RELATION BETWEEN SEA-LEVEL AND BAROMETRIC-PRESSURE DETERMINED FROM ALTIMETER DATA AND MODEL SIMULATIONS

The relation between sea level and barometric pressure and, specially, the validity of the inverted barometer (IB) approximation is examined over the global oceans, using nearly 2 years of TOPEX-POSEIDON altime ter measurements. Both crossover differences and collinear differences between consecutive cycles are utilized in this study. Linear regress ions between barometric pressure and sea level time series yield coeff icients between 0.8 and 1 cm/mbar poleward of 20 degrees and as low as 0.5 cm/mbar in the equatorial regions. Such deviations from the IB va lue of 1 cm/mbar can be due to the presence of data errors or to corre lations between pressure and adjusted sea level (i.e., sea level corre cted for IB effect). A simple error model for the pressure fields and a number of sensitivity tests are used to evaluate the changes in the regression coefficient possibly induced by data errors (pressure error s, altimeter measurements errors, and radial orbit errors). The combin ed (root-mean-square) effect of the different errors amounts to 0.8 mm /mbar poleward of 20 degrees and 1.8 mm/mbar within 20 degrees of the equator, in general smaller than the observed deviations from the IB v alue. Regression coefficients thus imply a correlation between adjuste d sea level and pressure. Results from a shallow-water, global ocean m odel forced by realistic wind and pressure fields corroborate this fin ding. The model is able to explain the observed coefficients: within m easurement errors, with wind-driven effects being most important in ac counting for differences from the simple IB model. Pressure-forced dyn amical signals cause maximum deviations of only 1 mm/mbar. The analyse s point to the general validity of the IB approximation over the deep oceans but also highlight the complex relation between sea level and b arometric pressure resulting from correlations between various sea lev el signals.