Extreme weather in Europe linked to meltwater from Greenland – Energy and Climate

Extreme weather in Europe linked to meltwater from Greenland – Energy and Climate

The short version:

  • When sea ice and ice melt, it becomes fresh water. Fresh water is lighter than salty sea water and forms a layer above it.
  • The younger surface layer affects how heat is transferred between the ocean and the air above it, affecting ocean currents and wind patterns.
  • This research shows that in the following summer, this effect leads to warmer and drier weather in Europe.

The researchers looked at observations over the past 40 years and discovered a pattern: In the winters before the 10 hottest summers, much greater amounts of fresh water appeared in parts of the North Atlantic than in the winters before the 10 coldest summers.

How can this actually be linked? According to Marilena Oltmans, lead author of the study, it all starts when fresh water enters the sea.

We're talking to

Marilena Oltmans He is a researcher at the National Oceanographic Center in Southampton, United Kingdom.

This interview is an abbreviated translation of a longer interview in English that you can read here.
This is an abbreviated translation of a longer interview in English. To read the original text, click here.

reference: Oltmanns, M., Holliday, N.P., Screen, J., Moat, B.I., Josey, S.A., Evans, D.G., and Bacon, S.: European summer weather is linked to freshwater anomalies in the North Atlantic in previous years, climate weather. Dynam, 5, 109-132, https://doi.org/10.5194/wcd-5-109-2024, 2024.

Marilena Oltmans: – When the ice melts, it becomes fresh water that is lighter than the surrounding sea water because it contains less salt. Therefore, an upper layer of fresh water is formed.

This fresh upper layer prevents heat exchange between the deep ocean and the atmosphere. This has consequences for the oceans and for atmospheric circulation.

In winter, the sea loses its heat and becomes heavier. This heavy water sinks and contributes to large-scale ocean circulation. However, in some winters, this fresh surface layer remains. It cools, but it does not sink or mix with the salt water below.

Cold, fresh water means hot, dry summers

<2°C: This sounds suspiciously similar to what's going on with AMOC, which many of us have probably heard about. But what is new in this study?

– It has long been known that fresh water can influence large ocean circulation. However, this new study suggests that the effect of fresh water on heat exchange may also have direct consequences for atmospheric circulation. And so it goes with our weather.

Normally, in this region in winter, the ocean surface is warmer than the atmosphere. Therefore, it gradually loses heat and the atmosphere warms. But when there is more fresh water, the surface layer is shallower and adapts more quickly to lower air temperatures. This can create areas of sharp heat fronts on the sea surface in autumn and winter, between areas with abundant fresh water and areas with abundant fresh water.

These fronts, in turn, promote the development of low-pressure systems. Since most of the fresh water occurs in the subarctic region, there is a large temperature gap between the northern and southern parts of the North Atlantic Ocean. We're getting stronger westerly winds also moving further north than usual. This creates pressure differences along the sea surface, which persist until next summer and displace the North Atlantic Current northward. Thus, the North Atlantic Current is an extension of the Gulf Stream.

How does this affect the weather in Europe?

– The thermal front at the sea surface, between the warm North Atlantic Current and the cold waters to the north, acts as a barrier to the winds. The following summer, the winds follow the North Atlantic Current and deviate northward. Also in spring and summer, there is an additional variation in temperatures along the European coast, because the country warms faster. The winds then shift northward, first over the North Atlantic and then along the European coast, bringing warmer and drier weather over Europe.

He analyzed data from 1979-2022

– Can you briefly describe the methods you used in this study?

We used a comprehensive set of observations, including ocean observations, satellite observations of sea surface temperature and ocean currents, and atmospheric reanalysis products dating back to 1979. The reanalysis products include most observations available from weather stations Weather balloons and satellites. These observations are then combined with the laws of physics to extract data points in places and times where there are no direct observations.

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Finally, we used different statistical analysis methods to distinguish patterns in the data. For example, we looked specifically at the difference between the ten hottest summers and the ten coldest summers in the past 40 years.

-And what did you find?

– When we subtract the ten coldest summers from the ten warmest summers, we see that hot and dry summer weather is associated with these fresh waters. We also see a link between sea surface temperatures and large-scale atmospheric circulation conditions in summer, and we find patterns in the ocean and atmosphere that typically lead to heatwaves and droughts.

All this supports our hypothesis. In addition, based on available observations and theories, we believe we can confirm that cold, fresh water in winter is the likely catalyst for the entire process. Previous studies have shown individual connections, but this study puts everything in order. Fresh water from melting ice produces cold anomalies that alter atmospheric circulation. This leads to changes in ocean currents. This in turn leads to new changes in sea surface temperature. Which ultimately leads to widespread changes in the summer atmospheric circulation, and thus the weather.

If we had known this whole mechanism earlier, we could have estimated the strongest heat waves of the past 40 years at least one winter ago.

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Location and severity can be estimated

– How accurate is this notification? For example, can we know where in Europe there is the greatest probability of extreme heat events?

We see that the localization and intensity of freshwater anomalies in winter are related to the localization and intensity of extreme heat and drought over Europe in the following summer. So, if we know where these freshwater anomalies are located, we can estimate how hot and dry Europe will be in the following summers, and we can estimate how severe the heat and dryness will be. However, the estimates are only approximate, because we only have a limited number of years of observations to build on. Ultimately, we want to design a model to capture these feedback mechanisms. Then we will be able to get more accurate seasonal warnings.

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-So what should we do now that we know all this?

– Current models reduce climate predictability, especially in the North Atlantic subarctic region. Freshwater is a complex variable in the climate system that depends on many different factors, and is therefore poorly represented in models. This study now suggests that freshwater is a good candidate to explain much of the unpredictability in the models. Thus we can get better models and better visibility.

Conveyor belt at sea

It is called the North Atlantic Ocean Circulation System Atlantic meridional overturning rotationOr amok. In Norwegian: North Atlantic upwelling circulation. Both the Gulf Stream and the North Atlantic Stream are part of this system.

In the AMOC, water moves northward in the Atlantic Ocean along the surface (red arrows). When surface water moves far enough north, it loses heat, becomes denser, sinks deeper, and flows south again (blue arrows). The white circles show where deep water is formed.

The southern deep ocean current compensates for the northern surface current. This is why AMOC is often compared to a conveyor belt: different currents “pull” each other.

source: Srokosh and Bryden 2015, Pretorius 2018
clarification: Galland

Dalila Awolowo

Dalila Awolowo

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