In order to evaluate the reliability and applicability of the Empirical Orthogonal Functions(EOFs)in the acoustic inversion of sound speed profile(SSP)and reduce EOF's dependence on the sample data,a methodology is proposed for the achievement of the basis functions for SSP's expansion.By analyzing the oceanographic dynamics which is the main cause of the SSP's variation,the basis functions are obtained naming the Hydrodynamic Normal Modes(HNMs).The HNM basis functions are almost the same as those derived from the EOF method,while HNMs has less dependence on the amount of the sample data.HNMs method has a physically meaningful interpretation,and it could give out the physical parameters which determine the basis functions for the expansion of SSP,and this makes it possible to analyze and evaluate the trustiness and applicability of EOFs.
