Gridded Sea Level Anomalies and Deviations
National Ocean Service, NOAA (N/OES11)
The data are here.
Silver Spring, MD 20910
This new "read_me" file documents some changes to our T/P
As of April 1996:
- the original JGM-2 orbit was replaced with JGM-3, reducing
radial orbit error from about 3.5 to 2.5 cm rms.
- data gaps of up to 10 seconds were filled by interpolation,
resulting in a more complete global data set.
- the local inverted barometer correction was augmented by a
non-local correction (usually less than 1 cm).
- sea level time series were expressed as deviations relative
to the 3-year mean, Jan 1993-96, rather than differences
relative to the first observation.
As of July 1996:
The net effect of most of these changes is to reduce the noise in
the altimeter sea level time series.
- Topex oscillator error (12-15 cm) was corrected.
- Topex internal calibration (less than 1 cm) was applied.
- CSR 3.0 ocean tide (Univ. Texas) was incorporated (instead
of Cartwright/Ray with Wagner upgrade).
- each Poseidon cycle was adjusted for an offset relative
to adjacent Topex cycles (usually less than 1 cm).
In contrast, the Topex oscillator and internal corrections alter the
large-scale, low-frequency sea level signal, having the combined effect
of reducing sea level at any given location by about 8 mm per year
(relative to the earlier, uncorrected Topex data).
It is recommended that previous versions of the T/P data be discarded.
The compressed file, global_2_136.lo_res.Z, is a collinear
difference analysis of Topex/Poseidon altimeter data for
cycles 2-136 (Oct. 3, 1992 - June 2, 1996). Results are
expressed as time series of sea level deviations, with respect
to a long-term mean, averaged along 1-degree latitude intervals of
the satellite track.
Upon request, similar analyses can be made available on a
regional basis for 0.2-degree latitude averages.
Each header gives N latitude and E longitude in degrees
for a particular 1-degree latitude segment of the satellite track.
The flag gives several pieces of information:
- value indicates a descending track (NW to SE)
+ value indicates an ascending track (SW to NE)
8 in the thousands column indicates that this is a header
remaining digits give the equator crossing longitude in degrees
For example, the first header above gives a location of 7.00 N, 198.10 E
and the 1-degree segment is part of a descending track that crosses
the equator at 200.60 E.
(b) Data Records
Following the header is the sea height time series at this location,
where the three columns are time (1985 day), H (cm), and n (the number
of 1-sec altimeter records averaged to obtain H). For a 1-degree latitude
segment, the maximum number of 1-sec records varies from about 21 at the
equator to 32 at 60 N.
In the example above, the first data record is at time 2833.28, has a height
of -7.15 cm, and is an average of 12 1-sec records. The time represents the
equator crossing of the pass, rounded to the nearest hundredth of a day. By
our definition, 0 = January 1, 1985, 00 hrs, so that 2833.28 corresponds
approximately to October 4, 1992, 0700 hrs UTC. The subsequent data records,
arranged in chronological order, give the sea level at ~10-day intervals,
where each height is the deviation relative to the long-term mean
(see next section).
(c) Reference Period
In the present file, each time series is expressed relative to the 3-year mean
(Jan 1993-96) at that particular location. As the data set becomes longer, this
reference period will be extended, but will always be an integer number of
years. In computing the long-term mean, there is no requirement for a
minimum number of observations. It is left to the user to check for data
gaps which could impact the significance of the mean.
Latitude/longitude positions of the time series are located along the
satellite track, but in the file they are organized into latitude bands
scanning from west to east between 0 and 360 E and from south to north
between -60 and +60. Ascending passes can be found in the first half
of the file, followed by descending passes for the remainder of the global
3. PROCESSING METHOD
The analysis includes both Topex and Poseidon data. The geophysical
data records (GDRs) are used for most of the mission while the interim
GDRs (IGDRs) are used for the most recent cycles to make the data set
useful for near real-time applications.
The present file contains:
GDRs for cycles 2-132
IGDRs for cycles 133-136
Orbit precision is about 2.5 cm for the GDRs and 5-10 cm for the IGDRs.
JGM-3 orbits are used throughout. The Topex data are those distributed
by JPL. The Poseidon data are distributed by CNES, with a small (< 1 cm)
offset applied to each cycle based on collinear differences with the
adjacent Topex cycles.
The data were processed in the following way using collinear differences:
The error discovered in July 1996 was corrected using the table provided
by NASA Wallops.
- Standard corrections:
- wet and dry troposphere
- tides, using the CSR 3.0 model of the University of Texas.
Solid earth tide includes pole tide corrections.
- local inverse barometer, based on the instantaneous atmospheric
- non-local inverse barometer, based on the global average surface
pressure for that particular 10-day cycle
- sea state bias: either Callahan's algorithm for Topex data or
Gaspar's 4-parameter correction for Poseidon data.
- OSU91A geoid to reduce errors resulting from slight cross-track
gradients within the 2-km collinear band.
Data were excluded if the satellite attitude was greater than 0.3 degrees
in cycles 1-8. GDRs have flags that allow exclusion of bad data.
Shallow as well as deep water data are allowed. If there are problems with the
altimeter sensor corrections, ionospere, sea surface height, or if sea
state conditions are invalid, data are rejected. In addition, land or other
geophysical conditions cause data to be rejected. Sigma_h, the root mean
square of 10/second sea surface heights, is limited to 110 for Topex data
and 170 for Poseidon data.
Data gaps in the GDRs of up to 10 seconds long are filled by linear
interpolation. The ionosphere correction is smoothed over 25 seconds
before being applied.
- Orbit adjustment
No adjustment was made for orbit error.
- Altimeter drift
The internal calibration correction supplied by NASA Wallops was applied.
- Oscillator (clock) drift