Anatomizing Synoptic Eddy–NAO Interaction through Eddy-Structure Decomposition

A method of eddy-structure decomposition is proposed to detect how low-frequency flow associated with the North Atlantic Oscillation (NAO) organizes systematically synoptic eddy (SE) activity to generate in-phase and upstream feedbacks. In this method, a statistical eddy streamfunction (SES) field which is defined by the three-point covariance of synoptic-scale streamfunction is introduced to characterize spatio-temporal SE-flow structures. The SES field is decomposed into basic and anomalous parts to represent the climatological SE-flow structure and its departure, respectively. These two parts are used to calculate the basic and anomalous eddy-velocity, eddy-vorticity, and thus eddy-vorticity flux fields, in order to elucidate those two SE feedbacks onto the NAO. This method is validated by the fact that the observed anomalous eddy-vorticity flux field can be reproduced well by two linear terms: the basic eddy-velocity field multiplied by anomalous eddy-vorticity field and the anomalous eddy-velocity field multiplied by basic eddy-vorticity field. With this method, we find that in the positive and negative phases the NAO flow tends to induce two different types of anomalous SE-flow structure, which are largely responsible for generating the net meridional and zonal eddy-vorticity fluxes that in return feedback onto the NAO. The two processes that are related to these two different types dominate in the in-phase and upstream feedbacks, respectively, which are delineated conceptually into two kinematic mechanisms associated with zonal-slanting and meridional-shifting changes in the SE structure. The present observational evidence supports the theory of eddy-induced instability for low-frequency variability and also provides insights into the reason of asymmetry between the SE feedbacks onto the two NAO phases.