OCEANOGRAPHIC INFORMATION FOR SUBMARINE CABLES 



1081 



taken in the same year and do not show these effects. The large fluc- 

 tuation on Profile 3 is probably due to variations with time. 



At a certain depth, often about 1,200 meters, a sharp change in tem- 

 perature takes place. An interface known as the main thermocline 

 separates the warm surface waters from the cold (2°-4°C) deep water. 

 This boundary shifts slightly from time to time and is affected by sub- 

 surface (internal) tidal waves. Thus, cable laid near the main thermo- 

 cline may undergo temperature changes of a few degrees. These changes 

 may be of different periods depending on their causes, e.g., subsurface 

 waves, seasonal changes, or long term changes in ocean regime. It is not 

 known if a long-term change in depth of the thermocline has occurred 

 but such change seems probable. 



The greatest percentage of any transatlantic cable route lies in depths 

 ! greater than 2,000 meters and thus the temperature changes in the 

 I deep sea are of primary importance to the engineering of a cable. The 

 I temperatures are low, averaging 3°C, and the temperature changes are 



Fig. 19 — Positions of Temperature Profiles 1, 2, and 3. 



