The researchers have gone through observations from the last 40 years and discovered a pattern: In the winter before the 10 warmest summers, significantly larger amounts of freshwater appeared in the subpolar North Atlantic compared to the winters preceding the 10 coolest summers.

A conversation with

Dr. Marilena Oltmanns is a research scientist with the National Oceanography Centre in Southampton, UK.

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

So how can this happen? According to Dr. Marilena Oltmanns, lead author of the study, it all starts when freshwater enters the ocean.

Marilena Oltmanns: – When ice melts, it becomes freshwater which is lighter than the surrounding ocean water because it contains less salt. It sits on top of the deeper ocean, forming layers of fresher water.

These fresher layers inhibit the heat exchange between the deeper ocean and the atmosphere. This has implications for both the ocean and the atmospheric circulations.

In winter, the ocean loses heat and becomes denser. This denser water sinks and contributes to the large-scale ocean circulation. In some winters, however, a fresh surface layer remains on top. The surface layer cools but does not sink or mix with the saltier water beneath.

Pushes the ocean current northward

<2°C: – Presumably like what happens with the AMOC that we all know and love. So, what is new with this study?

– It has been known for a long time that freshwater can impact the large-scale ocean circulation. This new study suggests that the influence of the freshwater on the heat flux can also have direct consequences for the atmospheric circulation – and hence for our weather.

Normally, the surface ocean in the subpolar region in winter is warmer than the atmosphere. So it loses heat and warms the atmosphere. However, when there is more freshwater, the surface layer is shallower and adjusts faster to the lower air temperatures. This can create regions of sharp sea surface temperature fronts in autumn and winter between regions with and without the increased freshening.

Sharp temperature fronts over the ocean, in turn, promote the development of storms. Since most freshwater occurs in the subpolar region, there is an enhanced north-south temperature gradient over the North Atlantic, between the subtropical and the subpolar North Atlantic. This leads to stronger – and northward shifted – westerly winds. The winds set up pressure gradients along the ocean surface. These are maintained through to the following summer and induce a northward shift of the North Atlantic Current, which is the extension of the Gulf stream.

Observations from 1979 to 2022

– But how does this affect the weather in Europe?

– The sea surface temperature front between the warm North Atlantic Current and the cold subpolar waters acts like a barrier to winds. Therefore, in subsequent summers, the winds follow the North Atlantic Current and are deflected northward. Also, in spring and summer, there is an additional temperature contrast along the European coastline, because the land warms up faster. And so the winds are deflected northward. First over the North Atlantic and then along the European coastline, giving rise to warmer and drier weather over Europe.

– Can you briefly describe the methods you’ve used for this study?

– We used a comprehensive set of observations. Including in-situ ocean observations, satellite observations for sea surface temperature and ocean currents, and atmospheric reanalysis products that date back to 1979. Reanalysis products include the most available observations from weather stations, weather balloons and satellites. These observations are then combined with the laws of physics, such as conservation of mass and momentum, to derive data points at locations and times at which no direct observations exist.

Finally, we used different statistical analysis methods to discern patterns in the data, including correlation and regression analyses, composite analyses, and even filtering and spectral analyses in the multi-taper coherence analyses. For instance, in the composite analyses, we specifically looked at the difference between the 10 hottest and 10 coolest summers over the last 40 years.

Identifies patterns that lead to heatwaves and droughts

– And what did you find?

– When we subtract the 10 coldest from the 10 warmest summers, we find a significant freshwater signal in the preceding winter. We also see significant signals in the sea surface temperatures and the large-scale atmospheric circulation conditions in summer, and identify those ocean and atmospheric patterns that typically lead to heatwaves and droughts.

The dynamical consistency in the obtained patterns across all investigated ocean and atmospheric variables supports the proposed mechanism. In addition, based on the available observations and theory, our study points to the occurrences of the colder and fresher water in winter as a potential trigger of the identified chain of events.

– “Chain of events”?

– Previous studies have demonstrated individual links – that European summer weather is linked to the atmospheric circulation, that the atmospheric circulation is linked to the sea surface temperature, and that the sea surface temperature is in turn linked to freshwater. This study now puts everything together and suggests that it’s all connected. Freshwater from melting ice can lead to cold anomalies, leading to changes in the atmospheric circulation, leading to changes in the ocean currents, leading to new changes in the sea surface temperature which in turn lead to changes in the large-scale atmospheric circulation in summer, and hence our weather.

Thus, we have a coherent chain of events, which starts with the freshwater and ends with European summer weather. So, if we had known this mechanism in advance, we might have predicted, or provided an estimate of, the strongest heatwaves over the last 40 years, at least a winter in advance. Unfortunately, however, many climate models still have difficulties capturing freshwater processes.

Can estimate location and intensity

– How precise a prediction could we possibly make? Could we, say, pinpoint which regions will have a higher chance of extreme heat events?

– We do see that the location and intensity of the freshwater anomalies in winter significantly correlates with the location and intensity of the warm and dry anomalies over Europe in the subsequent summer. So, by knowing where the freshwater is, we can estimate the location of an extent of the warm and dry anomalies over Europe in the subsequent summers. However, the estimates are only rough, because we only have a limited number of years in the observational records. Ultimately, we would like to models to capture the identified feedback loops. This will then lead to more accurate weather predictions in future.

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

– Current models underestimate climate predictability, especially in the subpolar North Atlantic region. Also, many models have a poor representation of freshwater because it’s a complicated variable in the climate system that depends on many different factors. This study now suggests that freshwater is a top candidate for a source of unaccounted predictability in the models, providing us with new insights and opportunities to improve models.