Observing mesoscale to submesoscale dynamics today –

& in the future with SWOT

Rosemary MorrowLEGOS, Toulouse

Lee-Lueng FuJPL - NASA

« Our current knowledge of mesoscale eddy activity in the ocean derived from altimetry is largely based on gridded AVISO data sets »

18 year time series (w. minimum 2 altimeters ) => monitoring the impact of mesoscale eddies on the large-scale climate system

Tracking large-scale non-linear eddies over years

Eddies mainly generated near E boundaries & bathymetry

Cyclones tend to the poles, anticyclones to the equator Morrow et al., 2004

Chelton et al., 2010

Cyclones : 84% poleward

Anti-cyclones : 74% equatorward

Strong zonal mean ocean jets – impact on atmospheric circulation

10 year mean zonal surface geostrophic velocity after Maximenko et al., 2008

Tracer released into an evolving 2D altimetric current field develops

filamentation patterns

Surface currents drive strong filamentation

F. D’Ovidio, LOCEAN, Paris

Day 0 Day 10 Day 15

Observed Tracer

=> Statistical techniques based on evolving altimetric currents now used to derive filament zones

Tracer filaments patterns can be observed from satellite SST or ocean color (Chlorophyll)

Filament positions derived from Lyapanuv exponants

Frontogenesis Mecanisms : Irminger Sea

Statistical model

Monthly mean SSS climatology SSS Field after deformation

Advection of fluid particles during 30 days by altimetric surface currents

Stirring of tracer field by mesoscale structures Good comparison with in-situ SSS obs - frontogenesis linked to horizontal advection

Temporal evolution of altimetric currents used to derive some filament structures

Despres et al., 2010

Today’s Challenges in Altimetry

1) Mesoscale processes (driving the horizontal dynamics) only partially observed with present generation altimeters (scales > 150 km)

2) High-latitude, coastal & near-shore processes missed

Ocean fronts and filaments can be studied with cloud-free SST and ocean colour

- Scales 1-50 km, 2-30 daysNeed high resolution sea level observations 1-100 km!

« Our current knowledge of mesoscale eddy activity in the ocean derived from altimetry is largely based on gridded AVISO data sets »

1st problem : Missing open ocean mesoscale eddies

Mesoscale processes (driving the horizontal dynamics) only partially observed with

present generation altimeters (scales > 150 km)

Chelton et al., 1998

Only larger eddies resolved by mapped data

2nd problem : Missing coastal structures

Coastal & high latitude processes only partially observed with

present generation altimeters (scales > 150 km)

Eddy scales & amplitudes increase offshore Example : Bay of Biscay

Space scales (km)

Standard altimetric maps smooth out smaller nearshore eddies, EKE reduced by 50%

Number of eddies

Eddy amplitudes (cm)

From Dussurget et al., 2011

AVISO MSLA 50 km

Improving mapping in regional seasUsing 4 missions : J1, T/P, GFO, ENV

• Satellite imagery provides clear-sky information on fine scale dynamics

50 km

XTRACK-OI 50 km

Steeper gradients Dipoles visible

near slope

From Dussurget et al., 2011

Nadir altimetry : 7 km alongtrack over oceans: resolves 30-50 km alongtrack, 150 km between tracks

SWOT : 1 km posting over oceans : resolve ~10 km in 2D (baroclinic Rossby radius)

Mesoscale eddies drive the ocean’s lateral stirring, advection & mixing

Sub-mesoscale fronts and filaments drive the vertical exchanges

R. Ferrari, MIT

Mesoscale Eddies, fronts & filaments

Reconstructing vertical velocities in the upper ocean

Under certain conditions, surface QG theory can be used to reconstruct the 3D ocean currents (u, v, w) in the upper ocean 0-500 m depth, starting from high-resolution SSH.

Observed W and Rel. Vorticity at 200 m

Reconstructed W and Rel. Vorticity at 200 m

P. Klein, Ifremer

Separating internal tide signature from mesoscale eddy processes

R. Ray, GSFC

100 10 km1000

Ocean dynamics with a SSH signature at 10-150 km wavelength, detected by SWOT:

• Smaller mesoscale eddies• Open-ocean fronts and filaments (geostrophic adjustment)• Internal tides• Coastal currents and filaments• Estuary – coastal exchange• Refined marine geoids and bathymetric maps• Sea ice freeboard and larger polynyas• …

Conclusions

Thank you!

Improving marine geoid accuracy GRACE and GOCE resolve only large-scale anomalies (spherical harmonic degree < 200, or wavelength > 200 km). This is because they measure the field at satellite orbital altitude.

Satellite altimeters measure the effect of the gravity field on sea level, so they resolve shorter scales.

SWOT’s 22 day repeat, global coverage of fine-resolution sea surface slopes will provide unprecedented 2D spatial resolution for marine geodesy.

Figure from the GOCO1S combined GRACE-GOCE model document, R. Pail et al., 23-07-2010.