Abstract
Abstract Utilizing a decomposition of anomalous eddy vorticity forcing (EVF) proposed by Song in 2016 and a modified Geophysical Fluid Dynamics Laboratory (GFDL) dynamical core atmospheric model, this study provides a different understanding of physical mechanisms that are responsible for the formation of the anomalous synoptic EVF (SEVF) associated with Pacific–North American teleconnection pattern (PNA) events. A series of short-term control experiments (CEs) and initial-value modified experiments (IVMEs) is conducted. In each case of CEs, there are no obvious PNA-like circulation anomalies. IVMEs are exactly the same as CEs except that appropriate small perturbations are introduced into the initial-value fields of CEs. The modified initial-value fields led to a gradual development of the PNA-like flow anomalies in IVMEs. Based on these numerical results, deformations of the synoptic eddy due to the emergence of the PNA pattern can be easily acquired by subtracting the synoptic eddy in CEs from the synoptic eddy in IVMEs . The anomalous SEVF associated with the PNA events in the model can be decomposed into ensembles of two linear and interaction terms (EVF1 and EVF2) and a nonlinear self-interaction term (EVF3). It is demonstrated that the physical essence of the anomalous SEVF associated with the PNA events is a competition result between EVF1 plus EVF2 and EVF3. Results also indicate that the different signs of SEVF associated with the positive and negative PNA events are not necessarily related to the different tilts of the synoptic eddy.
Highlights
Along with blocking (e.g., James 1994) and the North Atlantic Oscillation (NAO; van Loon and Rogers 1978; Kushnir and Wallace 1989), the Pacific–North American teleconnection pattern (PNA; Wallace and Gutzler 1981; Barnston and Livezey 1987) is one of prominent atmospheric low-frequency modes (LFMs) in the Northern Hemisphere
Extensive investigations have shown that the PNA is accompanied by significant anomalous synoptic eddy vorticity forcing (SEVF), which has an important effect on the PNA activity
We infer that the key factor in determining the sign of the anomalous SCEVF should be the relative spatial relationship between c0D and c0C that affects the intensity of hEVF1i 1 hEVF2i very sensitively
Summary
Along with blocking (e.g., James 1994) and the North Atlantic Oscillation (NAO; van Loon and Rogers 1978; Kushnir and Wallace 1989), the Pacific–North American teleconnection pattern (PNA; Wallace and Gutzler 1981; Barnston and Livezey 1987) is one of prominent atmospheric low-frequency modes (LFMs) in the Northern Hemisphere. Song (2016) proposed a procedure to decompose the nonlinear anomalous eddy vorticity forcing (EVF) associated with the LFMs by performing a series of initialvalue experiments based on an idealized atmosphere model. The small perturbations are the composite three-dimensional circulation anomalies of the positive and negative model PNA events at lag 28 days including vorticity, divergence, temperature, surface pressure, zonal wind velocity, meridional wind velocity, and vertical velocity. Compared with the composite anomalous HFEVF, the composite anomalous SCEVF has a slight eastward displacement with a weaker intensity Apart from these differences, the composite results of anomalous SCEVF and HFEVF are generally highly similar, suggesting that the anomalous SEVF associated with the model’s PNA events is largely insensitive to the methodology used to define the synoptic eddy fields. Since the synoptic eddy is more active in the upper troposphere and the focus here is on the large-scale characteristics of the anomalous SEVF, in the following analyses the HFEVF, SCEVF, EVF1, EVF2, and EVF3 are discussed in terms of the 300-hPa streamfunction tendency by inverting the Poisson equation
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