Decadal predictions outperform projections in forecasting winter precipitation over the Mediterranean region

The Mediterranean region is highly sensitive to climate change, having experienced an intense warming and drying trend in recent decades. In the context of decision-making processes, there is a growing interest in understanding the near-term climate evolution of this region. Climate change projections consistently indicate that southern Europe and the Mediterranean region will undergo significantly drier conditions in the latter half of the 21st century. However, if our focus shifts to the next decade or so, as is the case for many stakeholders and decision-makers, do climate change projections remain the best tool for understanding climate evolution during this timeframe? 

In this study, using retrospective forecasts from eight decadal prediction systems contributing to the CMIP6 Decadal Climate Prediction Project (CMIP6 DCPP) and the corresponding ensemble of non-initialized simulations (historical and projections), we compare the capabilities of the state-of-the-art climate models in predicting the near-term climate anomalies of the wintertime Mediterranean region for some key quantities so as to assess the added value of initialization. Our findings indicate that decadal predictions exhibit higher reliability than projections, particularly over the subpolar gyre for surface temperature and over southern Europe for precipitation. Therefore, their use should be preferred over projections for a decadal time horizon.

We also inspect the role of the North Atlantic Oscillation (NAO) index and subpolar sea surface temperature (SP-SST) in influencing the predictability of the Mediterranean climate. Therefore, we develop a statistical model to explore the potential impact of the NAO and SP-SST indices on Mediterranean precipitation predictability. The hybrid approach offers higher predictive skill, suggesting that both oceanic and atmospheric factors contribute to the increased skill in predicting European rainfall and can be potential predictors of Euro-Mediterranean rainfall variability on decadal timescale.