Observations of Turbulence Associated with Highly Nonlinear, Near-surface Solitons Over the Continental Shelf

Observations of Turbulence Associated with Highly Nonlinear, Near-Surface Solitons Over the Continental Shelf
T. P. Stanton Department of Oceanography, Naval Postgraduate School, Monterey, CA 93943 stanton@oc.nps.navy.mil

Abstract

A three week observation of upper ocean current, temperature, density and dissipation profiles near the shelf break off Northern Oregon has allowed the form and effects of solitons generated at the nearby shelf break to be studied. Strongly nonlinear Solitary Internal Waves (SIW) were continually observed on the leading edge of a semi-diurnal internal tide which propagated past our measurement site throughout a neap to spring tidal period. SIW timeseries of displacement, currents, dissipation rates and turbulent diffusivities from neap and spring tide forcing conditions are being used to characterize the effects of these energetic SIW/internal bore structures on the upper ocean.

The COPE Experiment

The Coastal Ocean Probe Experiment (COPE) was an interdisciplinary experiment designed to study the surface signatures and hydrodynamics of coastal internal waves over the continental shelf. The observation site, offshore from Tilamook Oregon, was chosen for the shallow pycnocline and strong surface signatures of internal waves which had previously been observed in satellite SAR imagery and visual observations from aircraft overflights prior to the experiment. High powered Ka and X band Doppler radars overlooked the ocean from a 750m high coastal mountain top, to measure radar surface backscatter properties (see Kropfli et al, 1998). The radars also made timeseries maps of 150m horizontal resolution surface backscatter levels and scatterer velocity spanning an 50 Km radius offshore from the radars. These map sequences provided a high resolution view of the two dimensional structure of the propagating internal wave surface signatures. FLIP was tri-moored in 140m of water near the edge of the shelf, providing a stable platform for atmospheric, surface interface and oceanographic measurements during a three week period in October 1994.

Observations of the upper ocean structure were made with an automated Loose-tethered Microstructure Profiler (LMP) which measured 0.1m resolution temperature, conductivity (and hence density) from the ocean surface to a depth of 35m while simultaneously measuring temperature and velocity microgradients, allowing thermal and turbulent energy dissipation rates to be estimated. LMP profile cycles were completed every 80 s resulting in 24000 temperature and density profiles during the observation period. An 8m length instrumented frame suspended from a boom extending southward from FLIP supported 5 acoustic travel time 3 component velocity sensors, and an inertial tilt and heave sensor package, while a BADCP extended the velocity profiles to 40m below the frame.

SIW Observations

The amplitude and nonlinearity of the SIW packets observed on the leading edge of the semidiurnal internal tidal bore generated offshore from the observation site were impressive. An example of a 24 hour temperature profile timeseries in Figure 1 shows two cycles of a semidiurnal internal tidal displacement of the of the pyconcline. The leading edge of the internal tide consists of a series of strong negative SIW displacements up to 20m downward from the initial 5m deep pycnocline depth at just prior to the arrival of the bore / SIW structure (barely resolved on the 24 hour timescale in Figure 1). Near the spring tide forcing, these SIW displacements had surface signatures with longshore coherent scales beyond the 40-60 Km range of the radars on Onion Peak. The leading edge of the SIW groups during periods of strong offshore forcing had largely parallel wave fronts propagating onshore, which can be seen in Figure 2, which is a snapshot of cross polarized radar backscatter intensity measured at yearday 269.54, near the middle of the timeseries in Figure 1. Onshore current pulses at the peak of the displacements reached 0.8 ms^-1, approaching the phase velocity of the SIW. Stanton and Ostrovsky, 1998, showed that the highly nonlinear SIW displacements were well modeled by a second order CombKdV equation, a form which also matches the observed weak dependence between soliton width and amplitude. The shallow initial depth and very large amplitudes suggest that these SIW observations have record breaking nonlinearity in geophysics.

Figure 1. A 24 hour profile timeseries of temperature at the COPE site showing two cycles of a semidiurnal internal tidal bore with solitons on the leading edge. This structure was characteristic of spring tide forcing at the measurement site near the shelf break off Northern Oregon.

Figure 2. A radar map of VVHH cross polarized radar backscatter measured from a coastal mountain top as the leading edge of the SIW set moved past FLIP at Yearday 269.54. FLIP can be seen as a small line at 28Km range at approximately 255 T bearing. (Courtesy of Bob Kropfli, NOAA ETL, Boulder Colorado).

Concurrent observations of turbulent thermal and velocity dissipation rates, c and e , made by microstructure sensors on the LMP have allowed changes in turbulence levels caused by the propagation of the SIW/internal bores to be estimated. Changes in turbulent diffusivity of the water column arise from a combination of modulation of the local buoyancy frequency and thermal gradients by the internal bore / SIW packets, a decrease in dynamic stability as vertical shear associated with the strong SIW current pulses acts against the modified density profile, and a range of interactions which occur between the strongly nonlinear SIW field, surface gravity waves, and background internal wave field. Figure 3 illustrates the net effects of these processes in changing the thermal turbulent diffusivity, K_T = c / T_z² /2, where the vertical thermal gradient T_z and thermal variance dissipation rate, c , are estimated over 5 min and 1m vertical averaging intervals in order to resolve changes within SIW displacement times, but with resulting marginal robustness of the turbulence estimates. The two hour timeseries was taken at the start of the 24 hour temperature profile timeseries in Figure 1, and has high resolution isotherms superimposed to show the position of the SIW displacements. This timeseries, which is representative of conditions during strong forcing, suggests a sequence of events where the first SIW simply displaces high diffusivity (low stratification) near-surface fluid downward, then even after a single displacement, the diffusivity of the water column between downward displacements increases significantly (keeping in mind the logarithmic scale used in Figure 3). Successive displacements further increase the turbulence (and therefore diffusivity) levels until the displacement amplitudes decrease.

Figure 3 A two hour timeseries of turbulent thermal diffusivity, with the logarithmic gray scale (log₁₀m² s^-1) shown on the right panel. 1 ° C interval isotherms have been superimposed to shown the position of the SIW displacements.

Conclusions

Cope provided an opportunity to measure long timeseries of the detailed structure of strongly nonlinear SIW associated with an internal tidal bore near their source region at the outer edge of the continental shelf. It is clear that these energetic SIW pulses have a significant effect on vertical diffusive processes, net displacements of the surface layer, and large effects on near-surface acoustic propagation on the shelf. Contrasts in structure, directionality, net displacement, turbulent dissipation rates and modulation of turbulent diffusivity for both strongly and weakly forced SIW packets are further discussed in Stanton, 1998, and will be summarized at the SIW workshop. Limitations of the existing data set and suggestions for future SIW observations to better quantify and model SIW generation, propagation, shoaling, and breaking will also be discussed.

References

Kropfli, R. A., L. A. Ostrovsky, T. P. Stanton, E.A. Skirta, A. N. Keane and V. Irisov, 1998. Relationships Between Strong Internal Waves in the Coastal Zone and their Radar Signatures. JGR, accepted.

Stanton, T. P. and L. A Ostrovsky, 1998. Observations of Highly Nonlinear Internal Solitons Over the Continental Shelf. Geophys. Res. Lett., 25, 14, 2695-2698.

Stanton, T. P., 1998. Upper Ocean Mixing by Highly Nonlinear Internal Solitons Over the Continental Shelf. Submitted to JGR.