542 



Internal Waves 



with true internal waves or only with displacements of the system, forced by 

 external causes. 



Seiwell (1937) analysed the extensive observations of the "Atlantis" 

 cruises in a region with very small horizontal gradients, north-north-west of 

 Bermuda. He showed that the frequently large oscillations of the oceano- 

 graphic properties can be presented essentially by waves of 24, 1 2, and 8 h 

 period, where generally the 8 h wave had the smallest amplitude. Conse- 

 quently, we are dealing here certainly with true internal waves. 



Serial observations repeated in short intervals and extending over several 

 days were made on the anchor stations of the "Meteor" in the Atlantic Ocean 

 and of the "Snellius" Expedition in the Indonesian waters. They formed the 

 basis for exhaustive investigations about internal waves by Defant (1932) 

 and Lek (1938). Simultaneous current observation at the various depths 

 were made at these stations. Therefore, the observations are quite complete. 



The internal waves on the anchor station 385 of the "Meteor" can be 



(hr4 68I0 02468I0 02468I0 02468I002468I00 



16 20 4 



en. i3.n. 



MGZ 



12 16 20 4 8 12 16 20 hr 



14 n. '5.n 



Fig. 225. Depths of the iso-halines 36-65-3700%, corresponding to Fig. 224. 



taken as a more detailed example (German North Atlantic Expedition, 

 12-14 February 1938, 16°48-3'N., 46°171'W., depth 2900 m). Schubert 

 (1944) reports that altogether sixty repeated series were made to a depth 

 of 150 m in hourly intervals. The current measurements with Bohnecke's 

 current meter were extended to a depth of 800 m. The station was situated 

 over great depths on the mid-Atlantic ridge, at a distance of more than 700 

 nautical miles from the mainland. The thermo-haline distribution showed 

 the usual subtropical thermocline below an isodense mixed layer of about 

 60 m depth; the salinity maximum is in 100 m depth. The greatest vertical 

 density gradient is found at 100 m; from 90 to 300 m the density increases 

 rapidly from 24-7 to 271. This discontinuity layer was the seat of particularly 

 well-developed internal waves of a period of 12 lunar hours. Figures 224 

 and 225 show the variations in depth of the isothermes from 18° to 24-5° C 



