Amplified Asymmetric Impact of ENSO Events on the Wintertime Pacific-North American Teleconnection Pattern

El Nino/Southern Oscillation (ENSO) as the strongest tropical interannual signature has the most prominent impact on wintertime Pacific-North American (PNA) teleconnection pattern. ENSO exhibits an increasing asymmetry in recent decades. This study quantifies its extratropical asymmetric impact on the amplitude of the PNA pattern, using a normalized asymmetry index defined as a ratio of asymmetric versus symmetric anomalies for El Nino and La Nina. Relative to the ENSO asymmetry, the extratropical asymmetric impact is largely amplified especially downstream by up to 82%. Such an amplification is attributed to noticeable increases, 83.6% (68.9%), in asymmetry of Rossby wave source (RWS) anomalies over two key regions of North Pacific, in which anomalous divergence induced by nonlinear condensational heating feedback and anomalous synoptic eddy forcing are two major contributors. The former contributes 39.3% (47.5%) over the western (eastern) North Pacific through increasing asymmetric RWS anomalies, while the latter contributes 29.5% (21.3%) through decreasing symmetric RWS anomalies.

Automated summary

This study analyzes the impact of El Nino/Southern Oscillation (ENSO) on the Pacific-North American (PNA) teleconnection pattern in wintertime. It finds that the asymmetry of ENSO has become more pronounced in recent decades. The study quantifies the extratropical asymmetric impact on the amplitude of the PNA pattern and attributes the amplification mainly to increases in asymmetry of Rossby wave source (RWS) anomalies over key regions in the North Pacific.