1. The California Current System: Comparison of Geostrophic Currents, ADCP Currents and Satellite Altimetry
LCDR David Neander, NOAA
OC3570, Summer 2001

2. Station Locations and Data Collection
CalCOFI lines 67, ”70s”, 77; 35 CTD stations
Casts to 1000 dbar or “near bottom”, except station 26 (3965 dbar)
Processed in Matlab using CSIRO Seawater library
ADCP data acquired continuously; processed as N-S, E-W components and rotated 30o

3. Satellite Altimetry Four distinct features 2 “lows” (ccw flow)
2 “highs” (cw flow)
Line 67: weak poleward flow and offshore flow near coast
70s Line: weak poleward flow along track
Line 77: poleward and equatorward flow
Source: CCAR website

4. T-S Diagrams
Line 67
70s Line
Line 77
Line 67: cool/fresh surface waters, warm/saline below ~100 dbar; distinct water masses ~ dbar – CC and CUC
Line 70s: variation in upper 150 dbar; well mixed below
Line 77: upper level mixing; distinct water masses ~ dbar – CC and CUC

5. CalCOFI Line 67
Low S core offshore – CC equatorward transport of subarctic waters
High S, T nearshore – CUC; upwelling also evident
Low T, high S – higher density, lower sea surface height
Strong horizontal T gradients – rapid change in density

6. CalCOFI Line 67 - Currents
Core of CUC clearly visible
Computed geostrophic velocities show greater variations in poleward/equatorward flow – related to strength of density gradients

7. CalCOFI 70s Line
Downwelling of surface waters – decrease in S and density, higher SSH
Upwelling of deeper waters – increase in S and density, lower SSH
Low S cores offshore; beginning of CC transition zone?
Transition zone of CC; mesoscale eddies and energetic meanders

8. CalCOFI 70s Line
Geostrophic; alternating flow across track, greater variation
Little correlation with ADCP velocities
Offshore variations attributed to transition zone of CC

9. CalCOFI Line 77 Low S core offshore, upper 100 dbar; subarctic waters
High T, S along slope: CUC
Upwelling, possible internal waves; complex bathymetry

10. CalCOFI Line 77 General correlation in location of CUC
Variations in poleward/equatorward flow
Complex topography – upwelling events, internal waves

11. Calculated Velocities vs. Altimetry Data
Line 67: Upwelling, offshore flow near coast; poleward flow at offshore end; CUC not evident in imagery
Line 70s: poleward flow at north end (ADCP) ; variations in E-W flow at south end
Line 77: Poleward flow – CUC; equatorward flow nearshore not observed

12. Volume Transport Computation
Principle of conservation of volume
Neglect compressibility
Volume transport “in” and “out” conserved
VT = L ∫ (ν – νr) dz
L = distance between stations
(ν – νr) = geostrophic velocity relative to ref level
dz = depth limits

13. Volume Transport Computation
North transport on line 77 (“into box”): Sv
North transport on line 67 (“out of box”): Sv
West transport on 70s line (“out of box”): Sv
Difference “into and out of” CenCal Box: Sv
Remaining 0.17 Sv?
Volume conservation suggests this flows out elsewhere…

14. Conclusions
General agreement: geostrophic, ADCP currents and satellite altimetry – all revealed CUC
High SSH reflected in ADCP and geostrophic currents; weak SSH gradients poorly correlated
Significant variation in geostrophic currents – tend to reflect changes in density gradients
Changes in bathymetry result in enhanced mixing due to variations in upwelling and formation of internal waves
CC not identified: Complex structure; core ~ km offshore; transition zone consists of mesoscale eddies, filaments and energetic meanders