The performance of line arrays is significantly affected by the irregular spatial variations that develop in acoustic waves when they propagate in the ocean. A numerical technique that has been successful in simulating the spatial variations observed in ocean acoustic experiments has been used to study their effect on array performance. Analytical expressions, helpful for describing the output of an array in such a spatially varying sound field, have been derived on the basis of the parabolic second‐ and fourth‐moment equations. The present paper is confined to treating vertically deployed arrays.