High‐resolution, high‐accuracy altimeter derived mean sea surface in the Norwegian Sea

Abstract

The mean sea surface topography (MSS) can be optimally estimated from satellite altimetry using a least‐squares inverse method, which uses a complete description of the statistical properties of the various altimetric errors and the signal, and allows an accurate assessment of the error limits. However, it requires the resolution of very large systems. Here, we suggest to degrade this rigorous one‐step strategy to a sequential recovery of wavenumber limited bands of the MSS. As a result, a high‐resolution, high‐accuracy MSS (long‐ and medium‐wavelength signatures) may be mapped onto a large area by splitting it into smaller domains. Such an iterative inverse method has been applied to Geosat altimeter data in the Norwegian sea. During the first step, the MSS is estimated relative to a long‐wavelength a priori geoid, the error spectrum of which is well documented. This allows the deconvolution of the MSS from both the orbit error and the ocean mesoscale signal, provided the area is large enough and the data subsampling dense enough in time and space. In our example, longer wavelengths (over 100 km) are then accurately recovered (7 cm rms under repeat satellite tracks and 14 cm rms elsewhere) over a large area (20% W‐15% E, 55% N‐70% N). In the second step, higher resolution (over 50 km) and higher accuracy (5 cm rms under repeat satellite tracks and 7 cm rms elsewhere) are achieved on a smaller domain (say 6 6%) using a denser data set referred to the MSS recovered in the first step. With this approach, adjacent small domains are coherent within error limits. Spectral analyses demonstrate that this second step significantly improves the signal to noise ratio in the 100#300 km spectral band.

Keywords:

• Mean sea surface
• Norwegian Sea
• altimetry
• inverse method