Onshore Section 



The temperature records for the shoreward tow from 1900 

 on 12 July to 0730 on 13 July are similarly presented in figures 

 6 and 7. This leg of the cruise started at point C, 30°15'N, 

 120°01'W, and ended at point D, 30°58'N, 118°31'W. Isotherms 

 12° to 16°C are present in the 100-to-300 foot record, which is 

 also broken up into three connecting parts. The apparent temper- 

 ature inversion in the 14°C isotherm that results in closed iso- 

 therms around 200 feet in the XY and YZ sections appeared to be 

 caused by malfunction of a thermistor. In the deeper section, 

 running from 450 to 650 feet, the 9° and sometimes the 10°C iso- 

 therms are present. 



Isotherms and Internal Waves 



The nature of the vertical changes in the isotherms in deep 

 water has not been investigated in such detail before. Likewise, 

 the cause of vertical oscillations has not been established. It is 

 possible, but not likely, that these changes in temperature are 

 balanced by changes in salinity, and thus that the density surfaces 

 are level. Another possibility is that the vertical temperature 

 changes are merely the result of standing waves in the thermo- 

 cline. Eckart, 4 referring to Vaisala, points out that a given den- 

 sity boundary may have its own normal oscillating frequency, the 

 Vaisala frequency. Still another possibility is that strong winds 

 may create convection cells in the upper layers of the sea, the 

 circulation of which causes the thermocline to be lowered more in 

 one area than another. There is, however, reason to believe that 

 the vertical variations observed with distance in the isotherms 

 are caused by internal waves moving in one or more directions. 

 Evidence to support the latter explanation lies in the fact that, at 

 all anchor stations where repeated measurements have been made, 

 the isotherms show vertical fluctuations with reference to time. 

 The progressive nature of these oscillations in shallow water has 

 been proved by results obtained off anchored ships and at the NEL 

 Oceanographic Research Tower. 6 In this report, therefore, the 

 oscillations are assumed to move as progressive internal waves. 



15 



