東京大学 · 環境科学
Tomoki Iwakiri教授の研究室は、気候変動と大気・海洋の相互作用に焦点を当てており、特にエルニーニョ・サバーデル・オシレーション（ENSO）の多周期的変動やそのメカニズムを、観測データ、再解析、気候モデルを用いて解明しています。特に、二度以上にわたるLa Niñaの持続や、ENSOの周期的反復現象の背後にある大気・海洋のフィードバック機構に注目しています。また、古気象記録と気候モデルを統合した研究により、過去の気候変動（例：8kaイベント）におけるENSOの変化のメカニズムを解明しています。
Figures are computed from collected data and may differ slightly.
El Niño-Southern Oscillation (ENSO), characterized by anomalous sea surface temperature in the central-eastern equatorial Pacific, is a dominant interannual variability, impacting worldwide weather and socioeconomics. The ENSO cycle contains irregularity, in which La Niña often persists for more than two years, called "multi-year La Niña". Observational records show that multi-year La Niña tends to accompany strong El Niño in the preceding year, but their physical linkage remains unclear. Here w
Abstract La Niña persisted from 2020 to 2023, but its mechanisms are still unclear. In this study, atmosphere and ocean reanalysis and 100‐member initialized forecasts using a state‐of‐the‐art climate model were analyzed to identify factors contributing to the persistence of the first‐ to second‐year La Niña during 2020–2022. We found that North Pacific high pressure anomalies in the winter of 2020/2021 forced a negative phase of the Pacific meridional mode through the following spring, forming
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 mult
Abstract El Niño–Southern Oscillation (ENSO) events occasionally recur one after the other in the same polarity, called multiyear ENSO. However, the dynamical processes are not well understood. This study aims to elucidate the unified mechanisms of multiyear ENSO using observations, phase 6 of the Coupled Model Intercomparison Project (CMIP6) models, and the theoretical linear recharge oscillator (RO) model. We found that multiyear El Niño and La Niña events are roughly symmetric except for case
Abstract This study presents modeling evidence that the Indian Ocean Dipole (IOD) is enhanced in its frequency and amplitude under mid‐Holocene (8‐ka) conditions. This enhancement is identified in a global climate model simulation driven by the 8‐ka insolation, in which negative sea surface temperature anomalies grow more than the present‐day simulation in the eastern Indian Ocean. The mechanism of IOD amplification is explained in terms of a positive feedback among sea surface temperature, prec
Abstract Off‐equatorial wind stress curl anomalies that mainly drive geostrophic transport are essential for the phase transition of the El Niño‐Southern Oscillation (ENSO) induced by a recharge‐discharge process. The ENSO‐induced zonal wind stress anomaly also drives surface Ekman currents, which may counteract the geostrophic transport, but its effect on ENSO periodicity remains unclear in the recharge oscillator theory. Here, we extended an ENSO diagnostic framework, called the Bjerknes‐Wyrtk
Abstract Paleo proxy records have suggested that El Niño–Southern Oscillation (ENSO) variability during the mid-Holocene [8200 to 4200 years ago (8.2–4.2 ka)] was weaker than during the instrumental periods, but the mechanisms remain unclear. We examined processes of ENSO suppression using a coupled general circulation model (CGCM) that simulates ENSO amplitude and skewness under the present climate reasonably well. Two long simulations were performed: one using the preindustrial condition (CTRL
Removing CO<sub>2</sub> from the atmosphere is emerging as a viable strategy to mitigate global warming, yet the responses of the climate system to CO<sub>2</sub> reduction remain uncertain. One of the most uncertain aspects of El Niño behavior is the change in periodicity in response to CO<sub>2</sub> forcing [O. Alizadeh, <i>Earth-Sci. Rev.</i> <b>235</b>, 104246 (2022)]. In this study, we show that climate models consistently project an abrupt shortening of El Niño periodicity once CO<sub>2</
El Ni&#241;o&#8211;Southern Oscillation (ENSO) events occasionally recur one after the other in the same polarity, called multiyear ENSO. However, the dynamical processes are not well understood. This study aims to elucidate the unified mechanisms of multiyear ENSO using observations, CMIP6 models, and the theoretical linear recharge oscillator (RO) model. We found that multiyear El Ni&#241;o and La Ni&#241;a events are roughly symmetric except in some cases. The composite multiy
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