Changing atmospheric circulations in a warming climate

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Pedro Herrera Lormendez | Technische Universitaet Bergakademie Freiberg

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In recent decades, we humans have strongly influenced the behaviour of climate given the enormous amounts of “heat-trapping” gasses that we have emitted since the beginning of the industrial revolution. These greenhouse gasses have generated an increase in the Earth’s temperature and this has had an implication for the climate as we once knew it. More extreme meteorological events have become more common and they are expected to continue to affect us more strongly in the future. Events like longer-lasting heatwaves, droughts or intense flooding taking place where they were not common in the past will  become the “new normal” of our daily news. But, why are these extreme events increasing and what causes them in the first place?

Atmospheric circulation patterns primarily determine the day-to-day weather we experience in our cities and regions, but what do you know about them? You usually see them displayed on the news during weather reports: High and low-pressure systems, warm and cold fronts as well as air masses are some of the most common words you might have heard. Anticyclones (pressure highs) and cyclones (pressure lows) are the two patterns that dominate the atmosphere. Many of the effects on the weather will depend upon the position, movement and intensity of these formations. While cyclones are mostly responsible for the rainy days we experience, they can also cause windy and snowy days during winter. They are also behind the formation of hurricanes in the tropical regions of the planet. Anticyclones, on the other hand, have the ability to inhibit precipitation and favour very cold periods during winter or heatwaves and droughts during summer.

Global temperature change since year 1850 (Ed Hawkins, https://showyourstripes.info/s/globe)

These atmospheric patterns have direct implications on the European climate and variables like temperature are strongly modulated by the occurrence of these circulations. Besides the main cyclones and anticylones mentioned above, we can also classify these atmospheric configurations depending on their dominant wind flow, which therefore relate to their influence on the dominant air masses that move over Europe. During winter (months of December, January and February, DJF) for example, two circulations overrule the contrast in temperatures that we experience. In principle, the constant flow of wind from the west (W) brings humid air from the Atlantic and promotes milder temperatures for most of the western part of the continent. However, the inverse occurs when the rarer events of (only 2.4% frequent) easterly winds (E) take place. They usually blow very cold and dry air masses from the continental part of Europe and Russia which give way to some of the coldest days that we endure during this season.

Influence of atmospheric circulations on winter (DJF) and summer (JJA) European temperatures (Herrera-Lormendez et al., 2021)

Unfortunately, these habitual climatic conditions that we are familiar with will likely bear significant changes in the coming decades given the progressive warming of the planet. Our current understanding of climatic science indicates that the tropical conditions that we usually observe below the 30º of latitude will tend to become the “new normal” for regions located further north (further south in the southern hemisphere). This means that given a general change in the global circulation we will also expect this to affect what happens in the smaller scale circulations. As an example, there is a high confidence that, by the end of the century, Northern Europe will experience more rainfall than usual during its winters. On the other hand, a significat reduction in precipitations future summers in the Mediterranean and Western Europe is expected.

Projected seasonal precipitation changes by the end of the 21st century (IPCC, 2021)).

Before we mentioned the increase of ‘heat trapping’ gasses, but what exactly is driving all these changes? To better grasp it, we evaluated past and predicted future variations in the dominant European atmospheric circulations from the 1900s to the 2100s. The future was assessed by using physical and mathematical numerical climate models that try to reproduce the behaviour of the climatic system under different possible scenarios. We assumed a worst-case scenario where we continue emitting greenhouse gasses without consideration. In this case, the results indicated that in winters, this projected increase in precipitation might relate to an escalation in the frequency of days characterised by westerly wind flow that brings humid air masses from the North Atlantic Ocean. In the summer, on the other hand, we would have more circulation changes. These suggest an increment in the easterly-wind dominated days (linked with warmer and drier summer days), whereas the frequency of milder and wetter westerlies will be coerced due to the increase of the first. We find that these long-term modifications to the climate as we know it will be inevitable by the mid 21st century if no actions are taken to reduce the emission of heat-trapping gasses.

 Past and future changes in synoptic circulations over Europe (a to d) and probable times of emergence of these changes (e and f) (Herrera-Lormendez et al., 2021)

Nevertheless, it remains hard to quantify the exact time when these transitions will emerge and how exactly they will affect the regional aspects of our weather and climate as these depend on much more complex interactions between other components like vegetation coverage, urban growth, soil humidity, among others.

 However, two things are clear:

First of all, we need more interdisciplinary studies in the matter to assess all the variables and interactions involved in the future impacts of climate change in the whole climatic system. Secondly, we need to take action now to try not to reach a situation anywhere close to the predicted “worst-case scenario” in our research. This would lead to a world and a society facing very difficult living conditions and fighting for the already under strained resources.

A more comprehensive and detailed discussion of this topic can be found in our recently published work in the International Journal of Climatology:

Herrera-Lormendez P, Mastrantonas N, Douville H, Hoy A, Matschullat J. 2021. Synoptic circulation changes over Central Europe from 1900 to 2100 – Reanalyses and CMIP6. International Journal of Climatology. John Wiley & Sons, Ltd. https://doi.org/10.1002/joc.7481

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