Naval Postgraduate School Monterey California

Department of Oceanography

OC3570 Project Report Determining Conditions Necessary for the

Generation of Solitons within the Monterey Bay

by

Preston Jacob Roland, LT, USN September 2006

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I. INTRODUCTION

Nonlinear, near-surface, internal waves, commonly referred to as solitons, evolve on

the leading edge of an internal tidal bore that has steepened due to dispersion effects

(Tjoa 2003). Solitons move in packets frequently bound to this steep front and are widely

observed in coastal regions when the water is stratified and generally propagate cross-self

(Apel 1983). These internal waves are strongly nonlinear and coupled with an internal

tide can generate strong current pulses and dissipates energy within the water column,

especially at the bottom boundary layer above the ocean bed as they propagate shoreward

and begin to shoal.

The dissipating effect caused by solitons can enhance the forcing due to surface

gravity waves, imparting necessary stress above the ocean bed that is required to suspend

sediment and organisms into the water column (Tjoa 2003). The current pulsing

contributions provided by solitons serve as a mechanism for cross-shore sediment

transport. Additionally, the baroclinic structure of the internal tidal bore can rapidly and

dynamically changes the characteristics of the acoustic environment.

A. Background

The offshore bathymetry of Monterey Bay, located along the central California coast

between 121°W-123°W longitude and 35°N-37°N latitude, is dominated with canyons

and ridges that run both parallel and perpendicular to the shore. The Monterey Inner

Shelf Observatory (MISO) was developed by Professor Stanton of the Naval

Postgraduate School’s Oceanography Department in order collect real-time data on

coastal phenomena including solitons. MISO is located approximately 600m from the

shoreline at the southern end of the bay, positioning it directly across from the segment of

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the 4000m deep Monterey Canyon that runs parallel to the beach. The observatory

consists of an instrument array frame that sits in 12m of water and is cabled to nearby

shore facilities.

B. Internal Tide Generation Mechanisms

Figure 1. Internal Tide Generation Mechanisms (Tjoa 2003). a) Ebb flow across a shelf break causes a depression in the pycnocline. b) Formation of a steep edge shoreward bore during slack tide. c) Internal bore propagates shoreward assisted by the flooding tide. Leading edge continues to steepen due to dispersive effects. d) The steep leading edge of the bore degenerates into solitons through the dispersive properties of internal waves.

A cross-shelf barotropic tidal current is the essential prime mover in the generation of

solitons. The flow of the ebb tide across the hydraulic jump created by the steep shelf

break (Figure 1a) becomes hydraulically critical, resulting in a depression in the

isopycnals and isotherms (Holloway 1987). This depression is seen as the thickening of

the upper layer of a stratified water column and will generate a tidal bore during slack

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tide (Figure 1b). This tidal bore propagates in both onshore and offshore directions. The

flood tide will interact with the onshore bore, causing its leading edge to steepen as a

result of dispersion effects (Figure 1c) (Tjoa 2003). The continued steepening of the

leading edge will cause nonlinearities that ultimately lead to solitons (Figure 1d).

C. Conditions Impacting Soliton Generation

As stated above, the barotropic tidal current of stratified water over a hydraulic jump

is the mechanism for generating solitons. Therefore, certain environmental conditions are

necessary to allow an internal tide and solitons to develop in coastal regions. These

conditions are: step shelf break, a strong, shallow stratification in the water column, a

wind pattern that promotes stratification, and a strong, barotropic cross-self current. The

premise of the field experiment was to determine if the presence of solitons in the near

shore region can be forecasted using observations of environmental conditions.

II. FIELD EXPERIMENT

A. Methodology and Physical Setup

Data on environment conditions was generated from various sources and compared to

observations of solitons from MISO for 26 July 2006 (207 Julian day) in order to validate

current theory on soliton generation and determine if the presence of solitons in the near

shore region could be forecasted using observations of environmental conditions (Figure

2).

1. Stratification

Stratification of the water column into a two-layer system, with one layer thicker

than the other, is necessary for the generation of an internal tide and thus solitary waves

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because it allows for the initial depression of isopycnals over the shelf break. In the

Monterey Bay stratification has a seasonal cycle driven upwelling due to wind driven

current. Summer months exhibit stratification that is stronger and shallower than winter

conditions (Tjoa 2003). The strong near surface layer present in summer has observed

high temperatures and low salinity above a lower layer of colder, saltier water.

The RV Point Sur surveyed a segment of the Monterey Bay using a conductivity,

temperature, depth sensor (CTD) in order to determine stratification. Stratification was

evaluated by using CTD observations to calculate potential density (σθ) and the Brunt

Vaisala or buoyancy frequency (N). The measurement started within the Monterey

Canyon and moved cross-self. The first CTD cast (093) was conducted within the

Monterey Canyon at 36° 46’N latitude 121° 59’W longitude at a depth of 733m. The cast

fell short of the charted bottom depth of 920m due to concerns of strong currents within

the canyon damaging the CTD on the canyon wall. The second CTD cast (094) was

conducted at the self break outside of the canyon wall at 36° 43’N latitude 121° 58’W

longitude at a depth of 100m. The final CTD cast (095) was conducted further cross-

shelf at 36° 41’N latitude 121° 56’W longitude at a depth of 90m.

2. Winds

Winds can positively or negatively contribute to the generation of solitons. Daily

wind patterns drive the surface circulation of a bay and have a large effect on surface

layer mixing. Near surface stratification can rapidly change due to a reversal of winds

(Tjoa 2003). Wind driven currents determine coastal upwelling, which influences

stratification as well.

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During the summer months the winds are consistently from the northwest and run

parallel along the central Californian coast, creating the southern running California

current. This produces an upwelling of cold, salty water that helps sustain a warm, fresh

shallow surface layer in Monterey Bay. During the winter months, as winds from the

northwest decrease, the Davidson current is dominate. The Davidson current runs

opposite the California current and upwelling ceases. Additionally, the intense winds of

winter storms result in a deepening of the mixed layer (Tjoa 2003 / Collins 2002). Thus,

the generation of solitons can be greatly affected by seasonal changes in wind forcing.

The weather station at Moss Landing Marine Laboratories provided data on wind

speed and direction from an RM Young manufactured anemometer located at 36° 47’N

latitude 121° 27’W longitude at an altitude of 40ft.

3. Tides

A strong barotropic tide is required in order to generate a cross-shelf current that

in combination with the self break creates the hydraulically critical isopycnal depression

within the two-layered system that results in a tidal bore that will eventually steepen and

evolve into nonlinear solitary waves. The magnitude of the tidal forcing is influenced by

the positions of the moon and sun relative to the earth. Spring tides, occurring when the

sun, moon, and earth are aligned, are stronger than neap tides that occur when the sun,

moon, and earth are in quadrature.

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MMIISSOO

RV PT SUR

CTD 094

CTD 095

CTD 093

MMLLMMLL

Figure 2. Experiment Area of Operations. a) EAO within the Monterey Bay. b) Locations of MISO, Cross-Shelf CTD cast conducted by the RV Point Sur, and Moss Landing Marine Laboratories weather station.

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In the Monterey Bay, spring tides generate stronger tidal surface currents at the

head of the Monterey Canyon and tended to be better aligned with bathymetry at the

southern end of the bay as observed by Petruncio (1993) and Paduan et al. (1995) (Tjoa

2003). Thus, tidal analysis can help to more clearly forecast increased steepening of the

leading edge of a tidal bore and consequently the formation of solitons.

Tidal data was acquired via the XTides tidal prediction model running the same

algorithm as the NOAA National Ocean Service. This data was used to asses the time of

flood and ebb tide as well as to determine whether spring or neap tide forcing was

occurring.

4. Bathymetry

The location and complexity of the self break directly impacts the formation of a

tidal bore. A continental self break that is directly offshore and parallel to the coast will

produce internal bores that propagate in a cross-shore direction. Increasingly complex

canyon shape will produce a more alongshore bore that could be refracted by the shallow,

inner shelf topography (Tjoa 2003). The cross-shore propagating bores will more likely

be steepened by an onshore flood barotropic tidal current, thus generating solitons more

readily.

Bathymetry data for the Monterey Bay was taken from NOAA mercater

projection chart 18685, Monterey Bay.

5. Soliton Observations

Soliton observations were conducted a MISO using a 16 element thermo-resister

string to measure a time series of isotherms.

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B. Experimental Results

1. Bathymetry

The southern edge of the Monterey Canyon located across from the MISO

provides a steep shelf break running parallel to the shore, generating cross-shore tidal

bores that more readily become solitons due to the dissipative effects of interaction with

the flood tide (Figure 3). It should be noted however that the Monterey Canyon has a

complex bathymetry that lends itself to generation of tidal bores in the alongshore

direction as well. Gaining better understanding of the alongshore propagating tidal bores

and their interaction with cross-shore propagating tidal bores is a valid focus for follow

on research.

2. Winds

Observed winds on and around 26 July 2006 were predominately from the

northwest, approximately 300º T, at a speed between 2-7 m/s (Figure 4). This is typical

of Monterey Bay in the summer months. The constant southeastern wind forcing ensured

coastal upwelling associated with the California current which transported cold, salty

water up from depth and trapped warm, fresh water in a shallow surface layer. This

produced the two layer stratification necessary for the generation of solitons.

Additionally, the consistent wind direction prevented deepening of the surface layer due

to mixing.

3. Tides

Consistent with model forecast and historical data for the Monterey Bay, a

barotropic semi-diurnal tide was observed during the month of the experiment (Figure 5).

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A spring tide was in effect on 26 July 2006 and contributed to increased tidal forcing

which in turn enhanced conditions necessary for the generation of solitons (Figure 5b).

Analysis of the time of occurrence of ebb and flood tide (Figure 5a) illustrates that

the solitary waves observed at MISO are indeed nonlinear.

4. Stratification

CTD from the RV Point Sur provided extremely accurate data on stratification

that proved to be consistent with historical summer observations of a shallow layer

consisting of warm, fresh water above a deep layer of cool, salty water caused by

upwelling. Figure 6 and Figure 7 display spatial plots of salinity, temperature, potential

density, and buoyancy frequency that clearly show a two-layer system in the Monterey

Bay. The shallow surface layer spans from the surface to a depth of approximately 20m

and remains consistent across the shelf break.

5. Presence of Solitons

A color contour time series of the temperature profiles for 26 July 2006 provided

by the thermo-resister string at MISO clearly shows the presence of solitons along the 17º

C isotherm in red (Figure 8). Analysis of tidal data shows that these waves are solitary,

nonlinear, near surface, internal waves that are traveling in packets.

III. CONCLUSION

Strong internal tidal bores and solitons were observed at the MISO site in conjunction

with observations of the environmental conditions necessary for the generation of those

phenomena. These coinciding observations practically demonstrate the current theory on

the generation of cross-shore propagating solitons. Thus, the presence of solitons in a

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near shore region can be forecast through correlation of bathymetry data, wind

predictions, tidal models, and stratification observations. By using a autonomous

underwater vehicle such as a sea glider equipped with wireless communications

technology, the manned research vessel could be replaced. This would allow for

persistent stratification observations in the near real time in a non-permissive

environment. That capability, coupled with remote sensing and 3D tidal prediction

models could enable naval planners to better exploit the battlespace by being able to

forecast solitons in a non-permissive environment.

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LIST OF REFERENCES

Apel, J.R. and F.I. Gonzalez, Nonlinear features of internal waves off Baja California as

observed from the SEASAT Imaging Radar, Journal of Geophysical Research, 88 (C7),

4459-4466, 1983.

Collins, C.A., Change in the hydrography of Central California waters associated with

the 1997-1998 El Nino, Progress in Oceanography, 54, 129-147, 2002.

Holloway, P.E., Internal Hydraulic Jumps and Solitons at a Shelf Break Region on the

Australian North West Shelf, Journal of Geophysical Research, 92, C5, 5405-5416,

1987.

Paduan, J.D., Wind-driven motions in the Northeast Pacific as measured by Lagrangian

drifters, Journal of Physical Oceanography, 25, 11, 2, 2819-2830, 1995.

Petruncio E.T., Characterization of tidal currents in Monterey Bay from remote and in-

situ measurements. M.S. thesis, Dept. of Oceanography, Naval Postgraduate School,

113pp. [Available from Naval Postgraduate School, Monterey, CA 93943-5000], 1993.

Stanton, T.P., www.oc.nps.navy.mil/~stanton/miso, 1998, January 2003.

Stanton, T.P., and L.A. Ostrovsky, Observation of highly nonlinear internal solitons over

the continental shelf, Geophysical Research Letter, 25, 14, 1998.

Tjoa K.M., The bottom boundary layer under shoaling inner shelf solitons. M.S. thesis,

Dept. of Oceanography, Naval Postgraduate School, 99pp. [Available from Naval

Postgraduate School, Monterey, CA 93943-5000], 2003.

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Figure 3. Steep wall of the Monterey Canyon parallel to the shore where MISO is located. Cross-shore tidal bore propagation observed.

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Figure 4. True wind data Monterey Bay from Moss Land Marine Laboratories weather station for July 2006. a) True Wind Direction. b) True Wind Speed.

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Figure 5. Tidal Information for the Monterey Bay from the XTides model for July 2006.

a) Flood and Ebb tides for 26 July 2006. b) Spring and Neap tides for July 2006. Note the Spring tide on and around 26 July 2006.

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Figure 6a. Salinity v. Depth for the Monterey Canyon. Note the shallow layer of fresh water near the surface (approx. 20m depth).

Figure 6b. Temperature v. Depth for the Monterey Canyon. Note the shallow layer of warm water near the surface (approx. 20m depth).

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Figure 6c. Density v. Depth for the Monterey Canyon. Note the shallow surface layer (approx. 20 m depth)

Figure 6d. Buoyancy Frequency v. Depth for the Monterey Canyon.

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Figure 7a. Salinity v. Depth for the Shelf Break south of the canyon wall. Note the shallow layer of fresh water near the surface (approx. 20m depth).

Figure 7b. Temperature v. Depth for the Shelf Break south of the canyon wall. Note the shallow layer of fresh water near the surface (approx. 20m depth).

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Figure 7b. Density v. Depth for the Shelf Break south of the canyon wall. Note the shallow surface layer (approx. 20m depth).

Figure 7b. Buoyancy Frequency v. Depth for the Shelf Break south of the canyon wall.

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Figure 8. Color contour time series of temperature profiles for 26 July 2006. Note presence of solitons in the 17º C isotherm.