THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

414 15. Underwater AcousticsThe seasonal thermocline lies below the surface layer. The term thermoclinedenotes a layer in which the temperature varies with depth. The seasonal thermoclineis usually characterized by a negative thermal or velocity gradient, meaningthat the temperature and the speed of sound decreases with increasing depth, andit does vary with the seasons. During the summer and fall, when the ocean watersnear the surface are warm, the seasonal thermocline is well defined and it becomesless so during the winter and spring and in the Arctic when it tends to becomeindistinguishable from the surface layer. Below the seasonal thermocline lies themain thermocline, which hardly varies throughout the seasons. It is this layer in thedeep sea, that the temperature changes the most. Underneath the main thermocline,reaching down to the sea bottom is the deep isothermal layer having an almost constanttemperature (generally about 3 ◦ –4 ◦ C) in which the speed of sound increaseswith depth because of the effect of pressure on the sound speed. At the saddle pointbetween the negative gradient of the main thermocline and the positive gradient ofthe deep layer, there occurs a velocity minimum toward which sound traveling atgreat depth tends to bend or becomes focused by refraction. In the more northernregions, the deep isothermal layer extends almost to the water surface. The regionwhere this minimum occurs is called the deep sound-channel axis.The existence and the thicknesses of these layers vary according to latitude, season,time of day, and meteorological conditions. Figure 15.2a displays the diurnalFigure 15.2. Diurnal and seasonal variation of a surface layer near Bermuda. In (a) temperatureprofiles at various times of the day show how surface temperature increases overthe temperature at 50 ft depth. The temperature profiles for different parts of the year aregiven in (b). Temperatures and temperature differences are given in Fahrenheit degrees.