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Luca Centurioni (SIO-PORD)

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.

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Luca Centurioni (SIO-PORD)

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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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