A. Simone et al.
54
ging (9]. In general, the rôle of the eddies, apart from the vertical transfer of
momentum, seems to vary with the location and to be only locally relevant [10].
In this general context the satellite altimeter is a powerful tool to observe sea
level variation globally and synoptically. Until now many studies [11-14] hâve
taken advantage of Geosat altimeter measurements to investigate sea surface high
variability in the Southern Océan and to reconstruct the mean sea surface high
across the ACC. T/P data in the Southern Indian Océan were analyzed by Park and
Gamberoni [15]; they found that altimétrie data show excellent agreement with
the numerical model solution and indicate an anticyclonic subtropical gyre north
of the ACC and two cyclonic gyres south of this current.
In this work we will examine the sea level variability of the Southern Océan
during the first 2 years of the T/P mission and compare the resulting pictures of
the surface circulation with previous altimeter data analysis and observational
évidences. The paper is organized as follows. Section 2 describes the processing.
Section 3 deals with repeat track analysis and large-scale circulation as observed
by T/P. Cross-over analysis and Reynolds stress are discussed in Section 4. The
results are summarized in the Section 5.
2 T/P Data Processing
Sea level anomaly (SLA) files, i.e., sea level measurements by the T/P altimeter,
were used to study the temporal variability of the ACC. Such a data set, provided
by the AVISO (Archivage Validation et Interprétation des Données des Satellites
Océanographiques) [16] on CD-ROM, has been produced applying ail the geophysical corrections, data validation, and quality control in order to allow direct
use of the data by the scientific community. No orbit correction was applied since
tests based on data fits, covariance analysis, and orbit comparison indicate that
the radial component of the T/P spacecraft is determined, relative to the Earth’s
mass center, with an RMS error in the range of 3-4 cm [1].
The altimétrie measurements made at two frequencies (5.3 and 13.6 GHz) are
combined to minimize the errors caused by the ionospheric free électrons. The
measurements at 10 Hz data rate hâve been averaged to obtain a sample each second (about 6 km in distance along a track), i.e., at 1 Hz data rate. This adjustment
has improved the altimeter noise figure by 20%. Since temporal and spatial resolution are inversely proportional (for a single satellite mission), the best choice is
a compromise: a 10-day repeat period has been chosen, which results in an équatorial cross-track séparation of 316 km (the average is about 200 km). The exact
orbit altitude is 1336 km. The RMS accuracy of a single pass sea level measurement is 4.7 cm for the TOPEX altimeter System and 5.1 for the POSEIDON System
[ 17]. The tollowing data processing is based on 2 years of T/P radar altimeter data,
from the start of a 10-day repeat period cycle, i.e., October 1992.
We analyzed the first 74 cycles of SLA (254 tracks each cycle), selecting a géographie window of the Southern Océan between 30° and 65°S. These data cover
the period October 1992-October 1994. We studied first of ail the spatial autocorrélation functions ot the tracks. Thus fitting the mean autocorrélation function
with a simple exponential we got an e-folding distance of about 100 km (Fig. 2).
54
ging (9]. In general, the rôle of the eddies, apart from the vertical transfer of
momentum, seems to vary with the location and to be only locally relevant [10].
In this general context the satellite altimeter is a powerful tool to observe sea
level variation globally and synoptically. Until now many studies [11-14] hâve
taken advantage of Geosat altimeter measurements to investigate sea surface high
variability in the Southern Océan and to reconstruct the mean sea surface high
across the ACC. T/P data in the Southern Indian Océan were analyzed by Park and
Gamberoni [15]; they found that altimétrie data show excellent agreement with
the numerical model solution and indicate an anticyclonic subtropical gyre north
of the ACC and two cyclonic gyres south of this current.
In this work we will examine the sea level variability of the Southern Océan
during the first 2 years of the T/P mission and compare the resulting pictures of
the surface circulation with previous altimeter data analysis and observational
évidences. The paper is organized as follows. Section 2 describes the processing.
Section 3 deals with repeat track analysis and large-scale circulation as observed
by T/P. Cross-over analysis and Reynolds stress are discussed in Section 4. The
results are summarized in the Section 5.
2 T/P Data Processing
Sea level anomaly (SLA) files, i.e., sea level measurements by the T/P altimeter,
were used to study the temporal variability of the ACC. Such a data set, provided
by the AVISO (Archivage Validation et Interprétation des Données des Satellites
Océanographiques) [16] on CD-ROM, has been produced applying ail the geophysical corrections, data validation, and quality control in order to allow direct
use of the data by the scientific community. No orbit correction was applied since
tests based on data fits, covariance analysis, and orbit comparison indicate that
the radial component of the T/P spacecraft is determined, relative to the Earth’s
mass center, with an RMS error in the range of 3-4 cm [1].
The altimétrie measurements made at two frequencies (5.3 and 13.6 GHz) are
combined to minimize the errors caused by the ionospheric free électrons. The
measurements at 10 Hz data rate hâve been averaged to obtain a sample each second (about 6 km in distance along a track), i.e., at 1 Hz data rate. This adjustment
has improved the altimeter noise figure by 20%. Since temporal and spatial resolution are inversely proportional (for a single satellite mission), the best choice is
a compromise: a 10-day repeat period has been chosen, which results in an équatorial cross-track séparation of 316 km (the average is about 200 km). The exact
orbit altitude is 1336 km. The RMS accuracy of a single pass sea level measurement is 4.7 cm for the TOPEX altimeter System and 5.1 for the POSEIDON System
[ 17]. The tollowing data processing is based on 2 years of T/P radar altimeter data,
from the start of a 10-day repeat period cycle, i.e., October 1992.
We analyzed the first 74 cycles of SLA (254 tracks each cycle), selecting a géographie window of the Southern Océan between 30° and 65°S. These data cover
the period October 1992-October 1994. We studied first of ail the spatial autocorrélation functions ot the tracks. Thus fitting the mean autocorrélation function
with a simple exponential we got an e-folding distance of about 100 km (Fig. 2).