Abstract
In the upper troposphere during winter, positive synoptic eddy (SE) feedback plays an indispensible role in maintaining the Pacific–North American (PNA) pattern that dominates climate variability on inter-annual timescales over the North Pacific and downstream regions. This study shows that the eddy forcing, induced by eddy-vorticity (EV) fluxes, is not only in-phase with, but also downstream to the PNA pattern in terms of its northeast Pacific lobe. We employ the eddy structure decomposition method to understand such an observed PNA-SEs feedback, and propose a kinematic mechanism that can depict dynamical processes associated with the eddy structure change and its induced positive eddy feedback relative to the PNA flow pattern. With this method, the winter-mean PNA-related SE structures are separated into climatological (basic) and anomalous SE structures, and these two parts can be used to represent the changes in SE structure in a statistical sense and then to calculate the EV fluxes in order to further elucidate the feedback mechanism. It is demonstrated that, on one hand, the winter-mean PNA flow tends to systematically deform the structures of SEs and induce anomalous EV fluxes, and these winter-mean EV fluxes primarily converge into the PNA cyclonic center, which, in return enhances the PNA flow. On the other hand, the PNA-related northeast Pacific flow is featured by a stronger zonal wind shear in the east than the west, which can induce larger zonal-slanting eddy structure change and then stronger meridional EV fluxes that converge to form downstream feedback. This kinematic mechanism may help to deeply understand the dynamical eddy feedback between the low-frequency PNA flow and high-frequency SEs.
Highlights
In the middle-high-latitude atmosphere, synoptic eddy (SE), which usually have a short lifecycle of 2–8 days, and low-frequency (LF) modes, which generally have their lifetime of longer than 2 weeks and always coexist together (Blackmon et al 1984a, b)
Using the ERA-Interim reanalysis data, this study shows that the eddy forcing, induced by EV fluxes, is positively in-phase with, and downstream to the northeast Pacific lobe of the Pacific–North American (PNA) pattern and further propose a kinematic mechanism to explain such a positive feedback between the PNA pattern and SEs, using the eddy structure decomposition method developed by Ren et al (2012)
The SE structure under the PNA conditions, which is constructed with the 3-point fields of covariance of the SE streamfunction, has been separated into two components: the basic eddy structure pattern and the anomalous eddy structure pattern as modulated by the PNA flow, which can be considered to physically represent the climatological and anomalous parts of SE flow in theory, respectively, suggested by Ren et al (2009, 2012)
Summary
In the middle-high-latitude atmosphere, synoptic eddy (SE), which usually have a short lifecycle of 2–8 days, and low-frequency (LF) modes, which generally have their lifetime of longer than 2 weeks and always coexist together (Blackmon et al 1984a, b). This framework provides a new idea to understand dynamical processes more deeply in the positive eddy feedback on LF flow by statistically separate eddy into its basic part and anomalous part., which represent a climatological measure of SE flow and anomalous SE activity induced by the LF flow, respectively. Ren et al (2012) further followed the basic idea of the framework of Jin et al (2006a, b) and proposed an eddy structure decomposition method to illustrate how the SEs feedback onto the NAO by examining the SE structure changes induced by NAO and the resultant SE structure anomaly patterns. In this study we will diagnose eddy feedback features associated with PNA and employ the eddy structure decomposition method to explore the physical processes deeply in the dynamical interactions between SEs and the PNA flow, which may Understanding positive feedback between PNA and synoptic eddies by eddy structure
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
