Vertical Thermal Oscillations 



The thermal structure off Cape San Lucas was investigated 

 on a number of cruises 6 - 9 using bathythermographs and revers- 

 ing thermometers, and the studies revealed the existence of ther- 

 mal fronts. * However, the nature of the detailed vertical changes 

 in the isotherms had not been measured with the thermistor chain. 



Definite density boundaries in the sea should exhibit certain 

 frequencies and modes of oscillation. It is pointed out 10 ' n that a 

 given density boundary may have its own normal oscillating fre- 

 quency (the Vaisala frequency) . Strong winds may create convec- 

 tion cells and eddies in the upper layers of the sea, and the re- 

 sulting circulation will lower the thermocline more in one area 

 than another. Vertical oscillations in the thermocline can be in- 

 duced by fluctuating winds at the air-water surface, which can 

 also generate waves on the thermocline. Tidal and other forces 

 that impel water movement around land boundaries and topograph- 

 ic features can also start oscillations in the thermal structure. 

 There is, however, reason to believe that the vertical variations 

 in the isotherms observed with distance can be internal waves 

 moving in one or more directions and causing reinforcement or 

 cancellation of the individual waves (fig. 4). The progressive 

 nature of these oscillations in shallow water has been established 

 by studies conducted from anchored ships and from the NEL 

 Oceanographic Research Tower. ^ 



The detailed recording of isotherm depths indicates the 

 complicated character of sea temperature structure and empha- 

 sizes the complex nature of the ocean in its temperature- related 

 chemical properties and biological populations. 



*The term thermal front is defined as the leading edge of a border 

 separating unlike water masses. The boundary is frequently ill- 

 defined, transient, and associated with motion causing a relative 

 vertical displacement. Fronts are commonly observed in the 

 region between water masses with marked gradients in temper- 

 ature, salinity, and other properties. The frontal zones are 

 important in sound transmission because of their strong horizon- 

 tal and vertical gradients in sound velocity. 



