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.