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Permanent Meanders in California Current: Near-Surface Observations and OGCM Comparison

Explore permanent meanders in the California Current System through near-surface observations and compare with OGCM solutions by Luca Centurioni. Collaborators Peter Niiler, Carter Ohlmann. Acknowledgments to Harley Hurlburt, Julie McClean, Jim McWilliams, Ruth Preller. Summary, bias problems, observation-OBCM comparison, conclusions included. Data analysis methods detailed, showing correlations and scatter plots of velocity fields. Explore model estimates, geostrophic velocities, and Ekman currents. Evaluate ROMS model realism. Investigate zonal flows, mass transport, and drifter acceleration calculations.

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Permanent Meanders in California Current: Near-Surface Observations and OGCM Comparison

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  1. Permanent Meanders in the California Current System and Comparison of Near-Surface Observations with OGCM Solutions Luca Centurioni (SIO-PORD) Collaborators: Peter Niiler, Carter Ohlmann Acknowledgments (PI):Harley Hurlburt (NLOM), Julie McClean (POP), Jim McWilliams (ROMS), Ruth Preller (HYCOM)

  2. Outline • Summary of observations from 15 depth drifters data; • the bias problem: best estimate of 15 m depth geostrophic velocity field; • comparison of some observation-derived quantities with OGCM solutions; • Conclusions.

  3. Number of 6-hrs interval observations in a 0.5º x 0.5º bin.

  4. MEAN VELOCITY FIELD At 15 m DEPTH

  5. FROM mean field at 15 m

  6. Momentum balance (mean) at 15 m depth (dissipation is ignored):

  7. 78% 80%

  8. Vector correlation and scatter plots of “geostrophic” velocity residuals from drifters and AVISO

  9. UNBIASED FIELD (VC): • (Niiler et al. 2003) • A running average (30 hrs) filter is applied to Lagrangian time series • Ekman currents (Ralph & Niiler 1999) are removed to compute geostrophic velocities from drifters; • Drifter geostrophic velocities (VDG) are binned in time (7 days) within each cell (0.5ºX0.5º) and anomalies are computed; • Geostrophic velocities anomalies from AVISO (VS) gridded maps are computed and interpolated at drifter locations and (binned) times; • Assume the following model: VG(ti;x)=A(x)VS(ti;x)+VC(x) • Estimate A and VC by minimizing {{(VG-VDG)2}} where {{}} denotes time average over concurrent drifter and satellite velocity data, i.e

  10. Slope of the linear model VG(t,x,y)=A(x,y)VS(t,x,y)+VC(x,y) A

  11. UNBIASED GEOSTROPHIC VELOCITY VECTOR FIELD AT 15 m DEPTH

  12. Unbiased geostrophic velocity field: zonal component (cm s-1)

  13. MEAN GEOSTROPHIC EKE0.5 FROM CORRECTED ALTIMETRY cm s-1

  14. MEAN SEA LEVEL (cm) HYCOM NLOM POP ROMS

  15. EKE0.5 FROM NUMERICAL MODELS (0-20 cm s-1) HYCOM POP NLOM ROMS

  16. EKE0.5 COMPARISON (0-20 cm s-1) ROMS FROM CORRECTED ALTIMETRY

  17. Conclusions1)Data confirm that the CCS (during the last 10 years and in the area examined) had 4 permanent meanders which are co-located with jets of zonal flow that extend nearly to Hawaii;2)Time biases from the drifter data can be removed with the aid of satellite altimetry; Comparison of observed quantities with OGCM outputs can be addressed;3)Preliminary comparisons show that ROMS is likely to be the model with the highest degree of realism;

  18. Number of 6-hrs interval observations in a 0.5º x 0.5º bin.

  19. Ageostrophic, non-linear velocity in ROMS and simple GFD model of cold eddy interacting with wind (Lee et al 1998)

  20. s-1

  21. s-1

  22. FOLLOWING THE DRIFTERS

  23. Ekman force ( ) is determined from (Ralph&Niiler 1999)

  24. cm s-1 MEAN EKE0.5 at 15 m DEPTH (from drifters)

  25. EKMAN CURRENT AT 15m DEPTH

  26. Can we explain the jets of zonal flow?

  27. Suppose that: And use the following barotropic model to compute the stream function of volume transport per unit depth: Acceleration of a drifter:

  28. From AVISO and Unb. Vel. Field. From drifters

  29. of zonal volume transport per unit depth from barotropic model. cm s-1

  30. Consider a one layer ocean of depth D=D0+D’ with a wind stress t acting over it: The stream function of the mass transport can be computed as:

  31. Absolute sea level 27 Oct, 1993 with drifter tracks that are 21 days long

  32. Acceleration of a drifter: (horizontal velocity)

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