ISOPYCNIC EOFS IN THE AZORES CURRENT REGION - A STATISTICAL TOOL FOR DYNAMICAL ANALYSIS AND DATA ASSIMILATION

An empirical orthogonal function analysis of the vertical space-time v ariability of the thermohaline structure of the ocean in summer and fa il 1993 in the Azores Current region has been performed. The analysis used data from CTD casts of the SEMAPHORE intensive experiment, referr ed to as a seasonally varying climatology. The relative merits of dept h-coordinate and isopycnal-coordinate representations have been examin ed in regard to the quality of identification of physical processes an d to the effectiveness of the extrapolation from a sea level anomaly i n an attempt to improve the treatment of the vertical dimension in oce anic assimilation problems. The isopycnal EOFs consistently proved mor e efficient in capturing the vertical structure of both dominant proce sses in the area of investigation: the variability linked to the Medit erranean Water, and the coherent physical system made up of the Azores Front and the Azo;es Current. The isopycnal analysis also proved more robust when meddles (Mediterranean Water lenses) were included in the analysis. In addition, isopycnal EOFs appeared to be more observable from altimetry: Residual sea level anomaly variance after projection o n the dominant mode (representing the Azores Front/Azores Current) was 8.4 cm(2) in depth-coordinate representation and 3.6 cm(2) in isopycn al-coordinate representation. As an attempt to estimate the vertical s tructure of errors needed by assimilation schemes, differential isopyc nal EOFs were calculated from pairs of casts dose in space and time. D espite that only 15 pairs were available for the chosen radii, the dom inant process was a quasi-homogeneous vertical displacement of isopycn als with quasi-conservation of water masses and potential vorticity on the isopycnal grid.