Impact of Madden–Julian Oscillation on Pacific Blocking Frequency during Two Types of El Niño

The influence of the Madden–Julian Oscillation (MJO) on Pacific blocking frequency during two kinds of El Niño (Eastern Pacific and Central Pacific El Niño) is investigated using a two-dimensional blocking index in winter, based on ERA-interim reanalysis of daily data from 1979 to 2019. In this research, phases 3 and 7 were chosen because they had a greater frequency. In MJO phase 3, during the Eastern Pacific and Central Pacific El Niño years (EP3 and CP3), the sites of MJO teleconnections are found to be comparable, correlating to a positive (negative) geopotential height anomaly in the polar area (the Bering Sea). Thus, during EP3 and CP3, there are positive blocking frequency anomalies in the high-latitude Pacific sector. Blocking frequency anomalies across the mid-high-latitudinal Pacific are notably positive in MJO phase 7 during the Eastern Pacific El Niño years (EP7) but not so much in MJO phase 7 during the Central Pacific El Niño years (CP7). Because the MJO-related anomalous Ross wave source is located northwest of the EP7 subtropical jet core region, the MJO teleconnection is located north of 50°N. This teleconnection relates to geopotential height abnormalities in the Pacific sector, which enhance the frequency of Pacific blocking. However, during the CP7, the MJO-related anomalous Rossby wave source was located in the subtropical jet core region. The associated teleconnection propagates across the subtropical jet stream, exerting a negligible impact on the geopotential height in the Pacific area. Thus, there are no statistically significant blocking frequency anomalies in the CP7 across the Pacific area. Finally, the ECHAM4.6 model is used to validate the results.