Thermal Stability of the World Ocean Thermoclines

Abstract

Because of the strong variation with temperature of the thermal expansion coefficient of seawater, both horizontal and vertical mixing that perturb the gradients produce changes of volume, usually a decrease, that shift mass relative to the earth's gravitational field resulting in significant changes of gravitational potential energy (GPE). For sufficiently large temperature gradients, these changes may serve as energy sources to supplement the tidal, internal wave, and other processes that mix the ocean waters and limit the gradient magnitudes. The hypothesis examined here is that for overall stable ocean stratifications of temperature and salinity, the GPE conversions must be positive with respect to local perturbations by diffusion, mixing, or other disturbances. Thus, for the long-term steady state of the oceans to exist, the GPE structure must be stable, including the nonlinear effects. A dynamical description of the conversion process for GPE changes to kinetic energy for mixing has not yet been developed. The evidence for the significance of the process is based on observational data. Examples are given for several oceans to show the limiting effects of the nonlinear Equation of State properties on the main thermoclines.