Relationship of TOPEX/Poseidon altimetric height to steric height and circulation in the North Pacific

Abstract

TOPEX/Poseidon altimetric height is compared with 20 transpacific eddy‐resolving realizations of steric height. The latter are calculated from temperature (expendable bathythermograph (XBT)) and salinity (expendable conductivity and temperature profiler (XCTD)) profiles along a precisely repeating ship track over a period of 5 years. The overall difference between steric height and altimetric height is 5.2 cm RMS. On long wavelengths (λ<500 km), the 3.5 cm RMS difference is due mainly to altimetric measurement errors but also has a component from steric variability deeper than the 800 m limit of the XBT. The data sets are very coherent in the long‐wavelength band, with coherence amplitude of 0.89. This band contains 65% of the total variance in steric height. On short wavelengths (λ>500 km), containing 17% of the steric height variance, the 3.0 cm RMS difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the XBT section. The 2.4 cm RMS difference in the basin‐wide spatial mean appears to be due to fluctuations in bottom pressure. Differences between steric height and altimetric height increase near the western boundary, but data variance increases even more, and so the signal‐to‐noise ratio is highest in the western quarter of the transect. Basin‐wide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement. The 1.9×104 m2 s−1 RMS difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current. A linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data. Errors diminish from 0.8°C at 200 m to 0.3°C at 400 m. Geostrophic volume transport, 0–800 m, shows agreement that is similar to surface transport, with 4.8 Sverdrup (Sv) (106 m3 s−1) RMS difference. The combination of altimetric height with subsurface temperature and salinity profiling is a powerful tool for observing variability in circulation and transport of the upper ocean. The continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized. This includes salinity measurements, which significantly reduce errors in specific volume and steric height.