MATER - CLS-GRGS publications - MATER
Journal of Geophysical Research, 100, 25163-25177, 1995.
Journal of Marine Systems (in press).
ABSTRACT : ERS-1 and TOPEX/POSEIDON (T/P) data have been combined to study the surface circulation variability in the Mediterranean Sea, from October 1992 to December 1993. The ERS-1 orbit error is corrected using T/P data as a reference which yields two consistent datasets. We combine them via a space-time objective analysis method. Comparison of Sea Level Anomaly (SLA) maps allows us to check that the specific contribution of ERS-1 consists in an improved mesoscale circulation description with respect to the analysis of T/P only. The Mediterranean circulation variability, as observed by T/P and ERS-1, is characterised by a wide range of temporal and spatial scales. As the various signals are superimposed and very likely interact with each other, it is difficult to isolate them. Moreover, as shown by the comparison of fall 1992 and fall 1993 maps, strong interannual signals are suspected to affect the seasonal circulation. This makes the reference to previous observations almost impossible, especially in the Eastern Basin. However, several well-known signals have been recovered and new interesting features are observed. The variability in the Western Basin consists of a wintertime intensification of the basin-scale cyclonic cell, with the acceleration of coastal narrow currents, and in mesoscale activity all year round in the southern part of the basin and in the Tyrrhenian Sea. The Alboran gyres temporal variations and eastward and seaward propagations of Algerian Current eddies are detected. The analysis of ECMWF monthly averages suggests that wind stress curl variations are responsible for the large-scale seasonal variability. In the Ionian Basin, the signals are more complex. In winter 1993, the eastward current along the coast of Africa is strongly intensified; in summer 1993, it is shifted to the north and forms a large anticyclonic meander extending up to the Otranto Strait. From April to December 1993, an anticyclone is detected around 17°E-34°N. In the Levantine Basin, the strongest signal reflects the seasonal variations of the Ierapetra gyre, southeast of Crete. No basin-scale features are detected there. On the contrary, strong mesoscale activity appears throughout the year, in the form of transient anticyclonic "eddies"; their development seems to be correlated with the basin topography. However, we can not identify distinct, isolated structures in the south (such as the expected Mersa-Matruh or the Shikmona gyres). It appears instead that the circulation in the southern part of the basin is composed of multi-centered anticyclonic systems with high temporal variability.
Nadia Ayoub, PHD Thesis, Universite Paul Sabatier
Pierre-Yves Le Traon and Philippe Gauzelin - CLS Space Oceanography Division
Journal of Geophysical Research, 102, 973-984, 1997
ABSTRACT : The response of the Mediterranean mean sea level to atmospheric pressure forcing is analyzed using 3 years of TOPEX/POSEIDON data. Coherence analysis between mean sea level and atmospheric pressure shows a significant departure from a standard inverse barometer effect at frequencies higher than 30 days-1. At high frequencies, the phase difference between sea level and pressure is about 100° while it should be 180° for a perfect inverse barometer response. This result is in agreement with previous findings and confirms the role of the straits of Gibraltar and Sicily in limiting the water exchange (and thus the response to atmospheric pressure forcing) at high frequencies. The response of the Mediterranean mean sea level is then investigated using the Candela (1991) analytical model which takes account of friction in the straits of Gibraltar and Sicily. The model explains a large part of the variance in TOPEX/POSEIDON mean sea level variations (50% for the western basin and 38% for the eastern basin). Compared to an inverse barometer correction, it gives a smoother response with a phase delay at high frequencies. It also explains more variance in TOPEX/POSEIDON mean sea level variations (5 cm2 and 7 cm2 for the western and eastern basins respectively). This demonstrates that this simple model provides an improved correction of atmospheric pressure effects in TOPEX/POSEIDON data. As the two corrections have an rms difference of 2 to 3 cm with maximum differences of up to 10 cm, the impact on the mapping of oceanic circulation is not negligible. This is exemplified through the comparison of sea level anomaly derived from the two corrections.
Journal of Atmospheric and Oceanic Technology, vol 25, 522-534, 1998
ABSTRACT: Objective analysis of altimetric data (Sea Level Anomaly) usually assumes that measurement errors are well represented by a white noise, though there are long-wavelength errors which are correlated over thousands of kilometers along the satellite tracks. These errors are typically 3 cm rms for TOPEX/POSEIDON (T/P) which is not negligible in low energy regions. Analyzing maps produced by conventional objective analysis thus reveals residual long wavelength errors in the form of tracks on the maps. These errors induce sea level gradients perpendicular to the track, and therefore high geostrophic velocities which can obscure ocean features. To overcome this problem, an improved objective analysis method which takes into account along-track correlated errors is developed. A specific data selection is used to allow an efficient correction of long wavelength errors while estimating the oceanic signal. The influence of data selection is analyzed and the method is first tested with simulated data. The method is then applied to real T/P and ERS-1 data in the Canary basin (a region typical of low eddy energy regions) and the results are compared to those of conventional objective analysis method. The correction for the along-track long wavelength error has a very significant effect. For T/P and ERS-1 separately, the mapping difference between the two methods is about 2 cm rms (30% of the signal variance). The variance of the difference in zonal and meridional velocities is roughly 30% and 60% respectively of the velocity signal variance. The effect is larger when T/P and ERS-1 are combined. Correcting the long wavelength error also considerably improves the consistency between the T/P and ERS-1 data sets. The variance of the difference (T/P - ERS-1) is reduced by a factor of 1.7 for the sea level, 1.6 for zonal velocities and 2.3 for meridional velocities. The method is finally applied globally to T/P data. It is shown that it is tractable at the global scale and that it provides an improved mapping.
Mean sea level and surface circulation of the Mediterranean sea from two years of Topex/Poseidon data
Gilles Larnicol, Pierre-Yves Le Traon, Nadia Ayoub and Pierre De Mey
Combining ERS-1 and Topex/Poseidon data to observe the variable oceanic circulation in the Mediterranean sea
Nadia Ayoub, Pierre-Yves Le Traon and Pierre De Mey
Variabilite du niveau de la mer et de la circulation en Mediterranee a partir des donnees altimetriques et de champs
de vent. Comparaison avec des simulations numeriques.
Response of the Mediterranean Mean Sea Level to Atmospheric Pressure Forcing :
An Improved mapping method of multi-satellite altimeter data:
P.Y. Le Traon, F. Nadal and N. Ducet - CLS Space Oceanography Division
