Repeat hydrographic surveys revealed that the warming already spread to the global abyssal waters (Kouketsu et al. The heat content of the global upper ocean above 700 m has been increasing since 1970s mainly due to the increase of carbon dioxide and other greenhouse gases in the atmosphere (e.g. Furthermore, the authors examined the changes of obduction and subduction related with the marine heatwave that occurred in the northeastern North Pacific around 2014. The method applied in this study also enabled us to assess the residence time and route of obducted and subducted water parcels below the ML. The waters entrained from the permanent pycnocline warm and salinize the ML in the high latitudes due to the dichothermal structure.
The salinizing rate due to obduction exceeded ± 0.02 psu/yr in these areas, and in the subarctic North Pacific and the eastern South Pacific. The ML heating rate due to obduction was smaller than ± 0.1 K/yr on average over most of the domain, but it was larger than − 0.5 K/yr in places around the Circumpolar Current, in the northeastern tropical Pacific, the southwestern tropical Indian Ocean, and the northeastern North Atlantic. They have successfully separated the effect of obduction on the ML from the entrainment of the seasonal pycnocline, and specified the regions where the obduction impact is noticeable. The authors have calculated the obduction and subduction rates with the Eulerian definition using a 1°-grid Argo dataset and incorporated the obduction rate into the ML budget analysis in order to assess the impact of the entrained permanent pycnocline water on the ML temperature and salinity. It is necessary for air–sea interaction and climate research to evaluate heat and salinity transport between the mixed layer (ML) and the permanent pycnocline and its contribution to the ML properties.
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