Topex/Poseidon: The 2-cm Solution
TOPEX/POSEIDON: The 2-cm Solution
J. Geophys. Res., in press, 1994.
Bob Cheney, Laury Miller, Russ Agreen, Nancy Doyle, and John Lillibridge
National Ocean Service, NOAA, Silver Spring MD 20910
ABSTRACT
The unprecedented accuracy of TOPEX/POSEIDON (T/P) altimeter data warrant a new
evaluation of the methods typically used to form time series of sea level change. Whereas explicit
removal of orbit error has always been required as a first step in altimeter data processing, the T/P
analysis presented here is based simply on unadjusted, monthly averages. This approach has the
advantage of retaining the large-scale ocean signal, which would be distorted by orbit adjustment.
Using 16 months of data, we have evaluated the T/P monthly means on spatial scales ranging from
mesoscale to global. In the tropical Pacific, comparisons
with 17 island tide gauge records and
dynamic height derived from 36 thermistor moorings (Fig. 4 below) show that the altimeter data have an accuracy
of approximately 2 cm rms when averaged over spatial scales of a few hundred km. On basin scales
in the northern hemisphere, similar agreement is found between the T/P data and the dynamic
height climatology of Levitus (1982).
The T/P data show that global mean sea level fluctuates with
an annual period and peak-to-peak amplitude of 3 cm (Fig. 8 below), consistent with the seasonal cycle of heating
and cooling.
Figure 4. Scatter plots of all T/P comparisons within the equatorial region, 10oN to 10oS. (a) 17
island tide gauges yield a tight envelope with rms difference of only 2.2 cm and correlation of 0.88.
(b) 36 thermistor moorings show more scatter with rms difference of 3.1 cm and correlation of 0.80.
Figure 8. Variation in global sea level from T/P. Measurements during the first 2 months may not
be reliable due to satellite pointing errors which were corrected in December 1992 (Fu et al., 1994).
Thereafter a regular seasonal cycle is evident with phase similar to the northern hemisphere.