colder atmosphere in winter and, being whiter than the ocean, reflects sunlight in the summer.

The NEMO modelling framework includes a sea-ice model component, known as SI³ (Sea Ice modelling Integrated Initiative). The sea-ice component is run along with the ocean component in a similar manner but using a different set of equations. To understand and prepare for climate change we need to account for the role of the ocean and sea ice.

How is the NEMO model developed?

Nucleus for European Modelling of the Ocean (NEMO) is a state-of-the-art ocean modelling framework. NEMO is developed by a European consortium with the objective of ensuring long-term reliability and sustainability of the code. In other words, the task of maintaining and developing such a complex computer programme requires a well-coordinated team effort, involves tens of developers and hundreds of users.

In the UK there are two member organisations: the Met Office and the National Oceanography Centre (NOC). Met Office Scientific Manager in Ocean Modelling, Ana Aguiar explains: “We work in partnership through the Joint Marine Modelling Programme, contributing to the development of NEMO. The code is publicly available for use in research and commercial applications. It is imperative to reach as many users as possible, to ensure the code gets tested and pushed to the limits of its usability. User requirements then prompt further advances.”

NEMO benefits from continual work to improve its performance (scientific and computational efficiency), to incorporate new scientific and process understanding, and to exploit the increase in supercomputer resources. When the developments are sufficiently mature and can provide significant scientific or technical improvements, a new NEMO version is released. Along with scientific upgrades (which tend to be increasingly computationally demanding), we must deliver code optimisation to make the best use of the available computing resources.
This video presents how NEMO is used by the Copernicus Marine Environment Monitoring Service.

What’s next?

The next NEMO release (expected to be rolled out this summer) will deliver significant improvements to model performance allowing it to run considerably faster. In the long term, among other things, we are also working towards porting the NEMO code to Graphical Processing Units (GPUs) to ensure continuity of the code in future mainstream High Performance Computing architectures

During April we are exploring the topic of the ocean and climate. Follow the #GetClimateReady hashtag on X (formerly Twitter) to learn more throughout the month.

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What’s the pollen outlook this year?



The Met Office launches this year’s Pollen Forecast today (Friday 22 March) which will run until mid-September.

One in five people in the UK suffer from hay fever, meaning spring can be the start of watering eyes, runny noses and keeping a keen eye on the pollen forecast, which is now available and provides a look at the pollen amounts and types in the air for the next five days.

This February was the warmest on record for England and Wales and winter too has followed this wet and warm pattern. This has an influence on the pollen season ahead.

Yolanda Clewlow is the Met Office’s Relationships Manager for Health and Air Quality, and the UK Pollen Forecast Manager. She said: “We’re constantly monitoring what’s in the atmosphere and what might be coming in terms of pollen in the air. We look at a very broad picture for the pollen outlook, including the weather in the previous year when pollen is formed on some plants to make a judgement on how heavy a season might be for pollen release. Rain, wind, temperature and sunshine all affect how much pollen will be produced and how much it will be dispersed.”

The Met Office works with the University of Worcester to monitor pollen levels. There are many factors at play when predicting levels, with much depending on how much pollen has formed on the plants and the weather at the time of release. Most people are allergic to grass pollen, which is generally released around mid-May to July.

Yolanda continued: “In terms of birch pollen, we are seeing the first grains in the atmosphere in the most southerly and easterly counties but overall, there’s currently generally low levels for tree pollen due to changeable conditions. We expect the season to be of mild to average severity depending on in-season weather.

“Grass pollen could start early, with some pollen airborne from late April. Average severity is most likely, but this will largely depend on the weather in April and early May. Warm sunshine and some rain is needed for good grass growth and pollen production. Dry conditions would inhibit growth.”

There are millions of hay fever sufferers across the UK. The Met Office pollen forecasts provide vital information to help reduce the impact pollen has on their health. 

Emma Rubach is the Head of Health Advice at Asthma + Lung UK. She said: “High pollen levels can be dangerous for people with lung conditions like asthma or COPD (chronic obstructive pulmonary disease), triggering symptoms such as breathlessness, coughing or wheezing. and could lead to hospitalisation or a potentially life-threatening asthma attack.

“To protect themselves, we would advise people with hay fever to use their preventer inhaler every day as prescribed, if they have one, and always have their reliever inhaler with them, in case symptoms do arise.  

“If you’re allergic to pollen, using steroid nasal spray every day can also help, together with non-drowsy antihistamine tablets to help reduce the allergic reaction. It is also a good idea to check pollen and air pollution forecasts in their local area, so they can avoid going outdoors as much as possible on high pollen days.”    

The three types of pollen

The pollen season generally has three main pollen type phases:

  • Tree pollen – late March to mid-May
  • Grass pollen – mid-May to July (most people are allergic to grass pollen)
  • Weed pollen – end of June to September

“The first thing to do is to make sure you understand the type of pollen you’re allergic to and monitor the forecast,” said Yolanda.  

“That way, you know when the very high days of pollen are coming and you can take precautions to try and avoid the pollen. It’s not possible to avoid it altogether though so for some, it’s about taking that preventative medication and planning your day to try and avoid being out in the pollen at the peak times.”

Pets can also be affected by hay fever and in severe cases, vets can often prescribe medication to help furry friends through the summer.

Advice on dealing with hay fever is available as part of WeatherReady with the Met Office.

Pollen levels and climate change

With global temperatures increasing, the pollen season in the UK isn’t immune to the effects of a changing climate. Climate models suggest an increasing likelihood of warmer, wetter winters and hotter drier summers, which will influence the release of pollen into the atmosphere.

A study conducted by the University of Worcester looked at pollen trends in the UK over the last 26 years to better understand the relationship between these trends and meteorological factors. It examined a range of UK pollen sites, with a focus on the key pollen types: grass, birch and oak. 

Dr Beverley Adams-Groom, Senior Pollen Forecaster at the University of Worcester and lead author of the study, said: “Birch tree pollen is showing a trend for increasing severity, particularly in the Midlands region, but grass pollen is not showing an increase over time. Seasons for all pollen types are tending to start earlier and earlier but there is no evidence that season duration is changing. Where changes are occurring, these are largely related to the effects of global warming, with similar patterns seen from research in nearby countries.”

Get pollen alerts directly to your phone on the Met Office app, or visit the pollen forecast on the Met Office website.


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Met Office scientists protecting our forests from pests and pathogens



Today is the United Nations International Day of Forests, and in this blog post we explore the importance of this work.

The Climate and Plant Biosecurity Climate Service, funded by the Department for Environment, Food and Rural Affairs (Defra), is a collaboration between the Met Office’s Vegetation-Climate Interactions team, Defra’s Plant Health Risk and Horizon Scanning team, the University of Exeter, Fera Science, the University of Warwick, Forest Research and The Royal Botanic Gardens, Kew.

Oak processionary moth caterpillars. This species is a non-native pest of woodlands. Picture: Adobe Stock.

Since 2006 the non-native and invasive Oak Processionary Moth has been spreading across England and Wales. Picture: Adobe Stock

The aim of the service is to provide analyses, tools and guidance to help manage the climate-related risks to UK plants, particularly trees and forests, from plant pests and pathogens.

Pest and pathogen outbreaks

Pests and pathogens present serious risks to our trees and forest habitats as well as the ecosystem services they provide. The number of new pest and pathogen outbreaks affecting trees has increased rapidly in recent years (see table, Source: Forestry Commission).

Year (since 1971)             New tree pest or pathogen outbreak

  • 1971      Dutch elm disease
  • 1983      Great spruce bark beetle
  • 1984      Phytophthora alni
  • 1995      Gypsy moth
  • 1997      Dothistroma needle blight
  • 2002      Phytophthora ramorum
  • 2002      Horse chestnut leaf miner
  • 2003      Phytophthora kernoviae
  • 2005      Bleeding canker of horse chestnut
  • 2006      Oak processionary moth
  • 2006      Phytophthora pseudosyringae
  • 2007      Pine tree lappet moth
  • 2010      Acute oak decline
  • 2010      Phytophthora lateralis
  • 2012      Ash dieback
  • 2012      Asian longhorn beetle
  • 2012      Sweet chestnut blight
  • 2012      Phytophthora austrocedri
  • 2014      Phytophthora sikiyouensis
  • 2014      Sirococcus tsugae
  • 2015      Oriental chestnut gall wasp
  • 2017      Elm zigzag sawfly
  • 2018      Eight toothed spruce bark beetle
  • 2021      Phytophthora pluvialis

Climate (variability and change) influences pests and pathogens in many ways, including i) the timing of life cycle events (such as emergence from egg to caterpillar), ii) the spatial distribution and spread, and iii) the introduction and establishment of non-native species.

Tools to help manage UK plant biosecurity

The UK Climate-Pest Risk Web Tool is one of the tools that has been developed by Met Office scientists Neil Kaye and Deborah Hemming (Vegetation-Climate Interactions team) in collaboration with biosecurity and forestry experts at Defra, Forest Research, Fera Science and the University of Warwick. It integrates ecological knowledge and models of known temperature thresholds for different pests/pathogens, with up-to-date climate observation datasets from the Met Office National Climate Information Centre.

Deborah Hemming, Scientific Manager of the Vegetation-Climate Interactions team at the Met Office, who leads this climate service, notes: “When tree pests and diseases become established, they can wreak havoc on our woodlands. Commercial forestry can be affected hugely, but they also affect the landscapes of our islands which many people love and cherish. In the 1970s, Dutch Elm Disease killed most of the UK’s stately elm trees with those plants remaining being small stands in isolated sections of hedgerow. Similarly, Ash Dieback since 2012 has decimated ash trees with similar devastating effect.

“When trees die, especially native broadleaf trees, there are impacts on landscape and wildlife. And in times of climate change there is also a reduction in the availability of carbon stocks because trees provide a hugely valuable ecosystem service by drawing down atmospheric carbon and locking it away. By joining forces with experts in plant biosecurity at Defra, ecological modelling at University of Exeter, forestry at Forest Research and ecological systems at Royal Botanic Gardens, Kew, we are able to provide scientifically robust and useful research, tools and services to help protect UK trees and forests now and into the future.”

The tool enables users to easily estimate the timings and locations of pest outbreaks across the UK, and inform actions to assess, survey, monitor and eradicate plant pests, helping to enhance UK plant biosecurity.

Simon Toomer, Curator of Living Collections at Royal Botanic Gardens, Kew, says: “As we develop strategies and plans to adapt and prepare our tree and shrub collections for changing climatic conditions, one of the most complex and least understood threats is that from pests and diseases. This research is going to help us understand how changes in general climate variables translate into changes in the specific conditions experienced by pest species, and how we may adapt our management accordingly.”

Healthy ash trees form an important part of the UK's tree canopy. Isolated trees are also a feature of hedgerows. Picture: Adobe Stock

Healthy ash trees form an important part of the UK’s tree canopy. Isolated trees are also a feature of hedgerows. Picture: Adobe Stock

Improving monitoring and modelling of microclimates

Pests and pathogens respond to microclimates within the habitats where they live. To improve the estimates of pest/pathogen risks, ecological modellers at the University of Exeter have developed mechanistic microclimate models to estimate temperature and humidity within relevant habitat locations e.g., under tree canopies, inside tree trunks or buried at various depths within the soil. In these habitats, microclimates can vary by 40-50°C and be significantly different to conditions observed at weather stations.

Ilya Maclean, Professor of Global Change Biology at the University of Exeter, says: “It is important to understand the climate as pests and pathogens experience it. This can be very different from the conditions measured by a weather station. My team is developing models that allow us to do this and the data we are collecting as part of this project will be invaluable in helping us improve our models”.

Daegan Inward, Senior Scientist at Forest Research, explains further: “We know that beetle outbreaks are often associated with sun-warmed stems, and understanding the under-bark microclimate is important to help predict the risk of insect establishment and population growth in Britain.”

Climate change and globalisation

To validate the microclimate models, in February 2024 the Met Office, the University of Exeter, Forest Research and Kew Gardens began a campaign of microclimate monitoring at five sites in different forest habitats across southern England (Cornwall, Dartmoor, Alice Holt, Kew Gardens and Wakehurst). The team installed a series of sensors to monitor temperature within the woodlands and inside tree trunks. The data collected will help improve the understanding and modelling of variations in the pest/pathogen microclimates, and therefore the estimates of biosecurity risks.

Defra Chief Plant Health Officer Nicola Spence said: “Climate change and globalisation are increasing the number and diversity of pests and pathogens we are exposed to, resulting in an ever-growing number of threats. This collaborative effort to develop climate modelling tools and improve our understanding of pest or pathogen climate interactions, will allow us to better plan for and improve our ongoing surveillance and monitoring. This work is crucial to adapt to a changing climate and better understand how the health and resilience of our trees could be at risk.”

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