During June and July of 1974, internal wave records totaling 21 days duration were made at a nominal depth of 350 m in a location 800 km offshore of San Diego, California. The measurements were made with a midwater capsule which profiles temperature while it drifts with the mean flow. The resulting vertical displacement spectra decrease generally as •o-: between the local inertial frequency •o• and the buoyancy frequency n with a pronounced peak just below n; outside these limits the spectra drop sharply. Vertical coherences are nearly frequency independent between •o• and n with the exception of a peak just below n corresponding to the peak in the displacement spectrum. A low-frequency (0 _< •o _< 0.5n) average of coherence (not squared) decreases linearly from 0.99 at l-m separation to 0.85 at 36-m separation. This result suggests that modal energy is distributed with mode numberj as (/': + j,:)-•, where j, -• 3. Phase spectra averaged over the same interval indicate little or no net vertical transport of energy. The wave spectra are consistent with those of a stationary Gaussian process for periods up to 12 hours.

An investigation of thermal structure fluctuations of tidal frequencies showed that an internal tidal wave profile becomes asymmetric as the wave enters shoaling coastal waters. The asymmetry is more pronounced with increased wave height, and it results in higher-amplitude waves, assuming the char,acteristics of internal tidal bores.

An instrument package has been developed that drifts along freely with the water while it repeatedly profiles ocean temperature. Profiling during drifting reduces the Doppler and fine structure effects that usually contaminate internal wave measurements. Six days of exceptionally clean internal wave records were acquired 470 km offshore of San Diego, California, in June 1973 at a nominal depth of 800 m. The resulting vertical displacement spectra decrease generally as ω−2 up to the local Brunt‐Väisälä (B‐V) frequency. Just below the B‐V frequency there is a spectral peak. Above the B‐V frequency the spectra drop sharply to low levels. Vertical coherence of internal waves over a separation of 100 m was found to be 0.8 and fairly independent of frequency up to the B‐V frequency. An internal wave model proposed by Garrett and Munk is in general agreement with the observed features. Both the measured coherence and the oscillatory nature of the spectrum near the B‐V cutoff are consistent with an energy concentration in the lower six or so modes. In addition to the internal wave records, continual changes in the thermal structure were observed by the drifting package. One repeating thermal inversion pattern occurred four times during the 6‐day thermal structure record.

No abstract

An investigation off the southern California coast has shown that a significant part of seasonal thermocline motion is caused by wind transport. A linear relationship for this motion, which takes into account the 14‐ to 16‐hour response time occurring between wind and depth changes, has been established. Through this relationship it is possible to compute and predict movements of the thermocline caused by the wind. Those found to occur regularly include a diurnal oscillation, an oscillation of 4½‐day period, and an over‐all upward trend throughout the summer. Tidal thermocline oscillations were studied with the wind effects removed. It was found that during the 4 months of nearly continuous thermocline depth data the internal tide lagged the surface tide by 3 to 5 hours and averaged 4 or 5 times as great in amplitude.

Details of the annual thermocline cycle in the coastal area off San Diego have been determined from temperature data recorded hourly for over a year at the Naval Undersea Research and Development Center Oceanographic Tower. Vertical thermal gradients averaged 0.59øC/m at their strongest part from late April through early September. The sharpest gradients, which occurred in late June, were as great as 1.27øC/m. Transient gradients averaging 0.29øC/m at their strongest part occurred frequently in late March and early April and from late September through early November. Weak thermal gradients averaging 0.15øC/m at their sharpest predominated from late November through early March. Water temperature varied from 9.7 ø to 23.6øC. Internal tides persisted throughout the year, contributing to monthly temperature variances that ranged from 0.2 (øC)' in January to 5.6 (øC)• in May and August.

No abstract

Observations in the coastal waters off southern California have revealed significant periodic oscillations in the magnitude of the vertical thermal gradient within the seasonal thermocline. These fluctuations occur with periods of approximately 6, 8, 12, and 24 hours and appear to be closely related to the internal tides.