Abstract
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.
Highlights
The North Atlantic Oscillation (NAO) is a major mode of interannual variability, and manifests as a seesaw in atmospheric mass between the subtropical Azores high and the polar Icelandic low, which is most pronounced during boreal winter [1,2]
Intercomparison project (CMIP3) have differences in the location of the sea level pressure dipoles compared with the observed NAO, and Davini and Cagnazzo [11] demonstrated that some climate models are unable to correctly simulate the physical processes connected to the NAO, including the frequency of high-latitude blocking over Greenland and the related zonal wind anomalies, which may react differently to distinct scenarios of climate change
We find that the individual model’s projected sea surface temperature (SST) pattern changes drive very different responses in the sea level pressure (SLP) anomalies associated with the winter NAO
Summary
The North Atlantic Oscillation (NAO) is a major mode of interannual variability, and manifests as a seesaw in atmospheric mass between the subtropical Azores high and the polar Icelandic low, which is most pronounced during boreal winter [1,2]. Intercomparison project (CMIP3) have differences in the location of the sea level pressure dipoles compared with the observed NAO, and Davini and Cagnazzo [11] demonstrated that some climate models are unable to correctly simulate the physical processes connected to the NAO, including the frequency of high-latitude blocking over Greenland and the related zonal wind anomalies, which may react differently to distinct scenarios of climate change. The comprehensive understanding of the mechanisms of NAO change, under future scenarios, is essential for producing more accurate climate model simulations [15]. The Cloud Feedback Model Intercomparison Project (CFMIP) aims to understand how the climate will change under future scenarios, using a number of idealized experiments. The improved assessment of cloud feedbacks combined with decomposed forcings can be helpful for better understanding of physical processes as well as mechanisms behind the changes of NAO variability.
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