Global Ocean Thermocline Weakening and Isothermal Layer Warming

Thermocline with a strong vertical gradient is the transition layer between the vertically quasi-uniform layer of temperature from the surface, i.e., isothermal layer (ITL) and the deep-water layer. Intense turbulent mixing near the ocean surface causes formation of ITL. The thermocline resists the turbulent mixing from the ITL due to its strong vertical gradient and in terns limits the heat exchange between the ITL and deeper layer. The heat content within the ITL (H_ITL) is a better parameter for climate change studies than the commonly used ocean heat content with fixed depths such as 0-700 m (H₇₀₀) (Levitus et al. 2009), because H₇₀₀ usually involves water in the ITL and thermocline, and the heat stored only in the ITL impacts the atmosphere directly. Strong thermocline gradient (THG) exhibits the downward heat transfer from ITL (non-polar regions), makes thin ITL, and causes less heat stored in the ITL (i.e., small H_ITL). On the other hand, weak THG makes thick ITL, and causes more heat stored in the ITL (i.e., large H_ITL). This letter shows the observational facts on such negative correlation and two opposite trends of the decadal thermocline weakening and ITL warming.