The Influence of Atlantic Multidecadal Variability on European Summer Climate: Competing Mechanisms and Implications for Prediction

Abstract. Skilful predictions of European summer climate are increasingly relevant due to an increasing probability of temperature extremes, but prediction skill beyond the forced trend has so far proven limited. Atlantic Multidecadal Variability (AMV), characterised at the surface by North Atlantic sea surface temperatures (SSTs), is both active and predictable during boreal summer, and previous studies have linked it to surface impacts in Europe. Current understanding largely relies on the relatively short observational record of decadal variability and the predictability of impacts and associated mechanisms are poorly studied. In this study, single model large ensemble historical and decadal hindcast simulations using the MPI-ESM-LR model are used to understand the role that AMV plays for North Atlantic-Europe sector climate prediction. It is found that strong AMV-associated SST anomalies in the subpolar gyre region are better represented in the initialised hindcasts than in the uninitialised historical ensemble, and they are highly predictable at lead years 1–7. The observed cyclonic response to positive AMV in the extratropical North Atlantic is not present in historical simulations, but it is found to be predictable in decadal hindcasts, although with underestimated amplitude. The hindcast pressure anomaly nonetheless skilfully predicts observations and highlights a potential role for AMV in the yet-unsolved "signal-to-noise paradox". The upper tropospheric (200 hPa) geopotential height response to AMV is analysed and it is found to differ in reanalyses and models. Further investigation reveals a high frequency component relating to tropical SST anomalies and resembling a Rossby wave train emanating from the Caribbean, and a low frequency component relating to the surface level response, with an imbalance between the two mechanisms in models due to the weak surface response.