Abstract. This study focuses on the improved estimation of mesoscale surface ocean circulation obtained by merging TOPEX/Poseidon (T/P) and ERS-1 and -2 altimeter measurements between October 1992 and May 1998. Once carefully intercalibrated and homogenized, these data are merged through an advanced global objective analysis method that allows us to correct for residual long wavelength errors and uses realistic correlation scales of ocean dynamics. The high-resolution (0.25 ø x 0.25 ø) merged T/P + ERS-1 and -2 sea level anomaly maps provide more homogeneous and reduced mapping errors than either individual data set and more realistic sea level and geostrophic velocity statistics than T/P data alone. Furthermore, the merged T/P + ERS-1 and -2 maps yield eddy kinetic energy (EKE) levels 30% higher than maps of T/P alone. They also permit realistic global estimates of east and north components of EKE and their seasonal variations, to study EKE sources better. A comparison of velocity statistics with World Ocean Circulation Experiment surface drifters in the North Atlantic shows very good agreement. Comparison with contemporary current meter data in various oceanic regimes also produces comparable levels of energy and similar ratios of northward and eastward energy, showing that the maps are suitable to studying anisotropy. The T/P + ERS zonal and meridional components of the mapped currents usually present comparable rms variability, even though the variability in the Atlantic is more isotropic than that in the Pacific, which exhibits strong zonal changes. The EKE map presents a very detailed description, presumably never before achieved at a global scale. Pronounced seasonal changes of the EKE are found in many regions, notably the northeastern Pacific, the northeastern and northwestern Atlantic, the tropical oceans, and the zonally extended bands centered near 20øS in the Indian and western Pacific Oceans and at 20øN in the northwestern Pacific. IntroductionIt is now generally admitted that at least two altimetric missions are needed to resolve the main space scales and timescales of the ocean circulation, in particular, However, merging multisatellite data sets is not an easy task. It first requires homogeneous and intercalibrated sea surface height (SSH) data sets. It is then necessary to extract consistent sea level anomaly (SLA) data from the different satellites. Finally, advanced interpolation techniques are needed to map SLA data onto a regular space/time grid. Gridded SLA data can be considered as a final merged product; they can be used directly for signal analysis and in comparison with numerical models and in situ measurements [e.g., Chao and Fu, 1995; Hernandez et al., 1995]. They may also be assimilated into ocean circulation models since they provide both data and associated error estimates at each grid point, although the assimilation of along-track data may be preferred for sophisticated assimilation schemes [Fukumori, 1995].Effective merging techniques have been developed at CLS Space Oceanograp...

DOwNLOADED FROM www.TOS.ORG/OCEANOGRAPHY Oceanography | Vol.23, No.4 14 T h e F u T u r e o F o c e a N o g r a p h y F r o m S pa c e eddy dyNamicS From SaTelliTe alTimeTry abSTr acT. Most of the kinetic energy of ocean circulation is contained in ubiquitous mesoscale eddies. Their prominent signatures in sea surface height have rendered satellite altimetry highly effective in observing global ocean eddies. Our knowledge of ocean eddy dynamics has grown by leaps and bounds since the advent of satellite altimetry in the early 1980s. A satellite's fast sampling allows a broad view of the global distribution of eddy variability and its spatial structures. Since the early 1990s, the combination of data available from two simultaneous flying altimeters has resulted in a time-series record of global maps of ocean eddies. Despite the moderate resolution, these maps provide an opportunity to study the temporal and spatial variability of the surface signatures of eddies at a level of detail previously unavailable. A global census of eddies has been constructed to assess their population, polarity, intensity, and nonlinearity. The velocity and pattern of eddy propagation, as well as eddy transports of heat and salt, have been mapped globally.For the first time, the cascade of eddy energy through various scales has been computed from observations, providing evidence for the theory of ocean turbulence. Notwithstanding the tremendous progress made using existing observations, their limited resolution has prevented study of variability at wavelengths shorter than 100 km, where important eddy processes take place, ranging from energy dissipation to mixing and transport of water properties that are critical to understanding the ocean's roles in Earth's climate. The technology of radar interferometry promises to allow wide-swath measurement of sea surface height at a resolution that will resolve eddy structures down to 10 km. This approach holds the potential to meet the challenge of extending the observations to submesoscales and to set a standard for future altimetric measurement of the ocean. b y l e e -l u e N g F u , d u d l e y b . c h e lT o N , p i e r r e -y V e S l e T r a o N , a N d r o S e m a r y m o r r o w Oceanography

Abstract. The Mediterranean Forecasting system Pilot Project has concluded its activities in 2001, achieving the following goals: Realization of the first high-frequency (twice a month)Voluntary Observing Ship (VOS) system for the Mediterranean Sea with XBT profiles for the upper thermocline (0-700 m) and 12 n.m. along track nominal resolution;2. Realization of the first Mediterranean Multidisciplinary Moored Array (M3A) system for the Near-Real-Time (NRT) acquisition of physical and biochemical observations. The actual observations consists of: air-sea interaction parameters, upper thermocline (0-500 m) temperature, salinity, oxygen and currents, euphotic zone (0-100 m) chlorophyll, nutrients, Photosinthetically Available Radiation (PAR) and turbidity;3. Analysis and NRT dissemination of high quality along track Sea Level Anomaly (SLA), Sea Surface Temperature (SST) data from satellite sensors to be assimilated into the forecasting model;4. Assembly and implementation of a multivariate Reduced Order Optimal Interpolation scheme (ROOI) for assimilation in NRT of all available data, in particular, SLA and VOS-XBT profiles;5. Demonstration of the practical feasibility of NRT ten day forecasts at the Mediterranean basin scale with resolution of 0.125 • in latitude and longitude. The analysis or nowcast is done once a week;6. Development and implementation of nested regional (5 km) and shelf (2-3 km) models to simulate the seasonal variability. Four regional and nine shelf modelsCorrespondence to: N. Pinardi (n.pinardi@ambra.unibo.it)were implemented successfully, nested within the forecasting model. The implementation exercise was carried out in different region/shelf dynamical regimes and it was demonstrated that one-way nesting is practical and accurate;7. Validation and calibration of a complex ecosystem model in data reach shelf areas, to prepare for forecasting in a future phase. The same ecosystem model is capable of reproducing the major features of the primary producers' carbon cycle in different regions and shelf areas. The model simulations were compared with the multidisciplinary M3A buoy observations and assimilation techniques were developed for the biochemical data.This paper overviews the methodological aspects of the research done, from the NRT observing system to the forecasting/modelling components and to the extensive validation/calibration experiments carried out with regional/shelf and ecosystem models.

Abstract. The ERS orbit error reduction method using TOPEX/POSEIDON (T/P) data as a reference [Le Traon et al., 1995a] was applied to ERS-1 cycles from phases C, E, F, and G and to the first 16 cycles of the ERS-2 mission (phase A). T/P M-GDR (geophysical data record) (version C) and ERS-1/2 ocean product (OPR) data were used. ERS-1/2 orbits are the D-PAF (processing and archiving facility) orbits and, when necessary, ERS-1/2 altimetric corrections were updated to make the T/P and ERS-1/2 corrections homogeneous. The adjustment method has been refined, and formal error on the estimation is now calculated. The ERS-1/2 orbit error estimation is thus estimated to be precise to within about 2 cm root-mean-square (rms). E-E crossover differences are reduced from 12 to 17 cm to only 6.5 cm rms for all processed cycles. Similarly, the T/P-E crossover differences are reduced from 11 to 14 cm to only 7 cm rms. The adjusted D-PAF orbit error varies between 6 and 12 cm rms. The adjustment has also been performed for the Joint Gravity Model 3(JGM 3) orbits of ERS-1 phases C, E, and F. The rms difference between the corrected orbits for the D-PAF and JGM 3 orbits is only about 1 cm rms, while it is about 11 cm before T/P orbit error correction. This shows that the adjustment is almost insensitive to the initial ERS-1 orbit used. It also confirms the 2 cm precision of the method. We also do repeat-track analysis on the 35 day repeat cycles of ERS-1 phase C. The mean difference in sea level variance before and after orbit error correction is 34 cm 2 (D-PAF orbit) and 17 cm 2 (JGM 3 orbit). The corrected ERS-1 and T/P sea level variabilities, however, are in excellent agreement. The study thus shows that ERS-1/2 orbit error must be corrected before analyzing large-scale oceanic signals and combining ERS-1/2 with T/P data. The proposed method provides a very effective correction and thus significantly enhances the quality of ERS-1/2 data. Corresponding data sets will be distributed to the scientific community by Archiving, Validation, and

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