The California Current System Comparison of Geostrophic Currents

The California Current System: Comparison of Geostrophic Currents, ADCP Currents and Satellite Altimetry LCDR David Neander, NOAA OC 3570, Summer 2001

Station Locations and Data Collection • Cal. COFI lines 67, ” 70 s”, 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 30 o

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

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

Cal. COFI 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

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

Cal. COFI 70 s 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

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

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

Cal. COFI 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 70 s: 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 70 s line (“out of box”): - 0. 29 Sv Difference “into and out of” Cen. Cal 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