Les tropiques vus du ciel, ou l'altimétrie satellitaire dans les régions équatoriales

Le rôle important de l'océan sur le climat de notre planète, et particulièrement le rôle de l'océan tropical, est maintenant bien admis par l'ensemble des scientifiques. Toutefois, comme beaucoup de sciences de la Terre, l'océanographie se heurte au problème crucial du manque de données. Au cours de la dernière décennie, les océanographes ont pu obtenir, grâce aux techniques spatiales, une nouvelle source d'acquistion de mesures. Les différentes missions altimétriques et les principaux travaux en océanographie physique, auxquels celles-ci ont donné lieu dans les tropiques, sont répertoriés et analysés ici. (Résumé d'auteur

Tropical Atlantic geostrophic currents and ship drifts

Historical ship drifts and geostrophic surface currents obtained from hydrographic data are compared in the Tropical Atlantic ocean. The major components of the current system (North Equatorial Current, North Equatorial Countercurrent, South Equatorial Current) are clearly depicted by the two data sets. The main difference between the two fields is the weakness of the geostrophic currents except in the near-Equatorial band, where on the contrary, they are much larger than the ship drifts. The amplitudes of the annual signals differ, but the phases are in rather good agreement. The uncertainty concerning the ship drift data, and the limitation of the geostrophic assumption, can explain a part of the differences encountered in this study. (D'après résumé d'auteur

Tropical Atlantic sea level variability from Geosat (1985-1989)

Geosat altimeter data between april 1985 and september 1989 are analyzed in the Tropical Atlantic ocean. First, improvements due to the use of new corrections and orbit computations are found to be effective, especially in the Gulf of Guinea, where part of the previously missing signal is recovered. Then, the variability of the ocean is examined using empirical orthogonal functions (EOFs). Only three EOFs are needed to describe 80% of the seasonal variance. The first one describes the meridional tilting of the Tropical Atlantic along the mean location of the Intertropical Convergence Zone, with an annual period. The second describes a mass redistribution due to the equatorial upwelling peaking in June-July. The third function presents semiannual signal. Looking at interannual variability, the first EOF reveals a mass redistribution between the Equatorial region (10°N to 10°S) and the Northern and Southern ones (10°N to 30°N, 10°S to 30°S°). In the Equatorial region the upper layer volume increases .... between 1987 and 1989. Occurring 1 year after the Pacific EL Nino, this phenomenon recalls the 1984 anomaly observed during the "Programme Français Océan et Climat en Atlantique Tropical"/Seasonal Equatorial Atlantic experiments. (Résumé d'auteur

Variability of the tropical Atlantic in 1986-1987 as observed by Geosat and in situ data

Launched in March 1985, the US Navy altimetric satellite GEOSAT gives the opportunity to the oceanographers to study the sea level variations with a high spatial and temporal resolution. We used 18 cycles of GEOSAT data from November 1986 until August 1987 to obtain the sea level variability in the Tropical Atlantic first results are in genet with those obtained through historical hydrographic data and evidence an interannual event concerning the equatorial upwelling which seems to be also present in simultaneous in situ data. (Résumé d'auteur

Low-frequency variability of the Tropical Atlantic surface topography : altimetry and model comparison

Altimetric data, climatological hydrological data, and numerical model results are compared over the Tropical Atlantic ocean between november 1986 and november 1988. All reproduce the seasonal cycle of the dynamic topography rather well, and the agreement is particularly good between altimetry and the primitive equation model. The study of the 1986-1988 period reveals interannual events evidenced by both the altimetry and the models, especially during spring 1988 in the Gulf of Guinea. (Résumé d'auteur

Comparison of the altimetric signal with in situ measurements in the Tropical Atlantic ocean

An intensive survey of XBT and surface salinity sampling was carried out during september/october 1988 to compare surface dynamic heigh anomalies and altimetric anomalies along a line close to a GEOSAT satellite track in the Tropical Atlantic ocean, 15°N-18°S. Hydrography and altimetry agree within 4 cm rms, except in the Southern part of the section (South of 5°S). In the Northern hemisphere, the correlation between the two data sets is about 0.80 and the two power spectra present the same energy level .... The hydrographic data show an unusual tongue of fresh water around 7°N that results in positive dynamic height anomalies of about 10 dyn cm. The GEOSAT data analysed during the same period also show these anomalies with a similar scale. (D'après résumé d'auteur

Toward altimetric data assimilation in a Tropical Atlantic model

We present three types of experiments of sequential assimilation of altimetric data in a linear tropical Atlantic model, using an analysis technique we developed. In the first experiments we assimilate dynamic height fields calculated from simulated data in order to study the impacts of assimilations. In the second ones we assimilate dynamic height anomaly fields calculated from simulated data, and finally, in the third ones we use sea level anomaly fields obtainend from altimetric data from Geosat. Perturbations are observed in the Eastern part of the basin, due to Kelvin waves artificially generated in the West Equatorial basin, due to Kelvin waves artificially generated in the West Equatorial basin. However, we show that altimetric data may be useful for improving model simulations in the tropics when appropriate assimilation techniques are used. (Résumé d'auteur

Geosat sea-level assimilation in a tropical Atlantic model using Kalman filter

We present preliminary results on Geosat altimetric data assimilation in a linear vertical mode model of the tropical Atlantic Ocean. The Kalman filter technique is used to assimilate altimetric data along one track at a time as the satellite over-flies the basin. Sensitivity and validation tests have been performed with simulated data. The results obtained with Geosat data are presented and compared on a monthly basis with objective analysis of altimetric data and oceanic general circulation model results

Altimetry for the future: Building on 25 years of progress

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology. The paper starts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion