Manifestation of Spiral Structures under the Action of Upper Ocean Currents

Abstract

The traditional approach to the interpretation of spirals observed in radar, optical and radiometric panoramas of a sea surface is based on equating the outer spiral scale with the scale of a manifesting eddy, but the validity of this approach has been poorly studied. Using the maximum cross-correlation (MCC) method for multispectral satellite images containing a spiral structure, we found a significant discrepancy between the structures of horizontal velocity fields and the geometrical characteristics of spiral structures in each band. Each velocity field demonstrated a pair of points of zero velocity with a km-scale difference between their positions in different bands. In order to describe the observed features, an analytical description of the upper-ocean current composed of a spiral eddy and of a homogeneous drift (related, in particular, to wind forcing) is proposed. This simple model states that the spiral characteristics and the position of the spiral center depend on a drift current even when the genuine characteristics of the marine eddy are fixed. The studied example shows that the diameter of an eddy core may significantly (2–3 times) differ from the outer scale of the spiral, which demonstrates the incorrectness of the traditional approach of spiral structures interpretation.