Eddy-Induced Instability for Low-Frequency Variability

Abstract

Synoptic eddy-mean flow interaction has been recognized as one of the key sources for extratropical lowfrequency variability. In this paper, the underlying dynamics of this interaction are examined from the perspective of a synoptic eddy-induced dynamic instability. To delineate this instability, a barotropic model is used that is linearized with respect to a stochastic basic flow prescribed with both climatologic-mean flow and synoptic eddy statistics. This linear model captures the dynamics of feedback between synoptic eddy and lowfrequency flow through a dynamic closure that relates the anomalous eddy vorticity forcing to low-frequency flow anomalies. After reducing this dynamic closure to its fundamental components, this stability is elucidated with analytical results under the most idealized consideration of basic flow. It is shown that through systematic alteration of the synoptic eddy structures in the basic flow, a low-frequency planetary-scale perturbation generates anomalous eddy vorticity forcing positively proportional to the vorticity of the perturbation. Such a perturbation amplifies itself; the energy source for its growth comes from the reservoir residing in the basic synoptic eddy activity. Thus, the growth rate of the synoptic eddy-induced dynamic instability depends primarily on the kinetic energy level of the basic synoptic eddy activity. Moreover, this instability is scale selective with preference for zonal symmetric and asymmetric planetary-scale modes, whose meridional and zonal scales are roughly in the range of those of the observed leading low-frequency patterns. Analysis of this synoptic eddy-induced instability provides insight into the origin of extratropical low-frequency variability.

The influence of boreal spring Arctic Oscillation on the subsequent winter ENSO in CMIP5 models

“…This implies that the intensity of the eddy-mean flow interaction is stronger in observation, CCSM4 and CNRM-CM5 than that in the LC models. Previous studies suggested that the eddy-mean flow interaction strength is related to several factors, including the intensity of the synoptic-scale eddy, the eddy spatial length scale and its life time (e.g., Jin et al 2006a, b;Jin 2010). Among those factors, the intensity of synoptic eddy is a key component in determining the strength of synoptic eddy feedback to low frequency mean flow.…”

“…Among those factors, the intensity of synoptic eddy is a key component in determining the strength of synoptic eddy feedback to low frequency mean flow. Specifically, the strength of the synoptic eddy feedback is proportional to the synoptic-scale eddy intensity, given the same magnitude of low-frequency mean flow (e.g., Jin et al 2006a, b;Jin 2010). Hence, it is reasonably hypothesized that the intensity of the synoptic scale eddy activity is stronger in observation, CCSM4, and CNRM-CM5 than that in the LC models.…”

Interdecadal changes in the storm track activity over the North Pacific and North Atlantic

“…Changes in storm track activity lead to not only global circulation changes through vertical and horizontal exchange of heat, water vapor, and momentum (Jin 2010;Kug et al 2010) but also significant changes in the hydrological cycle through anomalous evaporation from the warm ocean surface and precipitation (Nakamura et al 2002). Understanding of interdecadal changes in storm track activity in the past may give a hint of how a future mean state change will modify the storm track's behavior and how its change, in turn, will have an effect on the mean state.…”

Influence of the November Arctic Oscillation on the subsequent tropical Pacific sea surface temperature

“…In fact, previous studies suggested that the intensity of synoptic-scale eddy is a critical component in determining the strength of the synoptic eddy feedback (Jin et al, 2006a(Jin et al, , 2006b). In particular, the synoptic-scale eddy feedback strength is proportional to the intensity of storm track, given the same magnitude of low-frequency flow (Jin, 2010). Figure 10 shows the annual cycle of the North Pacific storm track intensity at 300 hPa.…”