Diagnosing Changes of Winter NAO in Response to Different Climate Forcings in a Set of Atmosphere-Only Timeslice Experiments

Changes of the winter North Atlantic Oscillation (NAO) variability in response to different climate forcings, and their possible causes, are decomposed and investigated using a set of atmosphere-only timeslice experiments forced by sea surface temperature (SST) from coupled runs. The results indicate that the effects of uniform SST warming and direct CO2 radiative forcing could enhance NAO variability, while SST pattern change could lead to large inter-model difference for model simulations. For the influences of uniform SST warming and the direct CO2 radiative effect, the most significant air temperature increases occur at mid-low latitudes instead of northern polar regions, which produces a greater meridional temperature gradient at mid-high latitudes, thus leading to enhanced westerly winds according to the thermal wind theory. The effects of uniform SST warming and CO2 direct radiative forcing could lead to intensification of winter NAO variability, although this result does not consider ocean-atmosphere coupling. The meridional temperature gradient decreases in most areas of the northern Atlantic under the forcing of SST pattern change, but with a larger inter-model uncertainty, which makes the change of winter NAO variability in response to SST pattern change an open issue.