The effect of model biases on the simulated future changes of the North Atlantic jet stream

Future projections of the European climate suffer from uncertainties in the changes of the North Atlantic jet stream. Previous studies of multi-model ensembles have suggested that the jet will shift poleward, and that the shift is anticorrelated with the simulated present-day latitude of the jet. Model basic state biases are a possible cause for the uncertainty, but their effect is difficult to assess because the spread in simulations may be caused by any inter-model differences. Here, we isolate the effect of model biases on future projections by modifying the basic state of a single atmospheric model with a run-time correction method aiming to adjust the model climatology towards those of three other models and a reanalysis. The effect of model biases was found to be strongly seasonal. In winter, changes in the frequencies of two of the three preferred positions of the jet were found to be sensitive to the model biases, but in summer the impact of biases is small relative to the magnitude of the changes. The results demonstrate that even though the anticorrelation of jet latitude and shift is only partly caused by biases, there is potential to reduce uncertainty in jet stream changes by improving model basic states.