1. Objectives and Requirements of SWOT for Observing the Oceanic Mesoscale Variability
(based on a workshop held at Scripps Institution of Oceanography, April 28-May 1, 2008)
Lee-Lueng Fu
Jet Propulsion Laboratory
C-K. Shum
Ohio State University
NASA SWOT Hydrology Workshop
September 15-17, 2008
The Ohio State University, Columbus, Ohio

2. ground tracks of Jason (thick) and T/P (thin) Tandem Mission
90% of the ocean’s kinetic energy is carried by eddies which are not well resolved by a nadir-looking altimeter
ground tracks of Jason (thick) and T/P (thin) Tandem Mission
10 km scale eddies
100 km scale eddies
100 km

3. A snapshot of ocean eddies from merged Jason/Envisat data provided by CLS (the best existing data)

4. D. Chelton (Oregon State U)

5. Effects of spatial resolution in observing ocean eddies
2 altimeters
4 altimeters
GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L02611, doi: /2005GL024633, 2006Improved description of the ocean mesoscale variability by combining four satellite altimetersAnanda PascualInstitut Mediterrani d'Estudis Avançats, Consejo Superior de Investigaciones Científicas/Universidad de las Islas Baleares, Mallorca, SpainYannice FaugèreSpace Oceanography Division, CLS, Toulouse, FranceGilles LarnicolSpace Oceanography Division, CLS, Toulouse, FrancePierre-Yves Le TraonL'Institut Francais de Recherche pour l'Exploitation de la Mer, Brest, FranceAbstractData from four satellite altimeters are combined with the aim of improving the representation of the mesoscale variability in the Global Ocean. All missions [Jason-1, ERS-2/ENVISAT, Topex/Poseidon interleaved with Jason-1 and Geosat Follow-On] are cross-calibrated previously to produce weekly gridded maps. In areas of intense variability, the rms differences between a classical configuration of two altimeters and the scenario merging four missions can reach 10 cm and 400 cm2/s2 in SLA and EKE, respectively, which represents an important percentage of the signal variance. A comparison with surface drifters shows that the four altimeter scenario improves the recovery of mesoscale structures that were not properly sampled with Jason-1 + ERS-2/ENVISAT. Finally, the consistency between altimetric and tide gauge data is improved by about 25% when coastal sea level is estimated with 4 satellites compared to the results obtained with 2 altimeters.
Comparison of altimetry and drifter data of a cyclonic eddy of the Gulf Stream. The white line represents the trajectory followed by a surface float between 14 May 2003 and 28 May The vectors correspond to the absolute velocity field (geostrophy + Ekman) and the background color field is the SLA + MDT (in cm) on 21 May 2003 from the (left) 2 (T/P/ERS)- and (right) 4 (T/P/Jason/ERS/GFO)- satellite configurations. (from Pascual et al, 2006)

6. The oceanic submesoscales have not been well observed
SSH Wavenumber spectrum from Jason altimeter data

7. The importance of oceanic submesoscales:
50% of the vertical motion in the world’s oceans responsible for heat and CO2 uptake takes place at the submesoscales
R. Ferrari (MIT)

8. Contours are relative vorticity
Feasibility of estimating the vertical velocity of the upper ocean from SSH measurement
Simulated W by an OGCM
Reconstructed W from SSH
Contours are relative vorticity
P. Klein (IFREMER)

9. SWOT SSH error spectral requirement
(to resolve signals down to 10 km wavelength) km
Jason pass 132 (147 cycle average)
Signal spectrum
k-3
Instrument noise
Power density (cm2/cycle/km)
k-2
error spectrum
3 cm noise at 1 km2 average
30 km
1cm noise at 1 km2 average
10 km
Wavenumber (cycle/km)

11. A family of orbits with an inclination of 78 degrees.
Selection of orbit with a desirable subcycle
There was a consensus among the workshop participants for the need of a special experiment of limited duration in which the repeat period is reduced to achieve higher sampling rates for studying the temporal variability of submesoscale processes, especially in coastal zones.
A family of orbits with an inclination of 78 degrees.
A 22-day orbit with a 3-day subcyle can be easily adjusted from an altitude of 970 km to 976 km to get into a 3-day repeat orbit.
Subcycle (days)
3-day repeat orbit
S. Nerem (CU)

12. Avoid tidal aliasing problem in orbit selection
We need to avoid aliasing the major tidal components into the low frequencies of interests to the mission (e.g., the annual signal), as well as to avoid aliasing multiple components into the same frequency.
Desirable orbits with 15-, 21-, and 22-day repeat periods for adequate tidal aliasing properties.
S. Nerem (CU)

13. Recommendations
SSH measurement performance of 1 cm noise at 1 km x 1 km averaging is essential for resolving sub-mesoscale features at wavelengths from km, depending on the location. Systematic errors should be at least one order of magnitude less than signals at wavelengths longer than 30 km.
A candidate orbit emerges with the following characteristics: 78 degree inclination, 22-day repeat period with a 3-day sub-cycle, km altitude. This orbit meets the following requirements: cover all of the major rivers of the world; minimize tidal aliasing effects; have a sub-cycle for resolving high-frequency signals with adequate spatial coverage.