Summer large-scale weather patterns in Central Europe have changed noticeably. Our analysis of June, July and August shows that southerly flow patterns occur much more frequently today than at the beginning of the study period in 1891. At the same time, cooler weather patterns from the west and northwest have become less important.
This shift affects more than just summer temperature levels. It can also favour longer-lasting periods of heat and drought and, when sufficient moisture is present, severe thunderstorms and heavy rainfall.
What are large-scale weather patterns?
Large-scale weather patterns describe the broad distribution of high- and low-pressure systems across Europe. They determine the direction from which air masses reach Central Europe and usually persist in their basic structure for several days.
Three overarching circulation patterns are distinguished:
- Zonal weather patterns: Air flows predominantly from west to east, often bringing mild, moist Atlantic air into Central Europe.
- Meridional weather patterns: The circulation has stronger north-south components, allowing both very warm and unusually cool air masses to penetrate far into Central Europe.
- Mixed weather patterns: These combine characteristics of zonal and meridional circulation patterns.
Southerly large-scale weather patterns are becoming more frequent
For our analysis, summer large-scale weather patterns were grouped according to their prevailing flow direction. The evaluation since 1891 shows a clear shift: at the beginning of the study period, just under a quarter of all summer days were associated with southerly weather patterns. In more recent decades, their share has at times reached nearly 50 percent.
This means that almost every second summer day can now be assigned to a southerly flow pattern. The frequency varies from year to year and from decade to decade, but the long-term increase is clearly visible in the analysis.
At the same time, maritime westerly and northwesterly patterns, which often bring cooler Atlantic air into Central Europe in summer, occur less frequently. This shift increases the likelihood of warm to hot weather periods.
How large-scale weather patterns influence temperatures
Temperature differences between individual summer large-scale weather patterns are substantial. In our analysis, particularly high temperatures occur with southeasterly flow. This often brings dry, continental and very warm to hot air into Central Europe.
On days with southeasterly patterns, the mean temperature is around 4.6°C above the average across all summer days considered. The opposite is true for northwesterly patterns: they bring comparatively cool and moist maritime air and average about 2.3°C below the summer mean.
This produces an average temperature difference of almost seven degrees between northwesterly and southeasterly patterns. When warm circulation patterns become more frequent and cooling westerly and northwesterly patterns become less common, the overall summer temperature level is directly affected.
However, the changing frequency distribution is not the only influencing factor. Against the backdrop of broader climate change, the air masses reaching Central Europe are also warmer today than they were several decades ago. These two developments can reinforce one another.
Why southerly patterns favour heatwaves
With a southerly or southeasterly flow, very warm air masses from the Mediterranean region, the Iberian Peninsula or North Africa can reach Central Europe. Whether this results in a brief heat spike or a longer-lasting heatwave depends primarily on how persistent the weather pattern is.
A southerly flow is not automatically associated with a blocking weather pattern. However, if a strong high-pressure system over Europe also remains in place, the exchange of air masses can be restricted for several days. The air and ground continue to heat up, while nights often provide only limited cooling.
We explain the role of a strongly meandering jet stream and blocking high-pressure systems in more detail in our article: “The Challenge of Meridionality: Why Extreme Weather Events Are Lasting Longer”.
Why southerly patterns can also bring heavy rainfall
Southerly large-scale weather patterns do not necessarily mean dry heat. If moist air from the Mediterranean is also transported into Central Europe, a high-energy, thunderstorm-prone weather situation can develop.
If a cold front or another Atlantic disturbance meets the heated, humid air, severe thunderstorms with heavy rainfall, hail and strong wind gusts are possible. If thunderstorms move only slowly, very high rainfall totals can accumulate locally within a short period of time.
The same large-scale circulation patterns can therefore favour heat first and severe thunderstorms afterwards. The actual development, however, depends on additional factors such as humidity, atmospheric stability, the position of fronts and storm movement speed.
What this development means for operational planning
For municipalities, infrastructure operators and operational managers, persistent summer weather patterns increase the need for planning:
- Heat: Working hours, heat protection measures and staffing or on-call arrangements need to be adjusted early.
- Drought: Irrigation needs, water demand and stress on green spaces increase.
- Infrastructure: Road surfaces, railway tracks and technical installations can be exposed to greater stress during prolonged heat.
- Heavy rainfall and thunderstorms: Staffing, event safety arrangements and protective measures need to be adapted to the current situation at short notice.
Large-scale weather developments can often be identified several days in advance. By contrast, where individual thunderstorms will form and how intense they will become can usually only be assessed reliably at short notice. The key is therefore to combine early situation assessment, continuously updated local forecasts and professional nowcasting
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This gives municipalities, infrastructure operators and operational managers more than just weather data: it provides an expert-interpreted basis for decision-making. Heat, drought and severe-weather situations can be identified earlier, resources can be planned more effectively, and measures can be adjusted quickly if conditions change.
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