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Hong-Li Ren, Fei-Fei Jin, and Li Gao


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, defined by the three-point covariance of synoptic-scale streamfunction, is introduced to characterize spatiotemporal SE flow structures. The SES field is decomposed into basic and anomalous parts to represent the climatological SE flow structure and its departure. 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, it is found 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, feed back onto the NAO. The two processes that are related to these two different types dominate in the in-phase and upstream feedbacks, 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 for the asymmetry between the SE feedbacks onto the two NAO phases.

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Hong-Li Ren, Fei-Fei Jin, and Jong-Seong Kug


Synoptic eddy and low-frequency flow (SELF) feedback plays an important role in reinforcing low-frequency variability (LFV). Recent studies showed that an eddy-induced growth (EIG) or instability makes a fundamental contribution to the maintenance of LFV. To quantify the efficiency of the SELF feedback, this study examines the spatiotemporal features of the empirical diagnostics of EIG and its associations with LFV. The results show that, in terms of eddy vorticity forcing, the EIG rate of LFV is generally larger (smaller) in the upper (lower) troposphere, whereas, in terms of eddy potential vorticity forcing, it is larger in the lower troposphere to partly balance the damping effect of surface friction. The local EIG rate shows a horizontal spatial distribution that corresponds to storm-track activity, which tends to be responsible for maintaining LFV amplitudes and patterns as well as sustaining eddy-driven jets. In fact, the EIG rate has a well-defined seasonality, being generally larger in cold seasons and smaller in the warmest season, and this seasonality is stronger in the Northern Hemisphere than in the Southern Hemisphere. This study also reveals a mid- to late winter (January–March) suppression of the EIG rate in the Northern Hemisphere, which indicates a reduced eddy feedback efficiency and may be largely attributed to the eddy kinetic energy suppression and the midlatitude zonal wind maximum in the midwinter of the Northern Hemisphere.

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