15 August 2024
Tokyo Climate Center, Japan Meteorological Agency (TCC/JMA)

Contribution of an unprecedented marine heatwave toextremely hot summer conditions over northern Japan in 2023
Unusually high ocean temperatures (known as marine heatwaves, or MHWs)
prevailing around northern Japan in association with prominent poleward
meandering of the Kuroshio Extension are likely to have played a part in theunprecedentedly hot summer conditions of 2023 in the region. This MHW
contributed to reduced amounts of low-level cloud and increased insolation,
atmospheric heating due to the warm ocean, and anomalous evaporation and anenhanced greenhouse effect.
In recent years, unusually high ocean temperatures (known as marine
heatwaves, or MHWs) have attracted focus because of their effects on marine
ecosystems and fisheries. The frequency and intensity of MHWs are expected toincrease in association with global warming (Intergovernmental Panel on Climate
Change, 2021).
In 2023, the Japan Meteorological Agency (JMA) Advisory Panel on Extreme
Climatic Events (comprised of prominent climate science academics andresearchers) identified MHWs around northern Japan as a potential contributorto northern Japan’s unprecedentedly hot summer in 2023 (Fig. 1) (Tokyo ClimateCenter, 2023; Takemura et al. 2024).

Fig. 1 Summer-mean surface air temperature anomalies over Japan in 2023.
(b) As in (a), but for sea surface temperature anomalies. From Sato et al. (2024
Scientific Reports).

1

Further analysis by a joint research team (comprising TCC/JMA and advisory
panel members Prof. Hisashi Nakamura from the Research Center for Advanced
Science and Technology (RCAST) at the University of Tokyo, Prof. Youichi
Tanimoto from Hokkaido University and Dr. Masami Nonaka from the Japan
Agency for Marine-Earth Science and Technology (JAMSTEC)) indicated that themarine conditions contributed to reduced amounts of low-level cloud andincreased insolation, atmospheric heating due to the warm ocean, andanomalous evaporation and an enhanced greenhouse effect (Fig. 2).

Fig. 2 (a) Schematic of processes involved in the impact of an unprecedented
marine heatwave (MHW) on record-breaking hot summer in 2023 over northern
Japan. (b) As in (a), but for atmospheric and oceanic processes thatcharacterize climatological summertime situation around northern Japan.
Based on Sato et al. (2024 Scientific Reports).
2

As the intensity and frequency of MHWs and high atmospheric temperatures
are expected to increase further as global warming progresses, it is crucial toclarify and be able to predict how MHWs will raise atmospheric temperatures insurrounding regions, as well as determining the mechanisms of MHWs
themselves. TCC/JMA remains committed to its analysis of extreme climate
events for enhanced prediction to support mitigation of impact of climate change.
The results of this survey were published online in Scientific Reports on 19
July 2024. See also a related article by RCAST at The University of Tokyo:
https://www.rcast.u-tokyo.ac.jp/en/news/report/page_00338.html.
Paper
Authors: Hirotaka Sato1,2, Kazuto Takemura1,2, Akira Ito1, Takafumi Umeda1,Shuhei Maeda1,2, Youichi Tanimoto3, Masami Nonaka4 & Hisashi
Nakamura5
Title:
Impact of an unprecedented marine heatwave on extremely hotsummer over Northern Japan in 2023.
Journal: Scientific Reports
DOI:
10.1038/s41598-024-65291-y.
1. Japan Meteorological Agency
2. Meteorological Research Institute, Japan Meteorological Agency
3. Faculty of Environmental Earth Science, Hokkaido University
4. Application Laboratory, Japan Agency for Marine-Earth Science and Technology
5. Research Center for Advanced Science and Technology, University of Tokyo

References
Intergovernmental Panel on Climate Change, 2021: Summary for Policymakers. In: Climate Change 2021:
The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of theIntergovernmental Panel on Climate Change [Masson-Delmotte, V., et al. (eds.)]. Cambridge University
Press,Cambridge,

UnitedKingdom

andNew

York,

NY,

USA,

pp.

3−32.

https://doi.org/10.1017/9781009157896.001
Takemura et al. 2024: Preliminary Diagnosis of Primary Factors for an Unprecedented Heatwave over Japan
in 2023 Summer. SOLA, 20, 69-78. https://doi.org/10.2151/sola.2024-010
Tokyo Climate Center, 2023: Climate characteristics and factors behind heavy rainfall during the Baiu season
in 2023 and extremely high temperatures from mid-July onward (press release)https://ds.data.jma.go.jp/tcc/data/news/press_20230928.pdf

3