TOWED ISOTHERM FOLLOWER 
A modification of the isotherm follower is 
a recently developed towed version. The prin- 
ciple is the same -- the follower seeks out a 
selected isotherm and traces it as the sea unit 
is towed from a ship. 
The sea unit is a torpedo-shaped device 
with fins (fig. 9) that are moved up or down by 
its small motor which, like the winch, is 
directed by a signal from a thermistor bead in 
the nose of the sea unit. The instrument thus 
follows the isotherm up and down as its depth 
by pressure is recorded on the towing ship. 
The towed isotherm follower is generally 
used in shallow water (where the thermistor 
chain cannot be towed) to determine the nature 
of internal waves as they cross the continental 
shelf and eventually impinge on the bottom. 
NATURE OF INTERNAL WAVES 
CHARACTERISTICS 
Internal waves have been measured in 
all oceans and in several lakes throughout the 
world. Off southern California in summer, the 
vertical oscillations of the temperature struc- 
ture were measured by use of thermistor beads 
suspended vertically at a depth of 50 feet? 
later by isotherm followers at a depth of 60 
feet,13 by the NEL thermistor chain in the San 
Diego Trough, + and in the deep, open sea. In 
the next sections are described the character- 
istics of internal waves that were determined 
in these experiments. 
WAVE HEIGHT 
The depth of a single isotherm in the 
thermocline was observed to fluctuate widely 
during one day in water only 60 feet deep. A 
day's record of smaller internal waves in the 
thermocline, as recorded by the isotherm fol- 
lower, is shown in figure 10. The short-period 
vertical oscillations are superimposed on the 
longer period tidal oscillations in the thermo- 
cline. Generally, the magnitude of the small 
vertical fluctuations was inversely propor- 
tional to the gradients through which they pro- 
truded. Small fluctuations were nearly always 
present. 
Long periods of continuous isotherm 
or thermocline depth measurements have been re- 
corded. It was found that a frequency plot of 
thermocline depth for one 7-day series, 
recorded in summer off Mission Beach, Califor- 
nia, had a trimodal distribution. The central 
primary mode (1) around 32 feet was interpreted 
to be the depth of the seasonal thermocline. 
The others, () and (3), around 18 and 44 feet, 
were caused by internal tide and amounted to 26 
feet at this time and place (fig. 11). 
During the periods of investigation, 
the maximum daily vertical migration of an iso- 
therm in the middle of the thermocline was 31 
Pecive 
139 
The frequency distribution of shallow 
internal wave heights at this location for part 
of 12 days throughout the summer of 1958, and 7 
consecutive days in 1959, is shown in figure 12. 
Only waves higher than 2 feet were considered 
since the lower ones were probably only random 
fluctuations. It was found that 50 per cent of 
the internal waves were higher than 5.6 feet. 
In the San Diego Trough 20 miles from 
shore in 600 fathoms of water, the upper thermo- 
cline during a June period contained vertical 
oscillations that were no higher than those ob- 
served from the nearby NEL Oceanographic Research 
Tower. The median vertical change for changes 
greater than 1 foot was only 4.6. This and 
other characteristics of the thermocline are 
shown in figure 13. 
During the same season, 200 miles from 
shore, the upper thermocline contained waves a 
little higher than those near shore. However, 
the vertical oscillations of constant tempera- 
ture were much higher at depths from 300-800 
feet, their height being inversely related to 
the vertical temperature gradients. Here they 
were from 50 to 200 feet (fig. 14). This in- 
verse relationship probably holds at greater 
depth where the vertical thermal and density 
gradients are even weaker. 
The tidal circulation described above 
also influences long-period wave heights and 
wave periods (fig. 15). In the deep sea areas 
as well as on the continental shelf, internal 
waves of tidal period are commonly found), 
However, the relative phase of surface aud in- 
ternal tide is not consistent. 
WAVE PERIOD 
The frequency distribution for the 
duration of 1061 shallow-water intrnal waves is 
shown in figure 15. Waves with periods of less 
than 2 minutes were excluded. Fifty per cent 
of all waves longer than 2 minutes had periods 
greater than 7.3 minutes. 
In deep water the internal-wave period 
is difficult to measure because of lack of suit- 
able platforms and adequate knowledge of the 
currents at different levels. An easier proced- 
ure is to measure the wave length or distance 
between crests with the towed chain. In the San 
Diego Trough, the upper thermocline oscillations 
greater than one foot occurred on the average of 
0.4 mile apart, whereas in the deeper and more 
open areas, they occurred about 1 per mile at 
depths of about 500 feet. 
SPEED 
In shallow water, the speed of internal 
waves was determined by measuring vertical os- 
cillations simultaneously in three locationsl©,12 
and deduced from the movement of their associated 
sea-surface slicks. 
Time-lapse films of surface slicks off 
southern California® showed that internal waves 
*Mission Beach, La Jolla, and San Diego Bay 
