2020 Volume 98 Issue 6 Pages 1245-1260
La Niña is the negative phase of the El Niño-Southern Oscillation (ENSO) cycle. It occurs in the equatorial Pacific, and events known as multiyear La Niña often persists for more than two years. During a conventional La Niña event, the seasonal cycle of surface temperature over Japan is amplified (i.e., hotter summer and colder winter than normal years), but the influence of multiyear events on temperatures over Japan is unclear. In this study, we evaluate the teleconnection associated with multiyear La Niña events. Our research uses composite analyses of observations, reanalysis data, and a large ensemble of atmospheric general circulation model (AGCM) simulations for 1951–2010, driven by observed boundary conditions. We propose two distinct mechanisms involved in multiyear La Niña events that cause hot summers over Japan.
Observational data composites show significant positive temperature anomalies over Japan in the boreal summer season preceding the two consecutive La Niña events reaching their mature phases. This robust summer signal can be reproduced by AGCM large-ensemble simulations, indicating that it is forced by multiyear La Niña. The time evolution of the anomalous summer temperature over Japan differs between the first and second years of multiyear La Niña. In the first summer, warm conditions occur in August–October in the southwestern part of Japan, due to anomalous southwesterly winds in the lower troposphere. This atmospheric circulation anomaly is attributable to a La Niña-induced decrease in precipitation over the equatorial Pacific. In the second summer, warm anomalies occur in June–August over northeastern Japan, and these are accompanied by an anomalous barotropic high-pressure induced by negative precipitation anomalies over the equatorial Pacific. The seasonal march in atmospheric background states and the delayed effect of a preceding El Niño may explain the distinct teleconnection during multiyear La Niña.