1. Ekman Divergence From Shipboard Wind Measurements
Tracey Delk, LT
16 Mar 2004

2. Outline
Objective
Background
Data
Methods
Results

3. Objective
Calculate Ekman Volume Transport within the area encompassed by the track for Leg 1 from R/V Point Sur wind measurements.

4. Background: Ekman Transport
Ekman transport is a wind driven circulation
Momentum from winds transferred to ocean by friction
Yields a relationship between wind stress and depth-integrated flow

5. Background: Ekman Transport
Wind-driven component of transport in the Ekman layer is directed perpendicular to the mean wind stress
Function of balance of Coriolis and friction
To the right (left) in the Northern (Southern) Hemisphere
Wind Stress
Ekman Transport

6. Background: Ekman Transport
Flow in Ekman layer transports mass
Ekman Transport (ME)
Integral of Ekman velocity from surface to Ekman depth (kg/m/s)
Volume Transport (Q)
ME divided by water density and multiplied by the width perpendicular to the transport

7. Background: Ekman Transport
Assumptions
Steady-state flow
Mass-transport is only a function of wind-stress at the surface and Coriolis

8. Data 20 s wind speed and direction measurements
From port and starboard anemometers
16.8 m above the sea surface

9. Methods Vector-averaged port and starboard measurements
Filtered wind data 2 ways
Decimate
Resamples data after low-pass filtering
Resampled to hourly averages
Smoothed
Filtfilt function with Hanning window
Zero-phase forward and backward filtering
Sampled value for every hour

10. U and V Components

11. Methods Calculated wind stress at surface
Used Dr. Garwood’s MATLAB routine for calculating fluxes (Large and Pond model JPO 1992)
Required other variables in addition to winds
Barometric pressure
Humidity
Air and Sea Temp

12. Averaged Winds Along Track

13. Methods Calculated Ekman Transport components
MEy = -τx/f
MEx = τy/f
Rotated to a local coordinate system in order to calculate transport relative to the box
X-direction parallel to northern and southern boundaries
Y-direction parallel to western boundary

14. Methods
Calculated Volume Transport (Q) based on the following sign conventions
+ transport into the box (CONV)
- transport out of the box (DIV)

15. Results Positive Transport A: southward transportB C
Positive Transport
A: southward transport
B: eastward transport
C: northward transport y’ x’

16. Results

17. Results

18. Results

19. Results Decimated value Smoothed value 0.4046 Sv (106 m3 /s)

20. References
Pond, S. and G. L. Pickard (1983) Introductory Dynamical Oceanography 2nd Ed., Butterworth-Heinemen.
Tomczak, M. and J. S. Godfrey (2003) Regional Oceanograpy: an Introduction 2nd Ed., Pergamon Press.
Stewart, R. H. Introduction to Physical Oceanography (2003), Open Source Text Book.