ECMWF’s global SEAS5 operational seasonal forecast for the months of June, July and August (JJA) 2023, initialised on 1 May 2023, was dominated by warm sea-surface temperatures (SST) in the tropical Pacific, which marked the onset of the current El Niño event. SEAS5 accurately predicted the warming of the central and eastern equatorial Pacific, along with many of its global teleconnections driven by the large-scale shift of the Walker circulation in the tropics.
Well-predicted El Niño–Southern Oscillation (ENSO) teleconnections include drier-than-normal conditions over the maritime continent, India and portions of Australia, the relatively dry and warm conditions over Central America and the dry season across large parts of South America.
In the northern extratropics, summer seasonal predictions are, in general, highly uncertain due to smaller magnitudes of the anomalies, overall reduced variability, and the absence of robust dynamical drivers. While warm near-surface temperature anomalies were correctly predicted over many land areas of the northern hemisphere, forecasting the atmospheric circulation proved more difficult.
Conditions over Europe
Over the North Atlantic, pronounced negative sea-level pressure anomalies extended from the east coast of the US across the ocean towards Ireland, Britain, and Scandinavia, according to ECMWF's ERA5 reanalysis (top-left panel). The eddy-driven jet over the North Atlantic shifted southward, leading to a band of stronger westerly winds at 850 hPa from western Europe to the Baltic Sea (middle-left panel). These circulation anomalies describe the negative phase of the so-called Summer North Atlantic Oscillation (SNAO), which is distinctly different from the winter NAO. The unsettled and wet weather experienced across western, northern and central Europe during July and August directly resulted from the negative SNAO circulation anomaly.
SEAS5 successfully generated ensemble mean forecasts of surface pressure and zonal wind anomalies that captured the observed southward shift of the jet over the North Atlantic (top-and middle-right panel). However, the model struggled to forecast the eastward extension of the jet into Ireland, Britain, and Scandinavia. Instead, the ensemble mean shows a weak signal of large-scale blocking over the North Sea and Scandinavia with easterly wind anomalies and suppressed precipitation.
To further investigate the performance of SEAS5, we define a simple SNAO index as the area-mean seasonal mean sea-level pressure anomaly over the box 45°N–65°N/15°W–15°E and analyse the ensemble distribution of the index (bottom-left panel). In comparison to the SEAS5 long-term climatological distribution during the 1993–2016 hindcast period, the forecast ensemble for JJA 2023 is slightly shifted towards more positive values. However, the ensemble spread is considerable, spanning a wide range of negative and positive values for JJA 2023. The observed negative index in ERA5 (red stars in the bottom-left panel) falls at the lower end of the climatological ERA5 distribution and is encompassed within the SEAS5 forecast distribution, despite its overall shift towards positive SNAO values.
The seasonal mean circulation anomalies outlined above mask some intriguing sub‑seasonal developments: ERA5 shows that May and June 2023 saw a strong blocking over Scandinavia, followed by large but opposite anomalies in July and more typical conditions in August (see the red stars in the bottom-right panel). SEAS5, however, failed to predict the distinct shift from blocking to a more westerly flow in July. Instead, it exhibited a tendency to persist the blocked flow conditions from early May through June and July. Consequently, the overly persistent model forecast led to a positive seasonal mean signal in contrast to the observed negative anomaly.
The Copernicus Climate Change Service (C3S) implemented by ECMWF publishes seasonal forecasts from a range of models. Similar to SEAS5, most other models also fell short in accurately predicting the circulation patterns and, consequently, the precipitation signals over Europe for the summer of 2023.
Tropical Atlantic SSTs
The SSTs across the tropical and subtropical North Atlantic exhibited an unusual warm signal in JJA 2023, which was well reproduced in the SEAS5 forecast. Warm SSTs favour the development of tropical storms and hurricanes in the Atlantic basin and thus counteract the tendency to suppress tropical storm formation over the tropical Atlantic during El Niño events. The observed and SEAS5-predicted above-average tropical cyclone season in the North Atlantic (19 observed and 17 predicted compared to an average of 14) supports the hypothesis that the warm SSTs dominated this season, outweighing the impact of the ongoing El Niño.