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

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

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

T-S Diagrams Line 67 70s Line Line 77 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

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

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

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

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

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

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

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

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

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…

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