Use of Surface Data to Estimate Geostrophic Transport

An extension of the use of altimetric data aimed at inferring the vertical structure of the geostrophic velocity field (and thereby to compute transports) is explored. The method is based on the assumption that altimetry provides a reliable measure of dynamic height (DH), and on the fact that DH and the density field can both be expressed in terms of the DH empirical orthogonal functions (EOFs). It is then argued that when altimetry is complemented by surface density data, it is possible to determine the amplitudes of the two leading EOFs of the mass field, which altogether usually account for a large percentage of the field variance. The method is tested in the western Mediterranean, where historical databases contain enough data as to compute statistically significant EOFs. Results indicate that with altimetric data alone (i.e., DH in the tests), the EOF-based method can estimate the actual velocity field with an uncertainty of about 60% (in terms of total transport). However, if surface density is also available, estimates have an uncertainty of only 20%. Limitations of the method such as the underlying statistical assumptions, data errors, and the assumed equivalence between altimetry and DH are also discussed.