An Objective Method for Computing Advective Surface Velocities From Sequential Infrared Satellite Images

Using cross correlations between sequential infrared satellite images, an objective technique is developed to compute advective sea surface velocities. Cross correlations are computed in 32 x 32 pixel search (second image) and 22 x 22 template (first image) windows from gradients of sea surface temperature computed from the satellite images. Velocity vectors, computed from sequential images of the British Columbia coastal ocean, generally appear coherent and consistent with the seasonal surface current in the region. During periods of strong wind forcing, as indicated by maps of sea level pressure, the image advective velocities are stronger and more coherent spatially and appear to cross surface temperature gradients; when winds are weaker, the advective velocities correspond better with the infrared temperature patterns, suggesting the increased contribution of the geostrophic current to the surface flow. Velocities determined from coincident, near-surface drogued (5-10 m) buoys, positioned every half hour by internal LORAN-C units in mid-June, show excellent agreement with the image advective velocities. In addition, conductivity, temperature, and depth (CTD) measurements (taken during the buoy tracking) confirm the homogeneity of the upper 10 m, and CTD-derived geostrophic currents are consistent with both buoy and sequential image displacement velocities.