moved toward shore at speeds of 0.11 to 0.6 knot. 
Other measurements from anchored ships with range 
markers indicated an average speed of 0.31 knot. 
More recent measurements averaged with three 
isotherm followers were 0.29 knot in 60-foot- 
deep water (fig. 16). 
The observed speed (c) agrees well 
with prior theoretical considerations~ when deal- 
ing with long internal waves as compared to their 
depth. 
where h' and h are the depth of a two-layer 
system having densities p and, respectively. 
In 180 feet of water in the Atlantic 
Ocean, internal wave speed still agreed with 
established equations and gave speeds of about 
1.0 knot t, thus it must be supposed that inter- 
nal waves in deep water, especially deeper ones, 
must travel at considerably greater speeds. 
DIRECTION 
The shape of internal waves varied 
with shoreward movement and refraction as they 
moved into shallower areas. Their shape was 
also influenced by their proximity to the sur- 
face sea floorl?. Nearly all internal waves 
proceeded from a west-to-southwest (mode 72.4°T) 
direction at the Mission Beach location (fig. 
17). Efforts to obtain information on internal 
wave direction were ineffective in deep water. 
One such action involved towing of the NEL 
thermistor chain in different directions, but 
no clear-cut doppler effect could be established 
It appeared from this experiment that deep-water 
internal waves are not in long parallel lines 
as they frequently are in shallow water. 
OTHER RELATED MOTIONS 
One approach to the study of internal 
wave motion in the sea is the assumption that 
they are progressive waves. In lakes and par- 
tially closed bodies of water, standing waves 
are found. The nature of progressive waves 
between two liquids of different densities has 
already been analyzed.~7? The nature of this 
circulation as applied to the sea has been in- 
vestigated by direct and indirect means. For 
example, sea surface slicks, which often re- 
present visible evidence of internal waves below 
are seen as streaks or patches of relatively 
calm surface water surrounded by rippled water. 
The absence of wavelets in a slick gives it a 
glassy appearance in contrast to the adjacent 
rippled water (fig. 18). These slicks hgye 5 
been studied in oceans, bays, and lakes.~’™”’ 
ef 
The occurrence of visible slicks is 
contingent upon proper wind, lighting, suffi- 
cient organic matter on the water, and the 
nature of internal waves. The concentration of 
surface film depends on the interrelation of 
internal wave height and period. The average 
depth of the internal wave and its relation to 
140 
water depth also influence the type of circula- 
tion and thus have a bearing on the formation of 
slicks. In 85 out of 105 cases, the slick was 
on the descending thermocline somewhere between 
the crest and the following trough (fig. 19). 
This relationship is undoubtedly the result of 
convergent water circulation created by internal 
waves. The significant motion is therefore a 
surface convergence over the trailing slope of 
the internal wave. 
Direct measurements of current asso- 
ciated with internal waves were made at the NEL 
Oceanographic Research Tower. By means of a 
closed-circuit television, observing dye and 
motion streamers, and a current meter that dam- 
pens surface wave motion, the existence of con- 
vergence-type circulation associated with a 
slick was confirmed. The reverse circulation 
over shallow crests and over troughs was also 
established, thus substantially confirming in 
part the above circulation theory of a two-layer 
internal wave system in shallow water. 
SUMMARY AND CONCLUSIONS 
Internal waves are now being measured by a 
number of instruments, including vertical 
strings of temperature sensors, which are sus- 
pended in one location, as with isotherm follow- 
ers, or towed, as with the NEL thermistor chain. 
The vertical oscillations observed in the 
thermal structure, commonly termed internal 
waves, are present in the sea virtually all the 
time. Their height, speed, direction, period, 
and other characteristics are found to vary 
widely with time, area, and depth. The larger 
waves are associated with weaker gradients and 
thus have larger amplitudes in the deeper layers 
where temperature changes are small. Studies of 
their motion, spatial distribution, and cause 
are continuing. 
REFERENCES 
1. Fjeldstad, J. E., 1933, INTERNE WALLEN 
GEOFYSISKE Publikasjoner, Vol. 10, No. 6, 
53 pp-, 1933 Oslo. 
2. Ekman, V. W., 1904, ON DEAD WATER, Sci. 
Results, Norwegian North Polar Exp. 1893-96, 
Volk. 5, No. 15, pp. 1-152. 
3. Shand, J. A., INTERNAL WAVES IN GEORGIA 
STRAIT, Trans. Amer. Geophys. Union, Vol. 
34, pp. 849-856, 1953 
4, LaFond, E. C.and Poornachandra Rao, VERTICAL 
OSCILLATIONS OF TIDAL PERIODS IN THE TEM- 
PERATURE STRUCTURE OF THE SEA, Andhra Uni- 
versity Memoirs in Oceanography, Vol. 1, 
pp. 109-116, 1954. 
5. .Munk, W. H., 1941, INTERNAL WAVES IN THE 
GULF OF CALIFORNIA, Jour. of Mar. Res., 
Vol. 4, pp. 81-91. 
6. Zeilon, N., 1934, EXPERIMENTS ON BOUNDARY 
TIDES, Goteborg Vetensksamh. Handl. Folj. 
5 ber. By Bd. 35 NO. LOF 
7. Zeilon, N., 1913, ON THE SEICHES OF THE 
GULLMAR FJORD, Svenska Hydrogr-Biolog. Komm. 
skrifter 5. 
