Mid-depth Circulation of the World Ocean: A First Look at the Argo Array

1. Mid-depth Circulation of the World Ocean:A First Look at the Argo Array Josh K. Willis and Lee-Lueng Fu jwillis@caltech.edu, llf@pacific.jpl.nasa.gov Jet Propulsion Laboratory, Pasadena, CA 91109 15 Years of Progress in Radar Altimetry Symposium Venice, Italy March 13-18, 2006

2. Overview • Data • The Argo Array • Satellite sea-surface height • Combining float displacements with altimeter data • Maps of general circulation at 1000m

3. The Argo Array

4. About 166,000 float displacements provide about 4300 float-years of data

5. Position error of O(5 km) implies < 1 cm/s error in subsurface velocity Data Hurdles: surface drift correction Extrapolation of surface and dive positions Technique from Davis et al., JAOT, 1992

6. Data Hurdles: different parking depths For floats that park at depths near 500m, 1500m or 2000m, velocities calculated using geostrophic shear from WOA01 were added to displacements prior to processing.

7. Satellite Sea Surface height data Sea surface height anomaly, Jan 5, 2005 • Mapped AVISO product • 2000-2005 mean removed

8. Is SSH complementary to float displacement data? If so, can we use it to reduce eddy-noise in the estimate of mean circulation?

9. Relating SSH anomalies to subsurface float displacements For eddies in the N. Pacific anomalous velocity decreases with depth Because SSH anomalies reflect surface velocity, they should be scaled-down for comparison with subsurface displacements From Roemmich and Gilson, JPO, 2001

10. Through the geostrophic relation: SSH anomaly implies anomalous geostrophic velocity at the surface. Pseudo-displacement Relating SSH anomalies to subsurface float displacements “Pseudo-displacements” can be calculated by advecting a particle with anomalous geostrophic surface velocity

11. Displacements from a few floats in the N. Pacific Note that slope < 1 From the slope we obtain α: the scale factor btwn. surface and subsurface velocity Relating SSH anomalies to subsurface float displacements “pseudo-displacements” from SSHA geostrophic velocity vs. actual displacements

12. Relating SSH anomalies to float displacements The scale factor for converting surface velocities anomalies to subsurface velocity is found by minimizing displacement variance in 10 x 10 deg. squares

13. Using scale factor and AVISO, we compute anomalous velocity at depth and subtract them from actual float displacements to get improved estimate of mean flow Blue arrows are raw float data Red arrows are ‘corrected’ Contours are cm of 1000/2000m steric height from WOA01 Examples for a few floats in the Southern Ocean

14. To test how well the correction works, consider the RMS variability of float displacements about the 10 x 10 degree mean: Before correction After correction difference

15. Applying the “pseudo-displacement” correction to the float data reduces the variance of float displacements by a factor of 1.5 to 2. Note that most improvement occurs in mid- to high-latitudes.

16. Bin-average of u and v on ~1° x 1° grid Exponential covariance fcn.: C(R) = ( 1 + R + 1/6 R2 – 1/6 R3 ) e–R R depends on f/H, to model topographic steering Mapped relative to WOA01 1000/2000m dynamic height Objective maps: Davis (1998) Lx = 500 km Ly = 300 km SNR: 1 Tiles were used to avoid inversion of large matrices

17. Results: 1000m dynamic height Objectively mapped from float displacements using techniques of Davis, JGR, 1992

18. Results: 1000m velocity Objectively mapped from float displacements using techniques of Davis, JGR, 1998

19. Maps of dynamic height with and without ‘pseudo-displacement’ correction Although very similar, the ‘corrected’ map shows slightly less noise in the S. Pacific subtropical gyre and a shaper gradient across much of the ACC

20. Difference between corrected and uncorrected dynamic height maps show largest differences in eddy-rich regions.

21. From Lavender et al., Nature, 2000 From Davis, JPO, 2005 Future work: can we detect changes in the deep circulation from float data?

22. Summary • Argo is now producing displacements (and profiles) at an unprecedented rate with near global coverage • SSH data is complementary to float displacement data and can be used to reduce error in time-averaged estimates of circulation • Coverage is now good enough to make preliminary maps of 1000m velocity field

23. And now for something completely different… “Recent Cooling of the Upper Ocean”J. Lyman, J. Willis, G. Johnson

24. The Ocean COOLED!!! Globally averaged upper-ocean Heat content from profile data From Lyman et al., Science, submitted

25. Globally averaged upper-ocean Heat content from profile data

26. References Rio and Hernandez, 2004 Rio, M.-H. and F. Hernandez, A mean dynamic topography computed over the world ocean from altimetry, in situ measurements and a geoid model. Journal of Geophysical Research109, C12032, 2004. Roemmich and Gilson, 2001 Roemmich, D., J. Gilson, Eddy Transport of Heat and Thermocline Waters in the North Pacific: A key to understanding interannual/decadal climate variability? Journal of Physical Oceanography, 31, 6757-687, 2001. Davis, 1998 Davis, R.E., Preliminary results from directly measuring middepth circulation in the tropical and South Pacific. Journal of Geophysical Research – Oceans, 103, 24,619-24,634, 1998. Davis et al, 1992 Davis, R.E., D.C. Webb, L.A. Regier, J. Dufour, The autonomous lagrangian circulation explorer (ALACE), Journal of Atmospheric and Oceanic Technology, 9, 264-285, 1992.

27. Comparisons with other velocity estimates Dynamic height at the surface Contours are 10 cm Mapped 1000m dynamic height + 0/1000m dyn. ht. mapped from Argo profile data. Combined mean dynamic topography from Rio and Hernandez, JGR, 2004