1. LCDR John Hendrickson 17SEP2008
An Investigation into a Sea Breeze Induced Cross-Shore Flow on Monterey Bay’s Inner Shelf
LCDR John Hendrickson
17SEP2008

2. Abstract
Diurnal Cross-Shore Exchange on the Inner-Shelf in Southern Monterey Bay, CA
John Hendrickson, Jamie MacMahan, Ed Thornton, Mike Cook, Tim Stanton, Ad Reniers
The effects of a strong diurnal sea-breeze on the cross-shore exchange on the inner shelf is investigated by comparing wind stress estimates and ocean currents over the vertical at three locations in southern Monterey Bay, CA . Cross-shore exchange on the inner shelf significantly impacts the ecosystem by transporting heat, nutrients, pollutants and phytoplankton between the inner-shelf and surf zone. Spectral analysis of surface winds at three coastal locations within the bay indicates a significant diurnal wind component. The observed subaqueous velocity profiles and pressure time series are measured by bottom mounted 1200-kHz Broad-band Acoustic Doppler Current Profilers (ADCPs) deployed at three separate alongshore locations in ~13 m water depth. The velocity and pressure signals were collected continuously at 1 Hz for all three locations for over 2 years. The cross-shore wind stress is significantly correlated to the cross-shore subaqueous velocity with onshore flow near the surface and offshore flow near the bottom. Cross-rotary spectral analysis is used to describe the rotational coherence and phase over the vertical with respect to the wind stress. It is further hypothesized that normally-incident sea-swell waves ( Hz) will modify net cross-shore transport. Cross-shore transport is evaluated for conditions that are dominated by either waves or cross-shore wind stress. Results indicate that when waves are small, the cross-shore wind stress associated with the diurnal sea-breeze is the primary forcing mechanism for cross-shore exchange on the inner-shelf.

3. Purpose of Experiment
Provide a better understanding of the cross-shore exchange and distributions of heat, phytoplankton, nutrients and pollutants between the mid-shelf and surf zone

4. Intro
formation and offshore displacement of the upwelling or downwelling front partitions the shelf into dynamically distinct regions with different stratification
full Ekman transport develops in the region offshore of the front because the strong stratification of the pycnocline acts to insulate the surface and bottom layers from each other
Onshore of the front, the stratification is relatively weak and the surface and bottom Ekman layers typically interact, causing a significant reduction in the net crossshelf transport

5. Data Collection
3 year long time series of winds, waves and water velocity profiles
3 locations just outside the surf zone:
Marina
Sand City
Del Monte Beach

6. Field Sites

7. Data Collection Bottom mounted ADCPs Approximately 12-14 m water depth
Sampling frequency is 1 Hz
Number of Bins: 28
Bin size: 0.5 m

8. How Does the ADCP work? Broadband ADCP
4 Beam Transducer w/ 20 degree beam angle
Doppler effect
“pings” at kHz
Backscatters
Doppler shift
Assumes currents are homogeneous/averaging/ trigonometry = u, v, w velocity components
Repetitive Sampling & “Gating” = “Profile” of the water column over a range of depths

9. Raw Data, Now What?
First we start with de-spiking and looking for bad data. This is not as much fun as it sounds like.
I then run a 600+ line m-file which I like to call “Workhorse”. It computes u, v, w, Hsig, different dt’s and much more!
We are getting close, but I can’t forget about the wind data I have.

10. More Data Crunching Normalize the ADCP data (z/h) Tides Tidal Analysis
Tidal Constituents ( S1 vs. K1)
Tidal Prediction

11. Coordinate System
Principal Component Analysis
Scatter Plots

12. Measured Surface Current Ellipses
(July 2003 – June 2004)
K1 (23.93 hrs)
S1 (24 hrs)
S1 currents are about 4 times the strength of the K1 currents
Rosenfeld et al. 2008

13. Rosenfeld et al. 2008

14. Spectral Analysis – Sand City

15. Spectral Analysis – Sand City

16. Spectral Analysis – Marina

17. Spectral Analysis – Marina

18. Coherence - Marina
gives a measure of the dependence of two random variables

19. Conclusions
Where to go from here…