Understanding Anomalous Eddy Vorticity Forcing in Pacific-North American Teleconnection Pattern Events

An alternative eddy-eddy interactions mechanism is proposed in this study to understand anomalous synoptic eddies vorticity forcing (SEVF) associated with the Pacific-North American teleconnection pattern (PNA) events. Using a modified Geophysical Fluid Dynamics Laboratory (GFDL) dynamical core atmospheric model which can realistically reproduces the general dynamical properties the PNA, a series short-term control experiments (CEs) is conducted. In each case of CEs, there are no obvious PNA-like circulation anomalies. Subsequently, a corresponding series PNA+_Exp (PNA-_Exp) is also performed by introducing some appropriate small perturbations into the initial-value fields of CEs. In PNA+_Exp (PNA-_Exp), there are noticeable positive (negative) PNA-like circulation anomalies. Utilizing the results of CEs and PNA+_Exp/PNA-_Exp and the anomalous EVF decomposing procedure proposed by Song (2016), the anomalous nonlinear SEVF associated with the positive/negative PNA in the model can be decomposed into two linear eddy-eddy interaction terms and an unimportant nonlinear eddy-eddy interaction term.

The analysis results show that, the deformed part of synoptic eddies (denoted as Ψ^'_d ) for PNA+_Exp (PNA-_Exp) locates to the downstream (upstream) of the original synoptic eddies of CEs (denoted as Ψ^'_c) slightly, forming a sandwich-like structure. Therefore, Ψ^'_c and Ψ^'_d interactions are inevitable. The behaviors of decomposing terms of the anomalous SEVF associated with the PNA events in the model can be properly explained by Ψ^'_c and Ψ^'_d interactions and the Ψ^'_d self-interaction, indicating that the anomalous SEVF associated with the PNA events is resulted from eddy-eddy interactions.