Impact of downward heat penetration below the shallow seasonal thermocline on the sea surface temperature

  • Shigeki HosodaEmail author
  • Masami Nonaka
  • Tomohiko Tomita
  • Bunmei Taguchi
  • Hiroyuki Tomita
  • Naoto Iwasaka


Observational data are used to investigate summer heat penetration into the subsurface ocean in order to quantify the heat capacity of the upper ocean with respect to surface heat exchange. Sea surface temperature is strongly modulated by the change in heat capacity, which could influence the overlying atmosphere and hence trigger climate variations, even during the warming season, when the ocean has been regarded as being rather passive. Few studies have focused on the heat exchange process in surface and subsurface layers because of the existence of a strong seasonal thermocline at the bottom of thin summer mixed layers (ML). By introducing the concept of the heat penetration depth (HPD), defined as the depth to which the downward net heat flux (Q net) distinctly penetrates, we here characterize the heat capacity in terms of the heat content above the HPD using a simple, one-dimensional vertical model during the warming season. Seasonal changes in the HPD indicate that the thermal effects of Q net gradually penetrate below the shallow seasonal thermocline due to vertical eddy diffusivity. Downward heat penetration into the layer below the shallow seasonal thermocline occurs widely throughout the North Pacific, and two-thirds of Q net penetrates below the ML. In a hypothetical analysis of the case where the observed Q net accumulates only within the ML, the change in SST is unrealistically larger than that of the observed SST. These results indicate that heat penetration plays a crucial role in climate variations during the warming season.


Heat capacity Seasonal variability North Pacific Argo J-OFURO2 Air–sea interaction 



We thank the participants in the regular meetings of the Mid- and High-Latitude Climate Predictability Research Team for their useful discussions. Members of the data management team of the Strategic Ocean Monitoring Research Team of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) helped with the use of Argo float data and the refinement of the dataset. Argo float data were obtained from the GDAC websites at and We obtained heat flux data from the J-OFURO2 website at This work was supported by Grants-in-Aid for Scientific Research (22106006, 22510023, 21540458, 24540476, and 23340139) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan.


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Copyright information

© The Oceanographic Society of Japan and Springer Japan 2016

Authors and Affiliations

  • Shigeki Hosoda
    • 1
    Email author
  • Masami Nonaka
    • 2
  • Tomohiko Tomita
    • 3
  • Bunmei Taguchi
    • 2
  • Hiroyuki Tomita
    • 4
  • Naoto Iwasaka
    • 5
  1. 1.Research and Development Center for Global ChangeJapan Agency for Marine–Earth Science and TechnologyYokosukaJapan
  2. 2.Application LaboratoryJapan Agency for Marine–Earth Science and TechnologyYokohamaJapan
  3. 3.Graduate School of Science and TechnologyKumamoto UniversityKumamotoJapan
  4. 4.Hydrospheric Atmospheric Research CenterNagoya UniversityNagoyaJapan
  5. 5.Faculty of Marine TechnologyTokyo University of Marine Science and TechnologyTokyoJapan

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