Multi-decadal North Atlantic Oscillation and the Interaction of Land and Atmosphere

The North Atlantic Oscillation (NAO) is the variability of the sea-level pressure difference between the Subtropical (Azores) High and the Subpolar (Iceland) Low. It has no particular periodicity, but do have significant variability in the multi-decadal timescale. It not only has large influence on the Northern Hemispheric weather, but has recently been shown to also drive part of the variability of Atlantic Meridional Ocean Circulation (AMOC) on timescales longer than decadal (e.g. [Delworth and Zeng, 2016]). Therefore, it is important to understand the mechanism for the NAO of long timescale. Utilizing the long simulations (>2000 years) of the model CESM1 by NCAR, it is shown that significant multi-decadal NAO is present in models with either prescribed sea surface temperature, slab ocean or dynamic ocean. This demonstrates that the variability is most likely due to interaction between land and atmosphere. We further carry out simulations with atmospheric component of CESM1, CAM4, coupled with its land component, CLM4, to test this hypothesis. In the simulations, the sea surface temperature is prescribed. Besides that, the snow depth is also prescribed to have only seasonal cycle. Our preliminary results show that the multi-decadal NAO is indeed suppressed to some extent.

Reference

Delworth, T. L., and F. R. Zeng (2016), The Impact of the North Atlantic Oscillation on Climate through Its Influence on the Atlantic Meridional Overturning Circulation, J Climate, 29(3), 941-962.