We present an extensive analysis of the first complete data set in the norther Ionian Sea collected during the Physical Oceanography of the Eastern Mediterranean (POEM) general circulation survey of September 1987. A four water mass structure of the basin is found to be represented by a first internal baroclinic Rossby radius of deformation of 11.8 km. The horizontal correlation scales decrease with depth, and the subsurface flow is dominated by anticyclonic gyres. Large-scale circulation trends of the temperature and salinity covariance matrices are compensated below 200 m, and only the gyre scales (~100 km) persist of intermediate and deep levels. The empirical orthogonal functions of the data set show that a horizontal scale separation exists between the first and higher vertical modes of the dynamic height field. ¿ American Geophysical Union 1993