A Retro on temperatures in September 2023

Jannes van Ingen

Published on: 2023/10/16

4 min

September 2023 will be remembered as an extraordinary month, marked by record-breaking warmth. As can be seen in our newly-launched dashboard, observed monthly anomalies in September 2023 were well above 2℃ over most of the European continent. The Beyond Weather forecast captured both the sign and pattern of the temperature anomalies well. However, like many other forecasting systems, we underestimated the intensity of the warmth. What was the main driver of these record-breaking temperatures and what is the role of AI in improving these forecasts?

℃ anomaly

compared to climatology + trend

Map of Beyond Weather of Europe temperature forecast september 2023

Fig 1: Beyond Weather forecast, 1 month ahead forecast of September 2023.

Image credit: Beyond Weather

℃ anomaly

compared to climatology + trend

Fig 2: Observed anomalies September 2023.

Image credit: Beyond Weather

For both figures, anomalies are shown with the climate change signal removed (by linear detrending), indicating that atmospheric dynamics played a crucial role.

We correctly foresaw the sign of warm weather and got the general pattern right. Interestingly, we correctly forecasted lower temperatures in Spain and Portugal and the right east-west gradient over Scandinavia. For quite some countries, particularly in central Europe, we predicted monthly anomalies exceeding 0.5℃, which is a pronounced signal for such a long-lead time (i.e., 1 month). While the anomaly patterns are reasonable, we did systematically underestimate the magnitude of the warming. Below, we investigate what other drivers, both global and more local, led to this pronounced warming.

Not only in Europe temperatures soared, but on a global scale, average temperatures were 0.93 ℃ above normal. The reason for this is still unclear and debated among climate experts. While El Niño certainly played a role, it couldn't have been the sole driver of the global anomaly. An often-used rule-of-thumb is that every degree of warming in the El Niño 3.4 region adds 0.07 degrees to global air temperatures, which means that El Niño usually only adds 0.1-0.2 degrees to mean global temperatures. Moreover, El Niño is still in a development stage and only expected to reach its peak in December.

Graph showing globally averaged surface air temperature anomalies relative to 1991–2020 for each September from 1940 to 2023.

Fig 3: Monthly global air temperature September anomalies 1940-2023. Climatology is 1991-2020.

Image credit: C3S/ECMWF

Another possible cause for globally increased temperatures is the eruption of the Tonga volcano in January 2022. While most volcanic eruptions cool the atmosphere because the emitted aerosols block sunlight, this underwater volcanic eruption sent an unprecedented amount of water vapor directly into the stratosphere. Since water vapor is the strongest greenhouse gas, climate experts say that this can increase temperatures in the coming years.

Another hemispheric effect that had a pronounced local effect on Europe, was a circumglobal wave train that resulted in a high pressure system over Europe. This circumglobal pattern can be characterized as a wave 3 index (Teng & Branstator, 2012) and is most often found in boreal winter. It is called wave 3 because it is characterized by three high pressure systems (see fig 4 below) over the northern hemisphere, located over Canada, Europe and the Northern Pacific ocean. The profound high pressure system over Europe attracted warm air from the south.

Fig 4: z500 anomalies in September 2023 over the Northern Hemisphere. Anomalies calculated using 1979-2000 climatology.

Image credit: ClimateReanalyzer.org

On the continental scale, we take a look at the North Atlantic ocean and dominant circulation patterns over Europe. Striking is the North Atlantic's persistent and widespread higher than normal temperatures (see fig 5 below). These homogeneously raised temperatures propagate into Europe, and might be a reason for systematic underestimation of our forecasts. The strong point of our AI model is systematically searching for teleconnections and combining these into a seasonal expectation.We believe that the homogeneous increase in the sea surface temperatures over the North Atlantic, possibly related to Tonga volcano, is not well accounted for in our forecast model. This insight helps us to work on further model improvements.

Fig 5: zSST anomalies September 2023. Anomalies calculated using climatology over 1979-2000.

Image credit: ClimateReanalyzer.org

Clearly, both global and regional effects have played a role in the exceptionally high European temperatures in September 2023. We believe that both numerical and AI weather predictions have a long way to go to tackle the challenges that are brought to our societies in a future climate.