著者
IWAKIRI Tomoki WATANABE Masahiro
出版者
Meteorological Society of Japan
雑誌
気象集誌. 第2輯 (ISSN:00261165)
巻号頁・発行日
pp.2020-064, (Released:2020-08-28)

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 persist for more than two years. During a conventional La Niña event, the seasonal cycle of surface temperature over Japan is known to be amplified (i.e. hotter summer and colder winter than normal years), but the influence of multiyear events on temperature over Japan has not yet been clarified. In this study, we evaluate the teleconnection associated with multiyear La Niña using 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, and propose two distinct mechanisms involved in multiyear La Niña causing hot summers over Japan.  Composites of observational data 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, which indicates 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 are found in August–October in the southwestern part of Japan, due to anomalous southwesterly winds in the lower troposphere. This atmospheric circulation anomaly can be explained by a La Niña-induced decrease in precipitation over the equatorial western Pacific. In the second summer, warm anomalies are found 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.

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A new graphical abstract! https://t.co/zEBsMaQuLM Iwakiri, T., and M. Watanabe, 2020: Multiyear La Niña impact on summer temperature over Japan. J. Meteor. Soc. Japan, 98, https://t.co/vuLvbJW6OX https://t.co/orr3xJkI2X
ラニーニャと日本の夏の天候について再検証したIwakiri and Watanabe (2020) https://t.co/c8czr7eiSF によると、ラニーニャが発達し出した年の秋には全国的に高温、さらにラニーニャが翌年まで持続した場合には、北日本で猛暑になることが、観測およびモデル実験から確認されました。

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