According to the Japan Meteorological Agency, SSTs in August 2020 in the southern region of Japan and the northwest Pacific Ocean reached previously unheard-of heights (JMA).

The record-high northwestern Pacific SST seen in August 2020 could not be anticipated to occur without human-induced climate changes, according to a recent study that was released in January 2021. Since then, the JMA has once more reported that record-breaking SSTs were seen close to Japan in July and October 2021 as well as from June to August 2022. However, it is still unknown how much climate change has affected the possibility that these regional extreme warming episodes would occur.

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“Impacts of global warming are not uniform, rather show regional and seasonal differences,” said a co-author Hideo Shiogama, the head of the Earth System Risk Assessment Section at Earth System Division, NIES. “A comprehensive analysis of regional SSTs for a long period may provide a quantitative understanding of how much ocean condition near Japan has been and will be affected by global warming. This better informs policymakers to plan climate change mitigation and adaptation strategies.”

The study, which was published in Geophysical Research Letters, examines the role that global warming plays in the distinct monthly intense ocean warming events that occur in Japan’s marginal seas and that, in the preindustrial age, might have happened less frequently than once every 20 years.

The Japan Sea, East China Sea, Okinawa Islands, Taiwan’s east coast, and the Pacific shores of Japan were among the ten monitoring zones that the JMA uses in its daily operations. With the exception of the area east of Hokkaido, the scientists confirmed that the observed SST variations between 1982 and 2021 were accurately simulated by 24 climate models taking part in Coupled Model Intercomparison Project (CMIP6sixth )’s phase.

In order to determine the contribution, exceptional ocean warming occurrences were then discovered in nine monitoring regions.

Extreme ocean warming and climate change
“In the present climate, every extreme ocean warming event is linked to global warming,” said corresponding lead author Michiya Hayashi, a research associate at NIES. The scientists estimated the occurrence frequencies of each event in the present and preindustrial climate conditions from January 1982 through July 2022 based on the CMIP6 climate models.

“We found that the occurrence probability of almost all the extreme ocean warming events has already at least doubled since the 2000s than the preindustrial era. It is increased more than tenfold in sizeable cases since the mid-2010s, especially in southern Japan.”

For instance, the extreme ocean warming episodes in July 2022 are identified as the anomalously high SSTs detected in five monitoring areas, including the Japan Sea (Areas 1, 3), East China Sea (Areas 5, 8), and south of Okinawa near Taiwan (Area 10).

The updated results based on the preliminary data downloaded from the NEAR-GOOS RRTDB website on September 15, 2022 (not included in the paper published) reveal that the events are also identified in six monitoring areas south of 35°N for August 2022: the East China Sea (Areas 5, 8), the south and east of Okinawa (Areas 10, 9), the southeast of Kanto (Area 7), and the seas off Shikoku and Tokai (Area 6).

“We estimate that, in all of these identified events in July and August 2022, the occurrence frequencies are increased at least doubled due to climate change, and more than tenfold for those in the south of 35°N except for the north of East China Sea,” stated Hayashi.

“Climate change impacts on extreme ocean warming events in northern Japan began to emerge relatively late compared to southern Japan,” noted Shiogama. The increased global aerosol emissions until the 1980s tend to cool the Earth’s surface, which is more substantial in the North Pacific especially near northern Japan via atmospheric large-scale circulation changes.

In addition, the year-to-year natural variability of SST is large in northern Japan so the global warming signal was less detectable than in southern Japan. Since in the last decades global aerosol emissions have been reduced, the cooling effect becomes less dominant to human-induced greenhouse gas warming.
“Our study indicates,” continued Shiogama, “that the contribution of climate change to SST extremes has been already discernible beyond natural variability even in northern Japan under the present climate condition.”

What about the predicted changes in ocean conditions? Using the results from 24 CMIP6 climate models from 1901 to 2100, the researchers further examined the likelihood of exceeding the monthly record high SSTs around Japan at various levels of global warming from 0°C to 2°C. Tomoo Ogura, a co-author and the chief of the Climate Modeling and Analysis Section at Earth System Division, NIES, stated, “Once global warming exceeds 2°C, all of nine monitoring regions are predicted to experience SSTs warmer than the past highest values at least every two years.”

He continued, “Limiting global warming below 1.5°C is important to prevent the new normal climate in Japan’s peripheral waters becoming the record warm circumstances.”

According to a quantitative investigation of SSTs near Japan, climate change is already the main cause of the majority of the recent record-high SSTs. Future analysis of the dynamics of each significant warming event must take into account both long-term climate change and annual natural variability, according to Hayashi.

“However, we anticipate that our statistical findings based on the most recent climate models will contribute to the implementation of climate change adaptation and mitigation strategies.”