Variation of the mean sea level from TOPEX/POSEIDON data

Variations of the 10-day-averaged sea level extracted from TOPEX/POSEIDON (T/P) altimeter data are examined from October 1992 to February 1995. Data are corrected from instrumental drifts, and recent precise orbit and ocean tide corrections are used. When data from the less accurate first eight cycles are excluded and atmospheric pressure effects are not corrected, the 10-day mean sea level varies by 5.0 mm rms. Its variations can be described by a 4.1 +/- 1 mm/yr drift, a small annual signal of 2.7 mm amplitude and a residual signal of 3.5 mm rms amplitude, and Gaussian statistics close to formal error estimates. The mean sea level rise is difficult to explain by drifts of geophysical corrections, through the case for the wet tropospheric correction and that for the orbit are uncertain. Pressure effects should be corrected relative to the global pressure averaged over the ocean. The latter shows a seasonal signal of 1 cm amplitude and apparent fluctuations of 30- to 70-day periods. These fluctuations could result from errors in the atmospheric pressure fields, but they seem to be mostly related to large regional pressure evolutions. Evidence is also found for a sampling effect by the T/P orbit of pressure signals at shorter than 10-day periods. Pressure effects on sea level are then assumed to be an inverse barometer relative to 10-day global atmospheric pressure over the ocean, which allows analysis of the geographical repartition of the mean sea level evolution due to other effects. The mean sea level rise is comparable in the northern (4.2 +/- 2.5 mm/yr) and southern (3.7 +/- 1.9 mm/yr) hemispheres. It seems mostly related to interannual variations of the tropical ocean (41% of the global signal) and of the northern hemisphere subtropical gyres (30% of the global signal); residual noise on these regional values seems inversely proportional to the square root of the number of data points in each area. The hemispheric seasonal pressure variation relative to the globally averaged pressure over the ocean is less than 3 mm amplitude; in our calculation it is implicitely assumed that it induces a small interhemispheric water mass exchange. The amplitudes of the seasonal hemispheric sea level variations are 27 and 18 mm in the northern and southern hemispheres, respectively. These amplitudes are very close to being inversely proportional to the ocean surface in each hemisphere. The maxima of sea level in these hemispheres are leached in the last weeks of September and March, respectively. At subpolar latitudes, semiannual components are observed in each hemisphere, which are consistent with those of sea surface temperature.