The California Current System: Comparison of Geostrophic Currents, ADCP Currents and Satellite Altim

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

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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
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T-S Diagrams
Line 67
Line 77
70s Line

• Line 67 cool/fresh surface waters, warm/saline
  below 100 dbar distinct water masses 100-500
  dbar CC and CUC
• Line 70s variation in upper 150 dbar well
  mixed below
• Line 77 upper level mixing distinct water
  masses 100-500 dbar CC and CUC

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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

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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

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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

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CalCOFI 70s Line

• Geostrophic alternating flow across track,
  greater variation
• Little correlation with ADCP velocities
• Offshore variations attributed to transition zone
  of CC

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CalCOFI Line 77

• Low S core offshore, upper 100 dbar subarctic
  waters
• High T, S along slope CUC
• Upwelling, possible internal waves complex
  bathymetry

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CalCOFI Line 77

• General correlation in location of CUC
• Variations in poleward/equatorward flow
• Complex topography upwelling events, internal
  waves

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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

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Volume Transport Computation

• Principle of conservation of volume
• Neglect compressibility
• Volume transport in and out conserved

VT L ? (? ?r) dz L distance between
stations (? ?r) geostrophic velocity relative
to ref level dz depth limits
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Volume Transport Computation

• North transport on line 77 (into box)
  1.81 Sv
• North transport on line 67 (out of box) -
  1.35 Sv
• West transport on 70s line (out of box) -
  0.29 Sv
• Difference into and out of CenCal Box
  0.17 Sv
• Remaining 0.17 Sv?
• Volume conservation suggests this flows out
  elsewhere

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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
  300-400 km offshore transition zone consists
  of mesoscale eddies, filaments and energetic
  meanders