Luca Centurioni (SIO-PORD)

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

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

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

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

19. 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;

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

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

22. s-1

23. s-1

24. FOLLOWING THE DRIFTERS

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

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

27. EKMAN CURRENT AT 15m DEPTH

28. Can we explain the jets of zonal flow?

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

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

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

32. 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:

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

35. Acceleration of a drifter: (horizontal velocity)