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How well do we know the mean ocean dynamic topography?

Femke Vossepoel(1,2), Peter Jan Van Leeuwen(1) and Radboud Koop(2)

(1) IMAU, Princetonplein 5, NL-3584 CC Utrecht, Netherlands
(2) SRON, Sorbonnelaan 2, NL-3584 CA Utrecht, Netherlands

Abstract

After more than 10 years of satellite altimetry, a spatially homogeneous, continuous data set of sea level has become available. Together with increasingly accurate estimates of the geoid, the altimeter data can be used to improve observational estimates of the mean dynamic topography (MDT) of the ocean. At the same time, advanced mixing schemes and increased resolution of numerical ocean models have led to realistic model estimates of MDT. In this study we investigate the present-day accuracy of MDT estimates from both models and observations, and quantify their mutual differences. A comparison of four observational estimates illustrates that rms differences in MDT vary from 4 to 19 cm at spatial scales of one degree, reducing to 3-8 cm for larger scales. Differences in data sources (geoid model, in-situ data) are mostly visible in the small-scale oceanic features, while differences in processing (filtering, inverse modelling techniques) are reflected at larger scales. The MDT estimates of seven different numerical ocean models are compared. Model estimates differ mostly in western boundary currents and in the Antarctic Circumpolar Current. These differences can be attributed to differences in wind-stress forcing, parameterization of sub-grid-scale processes, and spatial resolution. Rms differences between modeled and observed MDT are at best 19 cm at spatial scales of 1 degree, and reduce to a 5 cm level for spatial scales of 24 degrees. A comparison of low-pass filtered MDTs demonstrates that differences between MDT estimates reduce with increasing spatial scales. This reduction is smaller than expected, and suggests that GOCE will not only improve MDT estimates at small spatial scales, but also at the larger scales.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry