Multi-decadal initialized climate predictions using the EC-Earth3 global climate model

Abstract. Initialized climate predictions are routinely carried out at many global institutions that predict the climate up to next ten years. In this study we present 30 year long initialized climate predictions and hindcasts consisting of 10 ensemble members. We assess the skill of the predictions of surface air temperature on decadal and multidecadal timescales. For the 10 year average hindcasts, we find that there is limited added value from initialization beyond the first decade over a few regions. However, no added value from initialization was found for the third decade (i.e. forecast years 21–29). The ensemble spread in the initialized predictions grows larger with the forecast time, however, the initialized predictions do not necessarily converge towards the uninitialized climate projections within a few years and even decades after initialization. There is in particular a long-term weakening of the Atlantic Meridional Overturning Circulation (AMOC) after initialization that does not recover within the 30 years of the simulations, remaining substantially lower compared to the AMOC in the uninitialized historical simulations. The lower AMOC mean conditions also result in different surface temperature anomalies over northern and southern high latitude regions with cooler temperature in the northern hemisphere and warmer in the southern hemisphere in the later forecast years as compared to the first forecast year. The temperature differences are due to less transport of heat to the northern hemisphere in the later forecast years. These multi-decadal predictions therefore highlight important issues with current prediction systems, resulting in long-term drift into climate states inconsistent with the climate simulated by the historical simulations.