“ Combining Ocean Velocity Observations and Altimeter Data for OGCM Verification ”

1. “Combining Ocean Velocity Observations and Altimeter Data for OGCM Verification” Peter Niiler Scripps Institution of Oceanography with original material from N. Maximenko, M.-H.Rio, L. Centurioni, C. Ohlmann, B. Cornuelle, V. Zlotnicki,, D.-K. Lee

2. Method of Calculating Ocean Surface Circulation Combines Drifter and Satellite Observations Between 1/1/88 and 12/1/06 1988 10,561 drifters drogued to 15m depth were released in the global ocean, with array of 1250 since 9/18/05

3. Satellite Observationssea level = altimeter height - geoid height • Altimeters: GEOS, T/P, JASIN, ERS I&II • Data from 1992 - Present (rms noise: +/- 4cm relative to geoid) • GRACE’04: Estimated accuracy of geoid: +/-3 cm at 400 km horizontal scale • Sea level gradient, or geostrophic velocity, depends upon method and scale of averaging, or mapping, of sea level data

4. Drifter velocity observations are accurate (+/- 0.015 m/sec daily averages), but spatial distribution of data can result in biased averages in space and time

5. Altimeter data is used to calculate geostrophic velocity with “smoothing” scales (and amplitude correction) consistent with drifter data: e.g. AVISO * * • • • • • • • • N/S ; *E/W AVISO Correlation Scales • • B. Cornuelle

6. Drifter observed rms velocity variance [<u’2>+<v’2>]1/2 N.Maximenko

7. Log10 (Eddy Energy/ Mean Energy)1/2 N. Maximenko

8. The” simple method” of obtaining a velocity map

9. Vector Correlation between drifter and altimeter derived AVIO geostrophic velocity anomalies N.Maximenko

10. “East Sea”: 3 day average velocity from “simple method” vs drifter obs. 8/01-11/03 D.-K. Lee

11. Comparison drifter and ECCO 15m zonal velocity components in tropical Pacific B. Cornuelle

12. Vector correlation and scatter plots of “geostrophic” velocity residuals from drifters and AVISO in California Current L.Centurioni

13. C. Ohlmann

14. Unbiased drifter and satellite derived geostrophic 15m velocity (on left) and ROMS 5km resolution sea level (right) L.Centurioni and C. Ohlmann

15. Geostrophic zonal velocity from drifter and altimeter data L. Centurioni

16. Decadal MEAN SEA LEVEL (cm) in models of the California Current C. Ohlmann

17. HYCO POP NLM OGCM (Eddy Energy)1/2: California Current ROMS

18. Geostrophic EKE0.5 ROMS (left) corrected AVISO (right) (0-20 cm s-1) L. Centurioni and C. Ohlmann

19. Ageostrohic 15m velocity and MSL in 5km resolution ROMS of California Current C. Ohlmann

20. THE GLOBAL SOLUTIONS1. Time mean surface momentum balance for surface sea level gradient: • Observed drifter = “D” • Computed Ekman = “E”

21. 2. Compute sea level that minimized the global cost function in least square The solution is also minimized relative to parameters of Ekman force and GRACE altimeter referenced sea level, Go, is averaged on 1000km scales. Maximenko-Niiler

22. 3. Perform an objective mapping of sea level, with mesoscale based, geostrophic, correlation functions, as a linear combination of: • Levitus 1500m relative steric level, • GRACE referenced altimeter derived sea level• Drifter geostrophic velocity.RIO(05), Knudsen-Andersen

23. 1992-2002 Mean Sea Level: Maximenko (05)

24. Zonal, unbiased geostrophic velocity (-10,+10 cm/sec)

25. 1993-1999 Mean Sea Level: RIO (05) M.-H. Rio

26. Difference between Maximenko(‘05)-Rio(‘05) MSL with both data adjusted to 1993-1999 period M.-H. Rio : RMS difference of 5cm

27. Comparison of 15m velocity from SURCOLF and MERCATOR near real time maps of Gulf Stream region with drifter data M.-H. Rio

28. Mean Sea Level: Knudsen-Anderson V. Zlotnicki

29. ECCO-2 CUBE49 (18km horizontal, global assimilation with flux and diff.par.optim.) V. Zlotnicki

30. ECCO-2 cube 37 and 49 east velocity difference from Maximenko (05) and Knudsen-Anderson V. Zlotnicki

31. CONCLUSIONS • Combined drifter and altimeter derived velocity anomalies can be used to make regional, realistic, near real time maps of 15m ocean circulation. • Global, absolute sea level on 50km scale from combined data displays new circulation features. • OGCM solutions are most stringently tested with velocity fields derived from combined drifter and altimeter observations.

32. “Why does our view of ocean circulation always have such a dreamlike quality…”...Henry Stommel THE DREAM HAS COME TRUE… we are observing the circulation peter niiler

33. East-west average vorticity balance at 15m depth (black line is from Ekman’s 1906 model, shaded is drifter data; 100 km coastal and western boundary currents excluded)

34. The eddy transport of vorticity The eddy transport vector of vorticity is computed around the Gulf Stream eddy energy maximum.

35. North Atlantic: 0.25º resolution sea level (upper) and “simple” geostrophic velocity (lower).

36. The 1992-2000 time average quasi-geostrophic eddy vorticity flux vector in the Gulf Stream region.

37. The mean kinetic energy at 15m depth from drifters. This quantity graphed is (<v’•v’>/2g) and represents the sea level change caused by Bernoulli effect of ocean time variable eddies.

38. SST convergence (x10-7Cºsec-1) at 15m depth:

39. 1978-2003 Average drifter velocity with QSCT/NCEP blended wind-stress divergence

40. Conservation of vorticity in the Agulhas Extension Current

41. Drifter geostrophic velocity compared with ocean circulation model sea-level in California Current in POP (left) and UCLA/ROMS (right)

42. Global streamlines of 1992-2002 average 15m depth velocity

43. Zonal, unbiased geostrophic velocity (-40,+40cm/sec)