Deep Ocean
a. Surface Layer: from surface down to a few hundred meters. This layer is heated or cooled by the air above and/or sunlight. Significant mixing can occur that equalizes the temperature.
b. Main Thermocline: always characterized by a rapid drop in temperature to near 35 F. Essentially all the deep water is isothermal at 35 F.
c. Deep Layer: isothermal, but characterized by a positive gradient due to pressure affect on sound velocity. Provided the water is deep enough, there will be sufficient velocity increase to turn sound "rays" back around towards surface, leading to a convergence zone effect.
Of these three sections, only the surface layer will exhibit a seasonal variation. Figure 1. Illustrates the effects:
a. Winter: surface is essentially isothermal due to significant mixing caused by storms and wind. Pressure effect makes this layer slightly positive. Provides the best chance for a surface duct propagation.
b. Summer: maximum heating of surface water tends to create a significant negative gradient. The warm water will extend deeper through conduction and lower the point at which the main thermocline begins.
c. Spring/Fall: Transitional cases between summer and fall. Note, it is more difficult to produce a positive gradient by cooling the surface water in Fall. Any cooling is disrupted by convective upcurrents that destroy the stratification. This leads to the winter isothermal layer.