Why seasonal forecast skill is high in the tropics and low over Europe
Seasonal forecast models are far more skilful in the tropics than in mid-latitudes. What makes Europe hard, and which seasons and variables are more predictable.
Skill is not uniform
Every seasonal centre verifies its system against past observations and publishes maps of skill. They all show the same picture: high skill for sea-surface temperature and near-surface temperature in the tropics, moderate skill in the subtropics and parts of North America, and low skill over Europe, especially for precipitation and especially in spring and summer. A seasonal map should be read with that geography in mind.
Why the tropics are predictable
Tropical weather is tightly coupled to sea-surface temperature. Warm water drives convection, convection drives circulation, and the ocean changes slowly. A model that predicts the ocean therefore predicts the atmosphere above it. ENSO adds a large, slow, well-observed signal on top. The result is that tropical seasonal forecasts often show strong anomalies and high tercile probabilities, and they are often right.
Why Europe is hard
Mid-latitude weather is dominated by the internal variability of the jet stream, which is chaotic and only weakly tied to slow boundary conditions. The main mode of European winter variability, the North Atlantic Oscillation, has some predictability from the tropics, the stratosphere, Atlantic sea-surface temperatures and autumn snow and sea-ice, but the signal is small relative to the noise. Models capture the sign of the NAO more often than chance in recent years, but the signal in their ensembles is much weaker than in reality, and the effect on precipitation is harder to predict than on temperature.
Season and variable matter
- Winter has the most European predictability, because the NAO is most active and ENSO and stratospheric influences are strongest.
- Summer temperature has gained skill from the warming trend and soil-moisture feedbacks; summer precipitation remains close to unpredictable.
- Temperature is more predictable than precipitation everywhere, and sea-surface temperature more than either.
What this means for reading the maps
A confident-looking anomaly over Europe deserves more scepticism than the same anomaly over the tropical Pacific. Look for agreement across the five models and stability across runs, and treat a neutral tercile summary over Europe as the normal state rather than a failure. This site does not yet publish verification, so consult the skill maps on the Copernicus site for the model and season you care about, and read why a probability is not forecast skill.