ABSTRACT: A scheme to perform suboptimal intermittent assimilation in an open-ocean, quasi-geostrophic model is presented and applied to the assimilation of altimeter data in a domain of the northeast Atlantic west of Ireland. Both simulated and real altimeter data are used. Three-dimensional (3-D) synoptic observation fields and error variance fields are derived from synoptic dynamic topography anomaly estimates, using an empirical othogonal mode (EOF) vertical extension technique. The 3-D estimates are suboptimally combined with the time-dependent part of the forecast fields. Suboptimality here means that the combination does not directly use spatial correlation errors; however, the 3-D synoptic observation estimates used in the combination do contain error spatial statistical information. The resulting field added to a mean climatology are used as initial conditions for the model, which is integrated until a new dynamic topography anomaly estimate is available. Locally, the combination is optimal in the sense that it is based on the observational error variances, which reflect the measurement noise and the space-time distribution of data, and that it minimizes the error variance of the results. A similar combination scheme using past as well as future observations is used to update the boundaries during model integration. Simulated surface topography anomaly maps, typical of the northeast Atlantic, are assimilated every 20 days for a 300-day period, with different noise characteristics typical of altimetric sampling errors, in a three-level version of the model. The assimilated fields, especially the vorticity at deeper levels, converge toward the reference fields. The convergence is obtained after O(100 days), regardless of the observational noise level tested (up to -2 dB). Using a relevant observational error model seems to matter, in particular, the error level should not be underestimated. In the case of an 80-day gap in the data inflow, the model predictability limits the reliability of the forecast beyond 20 days. The scheme makes the model converge back as soon as new observations are entered, with limited convergence loss due to the gap. The case of an unknown model climatology is also studied. Finally, the scheme is applied to Geosat altimeter data, added to the Robinson, Bauer and Schroeder annual climatology, in the AthenA-88 cruise area, which is contained in the model box. The assimilation results compare favorably with hydrographical data from the cruise and help the synoptic interpretation of the observed phenomena.

ABSTRACT: The SEMAPHORE mesoscale air/sea experiment was conducted in the Azores-Madeira region from July to November 1993. TOPEX/POSEIDON (T/P) and ERS-1 were flying simultaneously at that time. The main purposes of this paper are to evaluate the estimation of the oceanic mesoscale circulation from the two different sets of altimetric data (T/P and ERS-1) and to compare the results with in situ measurements provided by the SEMAPHORE hydrographic surveys and surface drifters (three expendable bathytermograph conductivity-temperature-depth surveys in a 500-km² box and a set of 47 Lagrangian surface drifters drogued at 150 m). Comparisons are carried out through the maps obtained by objective analysis from the four data sets. The mapping accuracy of T/P, ERS-1, T/P and ERS-1 combined, and in situ data is investigated, as well as the sensitivity of the mapping to the correlation functions used. There is a good qualitative agreement between altimetric maps and corresponding drifter and hydrographic maps for the three hydrographic surveys. Correlations are about 0.8, and the regression fit is about 0.6-0.7; the lower values are due to the smooth climatology used to reference the altimetric maps. The correlation for time differences is better, with regression lines not significantly different from 1, especially when ERS-1 and T/P are combined. T/P mapping is almost as good as ERS-1 mapping, which was rather unexpected since the ERS-1 space-time sampling is better suited for the mesoscale. This may reflect the fact that the signal mapped by the hydrography and drifters does not contain the high frequency/wavenumber components. T/P and ERS-1 combined provide better results, although the improvement is not as large as expected, probably for the same reason.

ABSTRACT: Two years of Geosat data (11/86 to 12/88) are analyzed in the Azores-Madeira region of the North Atlantic. The objective is to study the eddy field associated with the terminal retroflection of the Azores current. The rms variability of Sea Level Anomaly is 4 to 8 cm, with maximum values between 33°N and 35°N being associated with the Azores current and located on its northern flank. Higher energy is observed during winter and during the year Nov. 1987 - Nov. 1988. Analysis of velocities at crossovers reveals anisotropy with larger meridional velocity variances. This is the signature of meanders and/or eddies elongated in the north-south direction. The eddy/mean flow interaction is studied through the analysis of the Reynolds tensor. Meridional and zonal accelerations of the mean by the eddy field are consistent with the retroflection of the Azores current to the south and to the west. Spectral analyses show that the dominant signals have periods and wavelengths larger than 100 days and 300 km respectively. Propagation velocities are westward but slow (1.5 km/day). Synoptic mapping of the eddy field shows the wavelike structure of the variability on the northern flank of the current. It is likely that the wave generation is not directly due to an instability of the Azores current, but is attributable to a cause more to the east. Once formed, the wave interacts with the current in winter and forms meanders. Seasonal variations of the surface circulation are finally analyzed from the 2-year long time series of synoptic maps. There is a clear seasonality in the surface circulation, which generally appears to be consistent with historical in-situ measurements. In winter, there is a strong meandering and a southward branching while in summer the mean front is narrower and penetrates further to the east.

June 1997

ABSTRACT: The ERS-1 orbit error reduction method using TOPEX/POSEIDON data as a reference (Le Traon et al., 1995a) has been applied to ERS-1 data from the first 35-day repeat mission and from the 168-day geodetic mission. The method has been refined and formal error on the estimation is now calculated. With the new T/P and ERS-1 data sets which include in particular JGM-3 orbits and CSR3.0 tidal model, the estimated accuracy of the ERS-1 orbit error estimation is now about 2 cm rms only. The adjustment has been systematically performed for the ERS-1 JGM-3 and D-PAF orbits. The E-E crossover differences are reduced from 17 cm (using the D-PAF orbit) or 11 cm (using the JGM-3 orbit) to only 7 cm rms for all processed cycles. Similarly the TP-E crossover differences are reduced from 13 cm (using the D-PAF orbit) or 10 cm (using the JGM-3 orbit) to only 7 cm rms. The adjusted D-PAF and JGM-3 orbit errors have a mean rms of 10 and 7 cm respectively. The corrected Sea Surface Heights for the D-PAF and JGM-3 orbits have an rms difference of about 1 cm rms only while it is about 11 cm before T/P orbit error correction. This shows that the adjustment is not sensitive to the initial ERS-1 orbit used. It also confirms the 2 cm accuracy of the method. Repeat-track analysis for the 35-day repeat cycles (cycles 6 to 18) is finally performed. Mean difference of sea level variance before and after orbit error correction is 34 cm2 (D-PAF orbit) and 17 cm2 (JGM-3 orbit). Without the correction, even with the recent ERS-1 JGM-3 orbits, the signal is thus still too much corrupted by orbit error to allow an analysis of the large scale oceanic signal and to be merged with T/P data. It is shown, on the other hand, that the corrected ERS-1 and T/P sea level variabilities are in excellent agreement.

ABSTRACT: The Semaphore mesoscale experiment took place from July to November 1993 in the northern Canary Basin, where the circulation is dominated by the eastward flowing Azores Current (AC). A large data set was acquired from three hydrographic arrays (phases 1, 2, 3, and 4), currentmeter moorings, surface drifters drogued at 150 m-depth, and 2000 m-depth deep Rafos floats. The analysis confirmed the large-scale observations previously made in this region, but also provided new insights of fine-scale dynamics of the Azores Current. The front was observed over the 6-month period. It was narrow (100 km width), and mostly surface intensified (velocities reaching 40-50 cm.s^-1). Whereas at the beginning of the experiment (phase 1) the Azores current was mainly zonal with weak oscillations, large meridional meanders were observed from phase 2 until the end of the experiment. They seem to be related to the arrival of two meddies, which interacted with the AC (Kase and Zenk, 1996, Tychensky and Carton, 1998). The front had a deep dynamical signature (down to 2000 m-depth), with a 16-18 Sverdrup (Sv) volume transport (0-2000 m-depth integrated). The southward recirculation branch of the AC near 22-23°W (Klein and Siedler, 1989) corresponds to a meridional transport of 5-6 Sv. It recirculates farther (along 31-32°N) to the west with a 4.5 Sv. Some interesting new oceanographic results were obtained by examining the Rafos float trajectories over the abyssal plain. They describe a circulation similar to that observed at the surface, with mean velocities of about 1-3 cm.s^-1 and EKE less than 4 cm².s^-2. In agreement with the analysis of currentmeter data, this reveals a significant barotropic component in the Azores-Madeira flow field of roughly 3-3.5 cm.s^-1.