# World Ocean Thermocline Weakening and Isothermal Layer Warming
## 5 Conclusions  
World ocean thermocline weakening with the rate of \((-2.11\pm 0.31)\times 10^{-3}\) (\({}^{\circ}\)C m\({}^{-1}\) yr\({}^{-1}\)), along with ITL warming with the rate of \((0.142\pm 0.014)\) W m\({}^{-2}\), were identified from the three open datasets NOAA/NCEI WOD18, WOA18, and the global ocean thermocline gradient, ITL depth, and other upper ocean parameters calculated from WOD CTD and XBT temperature profiles from 1 January 1970 to 31 December 2017 (NCEI Access 0173210) ([[https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc](https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc)]([https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc](https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc)): 0173210) using the new definition of the upper ocean heat content (i.e., \(\mathit{TIL}\) heat content \(H_{\mathit{ITL}}\)). Such two trends occur in the global as well as regional oceans except the Southern Ocean, where slightly decreasing in the \(H_{\mathit{TIL}}\) was identified. Weakening thermocline reduces the resistance to the ITL deepening that causes a thick ITL with more heat stored in the ITL (ITL warming). Thermocline weakening may play roles in global climate change in addition to the greenhouse effect from the atmosphere and in phytoplankton growth because strong thermocline inhibits the nutrient pumping. Besides, the global sea surface temperature is directly related to the ITL heat content through the ocean mixed layer dynamics. It is more reasonable to use ITL heat content than the heat content with fix-depth range in climate change studies.  
Figure 7: Comparison between ocean heat content (OHC) for the isothermal layer and fixed-depth layer.  
**Author Contributions:** [PERSON] developed the method, designed the project, conducted the data quality control, and wrote the manuscript. [PERSON] developed the code for computation and visualization. Both authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Acknowledgments:** The authors thank the NOAA National Centers for Environmental Information (NCEI) for providing the three datasets for this study. They are the World Ocean Database (WOD) expendable bathy thermograph (XBT) and conductivity, temperature, depth (CTD) temperature profiles, the World Ocean Atlas 2013 temperature and salinity fields, and the global ocean thermocline gradient, isothermal layer depth, and other upper ocean parameters calculated from WOD CTD and XBT temperature profiles from 1 January 1970 to 31 December 2017 (NCEI Accession 0173210).  
**Conflicts of Interest:** The authors declare no conflict of interest.  
**Data Availability Statement:** The three datasets were all downloaded at the NOAA/NCEI website with the synoptic temperature profiles from URL ([[https://www.nodc.noaa.gov/OC5/WOD/pr_wod.html](https://www.nodc.noaa.gov/OC5/WOD/pr_wod.html)]([https://www.nodc.noaa.gov/OC5/WOD/pr_wod.html](https://www.nodc.noaa.gov/OC5/WOD/pr_wod.html))), the 0.25\({}^{\circ}\) x 0.25\({}^{\circ}\) gridded climatological monthly mean temperature fields from URL ([[https://www.nodc.noaa.gov/OC5/woa18/](https://www.nodc.noaa.gov/OC5/woa18/)]([https://www.nodc.noaa.gov/OC5/woa18/](https://www.nodc.noaa.gov/OC5/woa18/))), and the global ocean thermocline gradient, ITL depth, and other upper ocean parameters calculated from WOD CTD and XBT temperature profiles from 1 January 1970 to 31 December 2017 (NCEI Accession 0173210) from URL ([[https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc:0173210](https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc:0173210)]([https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc:0173210](https://data.nodc.noaa.gov/cgi-bin/iso?id=gov.noaa.nodc:0173210))).