Distinctly Different Dynamical Processes in Maintaining the Intraseasonal NAO+ and NAO−

Abstract

AbstractPositive feedback between high‐frequency eddies and low‐frequency processes is believed to play an essential role in the NAO evolution. In this study, however, it is found that the previously well‐studied upscale forcing to the intraseasonal scale window is mostly limited during the phase NAO−, but almost disappears during NAO+. The maintenance of the intraseasonal NAO+ is by a strong barotropic instability of the basic flow, which has been overlooked previously. The divergence of the Reynolds stress tensor that decelerates the basic flow is found to account for this asymmetry. For NAO−, there also exist strong interactions between the basic flow and the intraseasonal component, but the energy transfer is in a dipolar form, which, if integrated over space, contributes insignificantly as a whole. These new findings from a dynamics viewpoint shed insight deeper into NAO, a climate mode playing a key role in the Northern Hemisphere warming.