INTERNAL WAVES 



An attempt was made in and near the ice area to determine 

 whether any vertical oscillations in temperature, commonly 

 known as internal waves, existed. At station N17 repeated 

 bathythermograph observations were obtained every 30 min- 

 utes for about 24 hours and, in addition, at shorter intervals 

 for 2 hours and 50 minutes. 



The plots of isotherms for the 24-hour series (fig. 42) 

 show the vertical temperature structure as a function of time, 

 resulting in an extremely complicated pattern. The analysis 

 of these data to determine internal waves is difficult for sev- 

 eral reasons. First, the drift of the ship into water of differ- 

 ent character produced an apparent vertical displacement of 

 isotherms when plotted against a time scale. Second, a dif- 

 ferential movement of the water at various depths distorts 

 the vertical column originally under the vessel and produces 

 apparent vertical fluctuation of the isotherms. In the analysis 

 of similar data from the open ocean these factors are small 

 compared to the effect of the large physical changes associ- 

 ated with internal waves. However, in the Chukchi Sea the 

 amplitudes of internal waves are so small that the changes 

 in temperature structure due to advection predominate. The 

 customary semi-diurnal internal tidal fluctuation in tem- 

 perature is small in the Chukchi Sea as compared to that 

 observed in the open ocean. A definite analysis of the di- 

 urnal and semi-diurnal internal waves cannot be conclusive 

 since it is believed that halfway through the series the ship 

 drifted into water of markedly different character. About 

 12 hours after the start of the repeated bathythermograph 

 observations, the USS NEREUS drifted out of the ice and into 

 a water mass where the thermocline was deeper. Thus, this 

 particular lowering of the thermocline (fig. 42) should be 

 attributed to the drift of the ship and not to internal waves. 

 Other small irregular vertical displacements, apparent 

 throughout the series, may be rapidly damped-out internal 

 waves caused by variations in wind velocity. The most strik- 

 ing feature of the thermal condition in the arctic, however, 

 is the occurrence of pockets of warm and cold water just above 

 the main thermocline. This series of observations shows one 

 pocket of warm water about 25 feet thick. This pocket of 

 warmer water (2 degrees F higher than the surrounding 

 water) lasted for 4-1/2 hours. Assuming a relative drift of 

 about 1/2 knot, the length of the pocket would be about 2 miles 

 long. Such pockets are further evidence that the changes in 

 temperature are advective rather than due to internal waves. 



68, 



