Oceans are heated by nature from the top by the incoming shortwave radiation called the solar irradiation, contrary to heating the vessels at home from the bottom – an unusual way of heating! As a result, ocean temperatures decrease from top to bottom. The temperature structure along a ship lane is shown in Fig. 1 and the vertical variation of temperature at a given point in Fig. 2. At the top 50m – 100m, the temperature remains almost constant as the waters in this layer are churned by the wind forcing. This layer is called the isothermal (iso for equal, thermal for heat) layer. The temperature decreases sharply below this layer, called the thermocline, after which the temperature remains almost constant.
For atmospheric phenomena, this isothermal layer is important as this is that part of the ocean which interacts with the atmosphere. The changes in the depth of this layer and its average temperature depends primarily up on the winds, incoming solar radiation, outgoing longwave radiation and sensible and latent heat fluxes. As the temperature decreases sharply in the thermocline, deeper the isothermal layer more the thermal energy stored in the top layer of the oceans and more the energy available for the atmospheric processes. Secondly, oceans have more thermal inertia (they resist to the temperature changes) as a result of this the day/night and summer/winter temperature changes in the oceans are much less compared to the land.
Hence, the winters of the coastal stations are not as severe as those of inland stations because the relatively more heat available in the oceans is transported to the land through the winds. Sometimes the isothermal layer is also called the mixed layer, though the later should have been defined as the layer where the density changes are minimal.
Besides for atmospheric studies, information on this layer is very critical for submarine communication and helps either in hiding our own ships or in detecting the enemy ships. The zone where ships get undetected by the sound waves is called the shadow zone.
