On the use of the forced sensitivity method in climate studies

Developed initially in a weather forecasting framework, in this study the forced sensitivity (FS) technique is applied in a climatological context. This technique makes it possible to provide a dynamically consistent description of weather under a prescribed persistently anomalous large-scale atmospheric circulation. Optimal model tendency perturbations are computed using the FS technique and applied in the EC-Earth Global Climate Model. These tendency perturbations are calculated subject to the requirement that the model circulation reproduces the prescribed circulation after an optimization time of the order of a few days. The time-averaged large-scale atmospheric circulation is shown to be consistent with the large-scale circulation pattern of interest, while maintaining variability on the synoptic scale. This is demonstrated by forcing the model to reproduce, on average, a pattern characterized by a more persistent westerly circulation over the North Atlantic sector (positive North Atlantic Oscillation (NAO)). The dipole pattern in surface pressure over the North Atlantic is spatially fairly well reproduced, albeit with a slight shift southwards and a larger amplitude. The net average surface-temperature field shows the characteristic quadrupole pattern associated with a NAO event, although a slight shift is observed. The amplitude of the net response exceeds the characteristic values associated with a NAO event. On the synoptic scale, the mean sea-level pressure (MSLP) closely resembles the typical MSLP pattern observed in a NAO+ event.