Dissipation of Internal Waves in Port Susan, Puget Sound

Transcription

1 Dissipation of Internal Waves in Port Susan, Puget Sound Proposal Draft Lauren Curry th Ave NE #A303 Seattle, WA (206) lbcurry@u.washington.edu School of Oceanography University of Washington February 28,

2 Project Summary Internal solitary waves (ISW) are waves that travel on the interface between two density stratified layers of water. There are many ways to create an ISW; most are tidally driven over topography on the continental shelves (Turner 1973, Bogucki et al. 1997, Knauss 1996). They cause much mixing in subsurface layers of the ocean that are important to ocean s structure, biological processes, and even acoustic properties (Turner 1973). Port Susan has a tidally driven ISW observed by Jeffery Harris in Port Susan is a quite basin in the northern end of Puget Sound with a large delta from the Stillaguamish River feeding it at the northern end and the mouth at the southern end. The waters in Port Susan are extremely stratified from the fresh water input and little mixing occurs in the basin. Harris theorized that the mechanism that might cause the internal wave would be the input of fresh water from the delta during the ebb tide released over the marine waters. This displacement initiates an internal wave that continues to the south end of the basin. Harris first observed the wave propagating out of the basin during flood tide and its dissipation during the ebb. The cause of dissipation is not known, and was speculated to be shear instability. Where internal tides break in Port Susan considerable mixing could occur that might have great impacts on the strongly stratified system. I would like to focus my research on the breaking of the internal wave. I would like to find out if it is shear instabilities that cause the wave to break, and if the tide is the main cause in changing the background flow to create the shear instabilities. I would also like to observe energy dissipation when the wave breaks to find some of the mixing properties. A general survey of Port Susan and taking direct measurements of the ISW isopicnal displacement will take place on two different dates. The general survey of the density structure 2

3 will be conduced from the Thompson in March. I will find the stratification of Port Susan at this time to help determine potential wave speeds of the internal wave. I will need a CTD system and niskin bottles to take salinity samples. Salinity samples will be used to calibrate the CTD, and lab analysis will be needed. There will be sampling at 5 stations, two PRISM sites, and 3 new sites within the basin. Time on the WeeLander will be spent observing the final propagation and dissipation of the ISW. Here we will once again use a CTD, but stations will be based more on an observational process when the wave is found. The small WeeLander boat will need a GPS to accurately record station locations, and other gear, such as wind meters and current metes will be needed. For the surface features to be visible and to obtain clean data, weather becomes an important factor for the cruise date. A calm, precipitation free day will be important to the success of this project. In addition the times of the tides will effect the days that we are able to go out so that the wave dissipation occurs during the daylight hours. I have chosen two windows in April to conduct the research. Introduction Background Every tidal cycle a packet of waves with amplitudes greater than 10 meters travels through Port Susan, Puget Sound almost unnoticed (Harris 2003). These waves exist just a few meters under the surface of the water, and except for a series of surface slicks from Langmuir Circulation, these waves are invisible from the surface. These waves are called internal solitary waves (ISW) and they propagate between two stratified densities layers. These waves can be large because the density difference between the two stratified layers is small. The reduced gravity lessens the pressure gradient force on the wave having slowing effects on the restoring 3

4 force and acceleration (Knauss 1996). Thus large slow waves can exist with small differences between density layers. The effects of internal waves on the environment can be substantial. Internal waves could be a large source for dissipating tidal energy by mixing stratified environments (Garrett and Munk 1979). Sandstorm and Elliott found that the Scotain Self had internal waves that were the main source for mixing nutrients to the euphotic zone. Bogucki et al. found internal waves that caused resuspension of particulate in costal waters off California without large currents, and this could be very dangerous because it was the nation s largest ocean dumping ground of DDT. Port Susan may not be a toxic dumping ground, but there are water quality factors for the recreational use of the parks and Indian lands on the estuary. The Port Susan ISW Port Susan is a quite fjord in the northern end of Puget Sound. There is a large delta from the Stillaguamish River, and the fresh water the River supplied to the estuary stratifies the entire basin. The fjord is relatively shallow reaching only 120 meters at its greatest depth, and has a characteristic sill near the mouth of the basin. The first study of the ISW in Port Susan was accomplished by Jeffrey Harris. He theorized the mechanism that initiates the ISW at Port Susan is an ebb flood. This allows the fresh water on the Stillaguamish Delta to pour very quickly into the basin in a pulse of fresh water, displacing marine water and causing an ISW. In his observations he found the wave speed to be 0.63 ± 0.04 m / s with amplitudes greater than 10m for a packet of five waves. Harris suggests an equation for the wave speed at Port Susan to be: c 2 = 4/ 2 * N 2 (h 2 + 4/9 * a 2 ) 4

5 This equation does not agree with the small amplitude Korteweg-de Vries wave theory, but seemed to explain the wave seeds observed at Port Susan. Wave speed for ISW is can normally be explained by the Equation from Bogucki and Garrett 1993: c = 2/ Nh (1 + 2/3 * a/h) This wave has not been completely characterized yet on its form and type. The generation of the ISW is also still speculation. The Decay of an ISW When ISWs reach a critical amplitude they decay from shear instabilities. The critical amplitude occurs when the Richardsons number falls below ¼ the wave becomes unstable and will decay. The critical amplitude is a function of the height of the wave. Bogucki and Garrett found that 0.82 times the height of the surface layer should be the critical depth. Harris did not find this to be the case, and suggests background flow to be a strong factor in the decay of the wave. In Harris observations the ISW propagated out of the basin while currents moved into the basin during flood tide. There were no observations during their theorized formation during ebb tide. The waves started to dissipate during the next cycle of ebb tide when the background flow of the tides changed and was in the same direction as the ISW. The energy an ISW propagates is the significant factor in how much the wave can mix the water column. It can be affected by the radial propagation of the wave as it moves away from a point source, the bottom shear stress from bathymetry or shear in the water column itself. The energy internal waves propagate is described in Knauss 1996 as: E = ½g a 2 where g = g( 1-2 )/ 2 5

6 Quantifying the amount of energy in the wave will help to find how much the ISW mixes the water column near the mouth of Port Susan when it dissipates. Proposed Work My research s potential finding would be able to repeat some of Harris data and will focus on the location and mechanism of dissipation. A second reading of wave speed, amplitudes and number of waves will be found to compare against Harris s observations. The focus will be to determine the dissipation location and correlate this to the tides, bathymetry, local density structure or some combination of these factors. There will be two cruises to Port Susan, the first on board the Thompson and the second on the R/V WeeLander. Information from the first cruise can be applied to equations to make better hypothesis of wave speeds and the medium that the wave must propagate through. The second cruise will focus on finding the wave and recording its dissipation. The time on Thompson will be spent taking samples from a transect through the basin to find the stratification of Port Susan. There will be five stations, two PRISM sites, and 3 new sites (Figure 1, Table 1). The time required at each station will be quite short, perhaps less than 30 minutes at each station. A CTD system will be used for data collection. Salinity samples for calibrating the CTD unit will also be taken and will need analysis done by a lab. This cruise will take place between March 21 st to the 25 th. The WeeLander cruise we will be spent Yoyo-ing the CTD in surface expressions of the ISW obtaining isopicnal displacements. An Operator from the University of Washington will be required to bring the boat to the Everett Marina for launching and driving the boat. From the Marnia we will drive out to meet the wave. We will then start taking frequent CTD casts until 6

7 the wave has dissipated. Sampling will take less than 5 hours on the water to complete. Sampling must be taken when surface expressions are visible; a calm precipitation free day will be required for a successful cruise. Also because the ISW is tidally linked and the WeeLander can only be on the water during daylight I have set up two time windows that are available in April to sample (Figure 2, Table 2). Tables and Figures Table 1. Locations for the 5 station transect in the March cruise on board the Thompson Table 2. Possible days for sampling on the WeeLander in April Figure 1. Location of sampling stations for the first March cruise aboard the TGT Thompson Figure 2. Tides on potential days to sample aboard the R/V WeeLander 7

8 Table 1 Repeating PRISM Stations: PRISM Description: Port Susan Latitude Longitude PRISM Description: Gedney Island Latitude Longitude New Stations: Sill Station Latitude Longitude Close to delta Latitude Longitude Middle Station * Take Salinity samples here from 10 depths (10 bottles) Latitude Longitude Table 2 Days in April Start observations at 7 th 11:30am 8 th 12:00pm 21 st 10:30am 22 nd 11:30am 25 th 12:30pm 8

9 Figure 1 Delta Station PRISM Port Susan Station Middle Station PRISM Gedney Station Sill Station 9

10 Figure 2 10

11 Budget cost per units Hypothetical real unit needed totals totals AUXILIARY EQUIPMENT A02A MESSENGERS, STD.- $55.00 REPLACEMENT $ 3.00/DAY A05 LEAD WEIGHTS $ 3.00/DAY COMPUTER EQUIPMENT G01 TOSHIBA PORTABLE $ 15.00/DAY NAVIGATION J04 GPS, NORTHSTAR 951XD $ 15.00/DAY PHYSICAL OCEANOGRAPHIC EQUIPMENT P04 CURRENT METER (UW) $ 15.00/DAY P07 CTD (INTEROCEAN) $ 15.00/DAY BOATS AND RELATED GEAR V02C TRAILER FEE $.20/MILE V03 EASYFISH WINCH WITH BATTERY $ 6.00/DAY V02A BEACHMASTER (WeeLander) $ 70.00/DAY /OPERATOR $ 45.00/HR Thompson WATER SAMPLING EQUIPMENT W03 U OF W BTL, (1.2, 2.5, 5.0, 10.0L, L) $ 3.00/DAY W08 SALINITY BTLS,IODINE(CASE,24 BTLS) $ 3.00/DAY REPLACEMENT: $8.00 EA Analyses of salinity samples $9.50 MISC. ITEMS ZTR TRUCK RENTAL $.65/MILE $ 15.00/USE

12 References Bogucki, D. and C. Garrett A simple model for the shear-induced decay of an internal solitary wave. J. Phys. Oceanogr. 23: Bogucki, D., T. Dickey, L.G. Redekopp Sediment Resuspension and Mixing by Resonantly Generated Internal Solitary Waves. J. Phys. Oceanogr. 27: Bourgault, Daniel and Daniel E. Kelley Wave-induced boundary mixing in a partially mixed estuary. J. Marine Res. 61: Choi, Wooyoung and Roberto Camassa Fully nonlinear internal waves in a two fluid system. J. Fluid Mech. 396: Garrett, Christopher and Walter Munk Internal Waves in the Ocean. Annual Review of Fluid Mechanics. 11: Grue, John, Atle Jensen, Per-Olav Rusas and J. Kristian Sveen Properties of largeamplitude internal waves. J. Fluid Mech. 380: Harris, Jeffrey C Internal Solitary Waves in Port Susan, Puget Sound. unpublished. Knauss, John A Introduction to Physical Oceanography. Prentice-Hall, Inc. Sandstorm, H., J. and A. Elliott Internal tide and solitons on the Scotian Shelf: A nutrient pump at work. J. of Geophysical Research. 89, C10: Turner, J.S Buoyancy Effects in Fluids. University Press. 12