The asymmetric impacts of ENSO modoki on boreal winter climate over the Pacific and its rim

Abstract

The El Nino–Southern Oscillation (ENSO) Modoki phenomenon has a substantial influence on regional climate. In this study, the results derived from observational and reanalyzed datasets show that the boreal winter climate anomalies over the Pacific and its rim in the different phases of ENSO Modoki are asymmetric during 1979–2017. During El Nino Modoki, an upper-level zonal “convergence–divergence–convergence” anomaly occurred in the Walker circulation over the Pacific sector, associated with anomalous ascending (descending) in the central side (both sides), leading to a wet “boomerang” pattern of rainfall anomalies in the tropical Pacific Ocean. Consequently, a drier winter occurred in Philippines, and warming and drying occurred in western Australia and northern South America. Meanwhile, a dipolar pattern with the wet south and the dry north in the United States occurred, accompanied by a Pacific/North American-like teleconnection. During La Nina Modoki, a roughly reversed Walker circulation anomaly and deeper Hadley circulation anomalies were associated with the strong air–sea feedback, which caused stronger rainfall anomalies in the Pacific Ocean. On land, anomalies of surface temperature and rainfall over the tropical Pacific Rim were more intense compared with El Nino Modoki. However, owing to the lack of a Pacific/North American anomaly, fewer anomalies occurred over the mid-latitude North America. In numerical experiments, the response to the different phases of ENSO Modoki basically reproduces the asymmetric climate anomalies in boreal winter, further confirming that the asymmetry can be partly attributed to tropical sea surface temperature anomalies.