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An Analysis of a Marine Heatwave Event and Its Triggers

A research group headed by Prof. Baoshu Yin from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS) accredited the record-breaking marine heatwave (MHW) event to the atmospheric weather pattern, global warming, and El Nino Southern Oscillation (ENSO).

An Analysis of a Marine Heatwave Event and Its Triggers
(a) Observed temperature anomalies in July 2021 relative to the July mean of 1982-2011. (b) Time series of July SST anomalies averaged over the rectangle region in (a) from different observations and CMIP6 ensemble simulations. Image credit: Institute of Oceanology of the Chinese Academy of Sciences

On January 30th, 2023, the study was published in Bulletin of the American Meteorological Society.

An unprecedented MHW occurred in the northwest Pacific Ocean (NWP) in July 2021, which had sea surface temperature anomalies (SSTa) larger than 4 °C in some coastal areas of the Okhotsk Sea and Japan (East) Sea. An unprecedented extensive outbreak of harmful algae in the coastal waters was triggered by this event, which resulted in grave damage to coastal fisheries and marine ecosystems.

To determine how anthropogenic climate change has impacted the possibility of MWH, the scientists integrated observations with climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), including historical all-forcing (natural plus all anthropogenic forcing) simulations and natural-only forcing simulations with no anthropogenic effects.

According to attribution analysis, human influence was estimated to have made such an event about 43 times more likely and the return period from more than two centuries in the counterfactual world to about five years in the present climate.

Prof. Baoshu Yin, Institute of Oceanology of the Chinese Academy of Sciences

Based on detrended SSTa, the experimental attribution analysis showed that the improved possibility of this MHW was directed by the warming mean climate rather than that of climatic changes in SST variance. In the future, it is anticipated that with more ocean warming, there will be more and more frequent MHWs.

Natural weather patterns or climate variability may also play significant roles in the occurrence of this type of event. Our conditional attribution analysis revealed that the anticyclone atmospheric patterns and developing phase of La Nina favor the likelihood of such an event by a factor of about two, which is weaker than the anthropogenic influence.

Dr. Delei Li, Study First Author, Institute of Oceanology of the Chinese Academy of Sciences

Journal Reference:

Li, D., et al. (2023) Attribution of the July 2021 Record-Breaking Northwest Pacific Marine Heatwave to Global Warming, Atmospheric Circulation, and ENSO. Bulletin of the American Meteorological Society.


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