How satellites have improved our knowledge of planetary waves in the ocean

1. How satellites have improved our knowledgeof planetary waves in the ocean Paolo Cipollini National Oceanography Centre, Southampton cipo@noc.soton.ac.uk Schematic of a 1st mode Baroclinic planetary wave Overview What we knew before altimetry Altimeter-based observations Improvements in the theory Present issues: waves vs eddies, meridional propagation, wave effects on biology,…. and more!

2. Acknowledgments • My coauthors: • Peter Challenor, David Cromwell, Ian Robinson, Graham Quartly • Provided invaluable theoretical support: • Peter Killworth & Jeff Blundell • Contributed material: • Dudley Chelton • Lee-Lueng Fu • Neil Holbrook • Angela Maharaj • Michael Schlax 34°N Longitude/time plot of TOPEX/Poseidon SSHA showing westward-propagating planetary waves

3. What we knew before altimetry • Nicely summed up by Pedlosky (1979), Gill (1982) • Dispersion characteristics, max f, max latitude… • Planetary waves dominate oceanic low-frequency variability • Responsible for east-west asymmetric response to the wind • Spin-up of the ocean: Anderson & Gill (1975) • Spin-up with topography: Anderson & Killworth (1979) • Theory of Equatorial Planetary waves (70s/80s) • (equatorial PWs not covered in this talk) • Very scarce observational evidence (despite valiant efforts) • Accepted description was the classic theory developed by Hough (1897) + Rossby (1939) • BUT… a hint of ‘fast’ waves Dispersion characteristics from Gill, 1982

4. observed cp classic theory cp Westward phase speed cp cm/s In situ efforts • Solid line: speed from classic theory plots courtesy of D.Chelton & M.Schlax

5. observed cp classic theory cp Westward phase speed cp cm/s Geosat 1985-1989 • Several studies • None global • Two main problems: • Orbit errors • Tidal aliasing M2 -> 317 days, 8 deg wavelength • Hints of Barotropic waves • Gaspar & Wunsch (1989) (model + altimetry)

6. observed cp classic theory cp Westward phase speed cp cm/s Modern Altimeter Era • ERS-Envisat & TOPEX/Poseidon-Jason series (+ GFO) • TOPEX/Poseidon accuracy was instrumental for the observational quantum leap in the 1990s • Lots of papers: • Schlax & Chelton 1994 • Nerem et al. 1994 • Wang & Koblinsky 1995 • Polito & Cornillon 1997 • Cipollini et al 1997, 1999 • Wang et al 1998 • White et al 1998 • Witter & Gordon 1999 • Zhang & Wunsch 1999 • Polito & Liu 2003 • Fu 2004 • Osychny & Cornillon 2004 • …. • First truly global study: Chelton and Schlax (Science, 1996) • Confirmed ubiquity of waves and speed up w.r.t. classic theory

7. ERS-based observations North Atl 34°N Cipollini et al 1997 (North Atlantic): Hughes et al 1998 ( Southern Ocean)

8. Global observations in SST Hill et al, 2000; Leeuwenburgh & Stammer, 2001 25°S 25°S

9. observed cp classic theory cp Westward phase speed cp cm/s Merged T/P+ERS • Chelton et al 2006 • Made possible by both remarkable improvement in ERS orbits (Scharroo et al 1998, 2000), and careful intercalibration + optimal interpolation techniques (Le Traon et al 1998, Ducet et al 2000) • Good example of synergy between different altimeters

10. The theoretical burst T/P T/P+ERS • Mainly prompted by observational results • Mean flow effects • Killworth et al (1997) • Dewar (1998) • Liu (1999) • deSzoeke & Chelton (1999) • Yang (2000) • Colin de Verdiere & Tailleux (2005) • Ocean-Atmosphere coupling • White + collaborators • Bottom topography effects • Killworth & Blundell (1999) • Tailleux & McWilliams (2000,2001) • Tailleux (2003) • Combined mean flow + topography • Killworth & Blundell (2003a,b) • Killworth and Blundell (2005a,b) Chelton et al 2006

11. Wavenumber/frequency spectra and dispersion curves • Empty circles are dispersion curves from classic theory • Black dots are dispersion curves from extended theory • Note distribution of energy: tends to be non-dispersive! Chelton et al 2006 from merged T/P+ ERS data

12. (satellite) Comparison over the North Atlantic, data filtered for long waves Long wave theory and observations now agree! cp m/s The speed issue seems resolved (for long waves) essentially owing to the formidable theoretical effort….

13. Unresolved issues & current research • …but there are still issues not completely resolved, for instance: • What are the generation mechanisms for the planetary waves? • Why are observed waves essentially non-dispersive over the range of wavenumbers resolved by altimetry (and therefore faster than expected at shorter wavelengths)? • Why are they broad-banded? • Why there are distinctive waveguides of enhanced westward propagation in the oceans? • Can we see barotropic waves? And the different baroclinic modes? • Why has the meridional component of propagation been rather elusive so far? • Is it possible to study waves as single events (and possibly, forecast them?) • How do we explain the presence of planetary waves in Ocean Colour?

14. Long wave speed (old theory) Long wave speed (new theory) ‘Fast’ westward propagating signals Short Waves • Radon Transform-based analysis of planetary waves across S Pacific • Significant part of energy is at wavelengths  500 Km (‘short’ planetary waves) • Shows various effects due to bottom topography • Recent short planetary wave theory (Killworth & Blundell, 2005) fits “fast waves” better • OR we could be seeing eddies here! Maharaj et al, 2005

15. Barotropic waves • Reported by Fu (JPO 2004) looking at ratio of W-ward to E-ward energy (plot on left) • Requires ad hoc gridding of the data

16. Meridional component of PW propagation • Theory does not forbid a N-S component • Previous studies (e,g. Challenor et al., 2001, using 3-D Radon Transform) did not find evidence for significant meridional propagation. • This seems confirmed by eddy tracking technique (Isern-Fontanet et al 2003) used globally by Chelton et al 2006 • However, Glazman and Weichman (2005) with a technique based on autocorrelation of ungridded data, present results showing a significant meridional component • Would have implications on wave characteristics estimated from 2-D plots • Need to resolve this discrepancy

17. Wave Tracking (and possibly, forecasting!) • Developed in 2-D so far • Cipollini et al 2006 • Fitting a ‘single wave’ shape model to subwindows of data… • …then reconstructing full wave events by joining single waves • Now being extended to 3-D • Can it be the answer to finding the meridional component? • Should allow ‘propagating’ planetary waves (forecasts)

18. Planetary waves and biology • Cipollini et al, 2001; Uz et al, 2001; Siegel, 2001; Charria et al., 2003; Killworth et al, 2004; Dandonneau et al., 2004; Charria et al, 2006 Chlorophyll Sea Surface Height Advection of chlorophyll gradients plays a significant role, but can part of the signal indicate an effect on production and Carbon cycle?  talk in session 6

19. Conclusions • Altimetry has prompted a substantial revision of our theoretical understanding of planetary waves • A number of issues remain, and are being investigated by a lively community of scientists • And… just try and imagine what we will shortly be able to do by combining altimetry with ARGO data and gravity fields from GRACE (+GOCE)… The Bottom Line: Planetary waves represent a glittering success story for altimetry… … but there’s still a lot more to come!!!