unknown because the direction and velocity of propagation of the 

 internal waves are unknown. However, the shift has no effect on 

 the amplitudes of the vertical variations in isotherm depths. 

 Second, small oscillations of the deep end of the chain are re- 

 flected in the depth changes of the isotherms. However, these 

 chain depth changes are very small compared to the vertical- 

 temperature-structure fluctuations which are retained through a 

 low-pass filtering process (discussed later). Third, the isotherm 

 depths for the entire record are not recorded simultaneously, and 

 some change in the thermal structure occurs in the beginning of 

 the data section by the time the end is being recorded. However, 

 the vertical changes in depth of a particular isotherm will be 

 nearly correct in any case. 



A broad frequency spectrum exists in the vertical variations 

 of the temperature structure with high frequencies superposed on 

 the lower ones, which is indicative of the complexities of oceanic 

 thermal structure. The sample section selected for analysis is 

 set off by vertical lines in figure 3. To the right of the selected 

 sample area and at a depth of about 200 feet, the 17 °C and 14 C C 

 isotherms display weak 1°C temperature inversions. Inversions 

 of this nature are not unusual in this area and have been dis- 

 cussed at length in studies of thermal fronts found off the southern 

 end of Baja California (LaFond and LaFond, 1966; Griffiths, 

 1962) . 14, 15 Such inversions were purposely omitted from the 

 selected sample section. The effects of temperature inversions 

 on vertical- and horizontal-temperature gradients are shown in 

 a more descriptive example of the summer temperature structure 

 of the deep Bering Sea, Sample Area 12 (see appendices to this 

 report) . 



In the thermistor chain data, the vertical excursions of a 

 particular isoltherm increase with depth as the vertical- 

 temperature gradient decreases. In areas where the vertical- 

 temperature gradient increases sharply, the amplitude of the 

 vertical displacement of the isotherms decreases . This inverse 

 relation of the amplitude of vertical displacement to the slope of 

 the vertical-temperature gradient is probably caused by the dif- 

 erence in the vertical stability between the more stable water in 

 the main pycnocline and the less stable water below it. 

 T. Hesselberg (1918) defined vertical stability by the expression 



1 dp 

 E - — — 



p dz (!) 



12 



