A certain dimensionless parameter F that characterizes the thermal state of the surface is determined from analysis of the instability mechanisms of the viscous and molecular heat conduction layers under the free surface of the ocean, to which a momentum flux and a negative heat flux are applied. Free convection develops in the subsurface layer below the molecular heat conduction layer if F < 1, and forced convection if it is not. Data from vertical sounding of thermal structure in a thin surface layer of the ocean that were obtained with a surfacing sonde in the equatorial Atlantic Ocean on the 29th cruise of R/V Akademik Kurchatov (FGGE-79 international program) are interpreted on the basis of this analysis.