Mid-depth Circulation of Mid-depth Circulation of the World Ocean: the World Ocean: A First Look at the Argo A First Look at the Argo Array Array Josh K. Willis and Lee- Josh K. Willis and Lee- Lueng Fu Lueng Fu [email protected][email protected], , llf@pacific.jpl.nasa.gov llf@pacific.jpl.nasa.gov Jet Propulsion Laboratory, Pasadena, CA 91109 Jet Propulsion Laboratory, Pasadena, CA 91109 2005 AGU fall meeting 2005 AGU fall meeting San Francisco, CA San Francisco, CA Dec. 5-9, 2005 Dec. 5-9, 2005
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Mid-depth Circulation of the World Ocean: A First Look at the Argo Array Josh K. Willis and Lee-Lueng Fu [email protected], [email protected].
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Mid-depth Circulation of the Mid-depth Circulation of the World Ocean:World Ocean:
A First Look at the Argo A First Look at the Argo ArrayArray
Josh K. Willis and Lee-Lueng Josh K. Willis and Lee-Lueng FuFu
[email protected]@caltech.edu, , [email protected]@pacific.jpl.nasa.govJet Propulsion Laboratory, Pasadena, CA 91109Jet Propulsion Laboratory, Pasadena, CA 911092005 AGU fall meeting2005 AGU fall meeting
San Francisco, CASan Francisco, CADec. 5-9, 2005Dec. 5-9, 2005
II.II. Combining float displacements with Combining float displacements with altimeter dataaltimeter data
III.III. Maps of general circulation at Maps of general circulation at 1000m1000m
The Argo ArrayThe 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 Position error of O(5 km) implies < 1 cm/s error in subsurface < 1 cm/s error in subsurface velocityvelocity
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.
Is SSH complementary to Is SSH complementary to float displacement data?float displacement data?
If so, can we use it to If so, can we use it to reduce eddy-noise in the reduce eddy-noise in the
estimate of mean estimate of mean circulation?circulation?
Relating SSH anomalies to subsurface float displacements
From From Roemmich and Gilson, Roemmich and Gilson, JPOJPO, , 20012001
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.
,y
gfu
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 dataRed 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:
Befo
re c
orr
ecti
on
Aft
er
corr
ecti
on
diff
ere
nce
22 )()( dydydxdx
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 Objective maps: Davis (1998)(1998)
• Bin-average of u and v on ~1° x 1° gridBin-average of u and v on ~1° x 1° grid
C(R) = ( 1 + R + 1/6 RC(R) = ( 1 + R + 1/6 R22 – 1/6 R – 1/6 R33 ) ) ee–R–R
• R depends on f/H, to model topographic steeringR depends on f/H, to model topographic steering
• Mapped relative to WOA01 1000/2000m dynamic Mapped relative to WOA01 1000/2000m dynamic heightheight
Tiles were used to avoid inversion of large matrices
Lx = 500 kmLy = 300 kmSNR: 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 Future work: can we detect changes in the deep circulation changes in the deep circulation
from float data?from float data?
From Davis, JPO, 2005
From Lavender et al., Nature, 2000
SummarySummary• Argo is now producing displacements Argo is now producing displacements
(and profiles) at an unprecedented (and profiles) at an unprecedented rate with near global coveragerate with near global coverage
• SSH data is complementary to float SSH data is complementary to float displacement data and can be used to displacement data and can be used to reduce error in time-averaged reduce error in time-averaged estimates of circulationestimates of circulation
• Coverage is now good enough to make Coverage is now good enough to make preliminary maps of 1000m velocity preliminary maps of 1000m velocity fieldfield
And now for something And now for something completely different…completely different…
““Recent Cooling of the Recent Cooling of the Upper Ocean”Upper Ocean”
J. Lyman, J. Willis, G. JohnsonJ. Lyman, J. Willis, G. Johnson
Globally averaged upper-ocean Globally averaged upper-ocean Heat content from profile dataHeat content from profile data
From Lyman et al., Science, submitted
The Ocean COOLED!!!
Globally averaged upper-ocean Globally averaged upper-ocean Heat content from profile dataHeat content from profile data
ReferencesReferences
Rio and Hernandez, 2004Rio and Hernandez, 2004 Rio, M.-H. and F. Hernandez, A mean Rio, M.-H. and F. Hernandez, A mean dynamic topography computed over the world ocean from dynamic topography computed over the world ocean from altimetry, in situ measurements and a geoid model. altimetry, in situ measurements and a geoid model. Journal of Journal of Geophysical ResearchGeophysical Research 109109, C12032, 2004., C12032, 2004.
Roemmich and Gilson, 2001Roemmich and Gilson, 2001 Roemmich, D., J. Gilson, Eddy Roemmich, D., J. Gilson, Eddy Transport of Heat and Thermocline Waters in the North Transport of Heat and Thermocline Waters in the North Pacific: A key to understanding interannual/decadal climate Pacific: A key to understanding interannual/decadal climate variability? variability? Journal of Physical OceanographyJournal of Physical Oceanography, , 3131, 6757-687, , 6757-687, 2001.2001.
Davis, 1998Davis, 1998 Davis, R.E., Preliminary results from directly Davis, R.E., Preliminary results from directly measuring middepth circulation in the tropical and South measuring middepth circulation in the tropical and South Pacific. Pacific. Journal of Geophysical Research – OceansJournal of Geophysical Research – Oceans, , 103103, , 24,619-24,634, 1998. 24,619-24,634, 1998.
Davis et al, 1992Davis et al, 1992 Davis, R.E., D.C. Webb, L.A. Regier, J. Dufour, Davis, R.E., D.C. Webb, L.A. Regier, J. Dufour, The autonomous lagrangian circulation explorer (ALACE), The autonomous lagrangian circulation explorer (ALACE), Journal of Atmospheric and Oceanic TechnologyJournal of Atmospheric and Oceanic Technology, , 99, 264-285, , 264-285, 1992. 1992.
Comparisons with other Comparisons with other velocity estimatesvelocity estimates