1. GEOSTROPHIC, EKMAN & ADCP VOLUME TRANSPORT
Objective:
Compute geostrophic, Ekman and ADCP volume transport
Verify whether Ekman covergence transport is balanced by geostrophic convergence
Estimate the net velocity profile (sfc to –20 m)
CalCOFI
Line 67
Line 70
CalCOFI
Line 77

2. REFERENCES
The Sea, Volume 11 – edited by Allan R. Robinson and Kenneth H. Brink
Regional Oceanography: an Introduction - Matthias Tomczak & J. Stuart Godfrey at Pergamon, New York, 1994
Introductory dynamical oceanography - Butterworth, Oxford, “Pond, S. and Pickard, G. L., 1983.
Mean structure of the inshore countercurrent and California undercurrent off Point Sur, CA. – C.A. Collins, N Garfield, T.A. Rago, F.W. Rischmiller, E. Carter
The nature of the cold filaments in the California current system – P. Ted Strub, P. Michael Kosro and Adriana Huyer

3. SCOPE Data processing & Analysis Methods & Results: Upwelling
Geostrophic Transport
Ekman Transport
Comparison with ADCP transport
Estimated net velocity (surface to –20 m)
Conclusion
Upwelling

4. DATA PROCESSING & ANALYSIS
Raw data analysis
- wind speed & direction
position & time
relative humidity
sea surface & air temp
CTD and ADCP data
Time-series plotted
Windward side of the anemometer readings used
Another approach was to use starboard anemometer readings
CalCOFI
Line 67
Line 70
CalCOFI
Line 77

5. WIND IS RELATIVELY CONSISTENT
WIND IS RELATIVELY CONSISTENT
Line 67
Line 70
Line 77
Max starboard wind and corresponding direction (knot / )
26.4 /
333.8
22.7 /
319.3
27.7 /
339.4
Max port wind and corresponding direction (knot / )
27.5 /
333.7
22.9 /
319.2
26.9/
318.0
Min starboard wind and corresponding direction (knot / )
3.4 /
223.7
2.0 /
66.7
0.6 /
173.0
Min Port wind and corresponding direction
(knot / )
3.6 /
51.0
2.2 /
45.7
0.6/
140.6
Percentage of samples with wind speed (%wd spd) < 10 kts 8%
0.6%
11%
14 <= %wd spd <= 20 kts
51%
71%
53%
%wd spd > 20 kts
15% 2%
28%

6. Time series of port wind speed Time series of starboard wind speed
TIME SERIES OF DATA
Time series of port wind speed
Time series of starboard wind speed
kts
kts
Vessel sheltered by mountains from station 34 to 35 along line 77

7. AUTOCORRELATION FUNCTION OF DATA
Length scale ~ 5 hrs (Zero-crossing method)
2-hourly averaged was chosen
More than 10 data points for each line – No Aliasing

8. GEOSTROPHIC VOLUME TPT COMPUTATION
Geostrophic velocities computed from D =
Reference level for D
Line 67 - computed down to 1000m depth except 1st & 2nd stations
Along line 70 - computed down to 1000m
Along line 77 - numerous variations in cast depth from station 26 to station 35
CalCOFI
Line 67
Line 70
CalCOFI
Line 77

9. GEOSTROPHIC VOLUME TPT THROUGH COASTAL BOXSv Sv Sv
Vol Transport into the box (+)
Vol Transport out of the box (-)
Net Geostrophic Volume transport: Sv

10. GESTROPHIC VOL TRANSPORT PROFILE BETW STATION 2 & 10
Local max value at a depth of about 200 m
Flows Northward along upper continental slope with its core at a depth of about 200 m (Hickey, 1979)
This feature can be seen in other plots of volume transport profile
Core of the California undercurrent

11. EKMAN VOLUME TRANSPORT COMPUTATION
Difficult to measure the perturbed motion (u’ and v’)
Solve for surface friction velocity based on drag coefficient and 2
Derived Wind Stress was resolved along 150 & 060

12. EKMAN VOLUME TRANSPORTSv Sv Sv
:Taux along CalCOFI line 67 & 77
:Tauy along CalCOFI line 70
(+): Vol Transport into the box
(-) : Vol Transport out of the box
CalCOFI
Line 67
2-hourly average was taken & multiplied to the distance traveled in each two-hourly segment
Line 70
CalCOFI
Line 77
Net Ekman Vol transport : Sv

13. Sum of Geostrophic & Ekman volume transport
CalCOFI
Line
Net volume into the box (Sv)
Net volume out of the box (Sv)
Net total (Sv) 67
(E)
(G) 70
(G)
(E) 77
(G)
(E)
Total
Net ADCP Vol Transport = Sv
Difference could be due to inertial & tidal currents

14. CONCLUSION FOR EKMAN AND GEOSTROPHIC TRANSPORT
Ekman divergence transport does not seems to balanced by geostrophic convergence
- Net Geostrophic Vol transport: Sv
- Net Ekman Vol transport: Sv
Plausible explanation:
Geostrophic volume transport does not calculate to the full water depth
Time-lagged in the data collection
Inertial and tidal current

15. ESTIMATED NET VELOCITY (SURFACE TO –20 M DEPTH)
De = 4.3*W/(sin||)½ (W:m/s) (1) Vo = **1.8*10-3*W2/(De*1025*|f|)
(2)
ue = Vo*COS(/4 + *z/De)*exp(*z/De) ve = Vo*SIN(/4 + *z/De)*exp(*z/De)
Vel Normal to line 67

16. ESTIMATE NET VELOCITY (SURFACE TO –20 M DEPTH)
Along line 70 – Ekman Velocity dominate
Vel Normal to line 70

17. Ekman velocity is weak through line 67 and 77
CONCLUSION
Ekman velocity is weak through line 67 and 77
- Wind is predominantly from NW
- Steepest isopycnal lines is cross-shore
Sfc velocity profile normal to 67 & 77 similar to geo velocity
Vel Normal to line 67
Vel Normal to line 70