Mid-depth Circulation of the World Ocean: A First Look at the Argo Array Josh K. Willis and Lee-Lueng Fu Jet Propulsion Laboratory, Pasadena, CA AGU fall meeting San Francisco, CA Dec. 5-9, 2005

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

The Argo Array

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

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

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.

Satellite Sea Surface height data Mapped AVISO product mean removed Sea surface height anomaly, Jan 5, 2005

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

Relating SSH anomalies to subsurface float displacements From Roemmich and Gilson, JPO, 2001 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

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

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

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

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 Examples for a few floats in the Southern Ocean Blue arrows are raw float data Red arrows are corrected Contours are cm of 1000/2000m steric height from WOA01

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

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.

Objective maps: Davis (1998) Bin-average of u and v on ~1° x 1° grid Bin-average of u and v on ~1° x 1° grid Exponential covariance fcn.: Exponential covariance fcn.: C(R) = ( 1 + R + 1/6 R 2 – 1/6 R 3 ) e –R R depends on f/H, to model topographic steering R depends on f/H, to model topographic steering Mapped relative to WOA /2000m dynamic height Mapped relative to WOA /2000m dynamic height Tiles were used to avoid inversion of large matrices L x = 500 km L y = 300 km SNR: 1

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

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

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

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

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

Summary Argo is now producing displacements (and profiles) at an unprecedented rate with near global coverage 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 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 Coverage is now good enough to make preliminary maps of 1000m velocity field

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

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

Globally averaged upper-ocean Heat content from profile data

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 Research 109, C12032, 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, , 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, 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, , 1992.

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