Distinct features of mid-winter North Pacific storm track suppression associated with central and eastern Pacific El Niños

Abstract

The North Pacific storm track (NPST) represents intense activity of synoptic-scale eddies that are important for Northern Hemisphere mid-latitude weather systems. According to the linear baroclinic instability theory, the strength of the NPST should peak in January and February but is substantially weaker than that in late fall and early spring, a phenomenon termed mid-winter suppression (MWS). This study investigates the characteristics of the MWS associated with central-Pacific (CP) and eastern-Pacific (EP) El Niños mainly based on composite analyses in the HadISSTv1.1 and ERA-5 datasets. It is found that over the southern North Pacific (NP; south of 40°N), the peak of the NPST delays until January and thus the MWS does not appear during CP El Niño as that in climatology. In contrast, a prominent MWS-like feature is identified in January over this region during EP El Niño. Over the northern NP (north of 40°N), the MWS is largely enhanced in January and February during CP El Niño. Quantitative diagnosis of eddy kinetic energy conversion further reveals that different features of the MWS are mainly caused by baroclinic conversion anomalies. The two types of El Niño may affect the NPST and MWS by modifying the atmospheric thermal structure and exciting cross-Pacific wave trains, which are associated with different convective anomalies.