Some mechanisms of oceanic mixing revealed in aerial photographs

Estimates of circulation rates within the mixed layer of the ocean have been made based on aerial photographs of a variety of dye injections and floating cards in the surface and near-surface levels under different wind, sea state, and thermal profile conditions. The following are the main features derived from our studies of the circulation patterns in the mixing layer in the Bermuda area. (1) Except under calm conditions, the first few meters of the sea are subjected to helical flow of small-size Langmuir cells with 3 to 6 meter spacing between the convergence lines. (2) Under moderate to strong wind conditions (10-30 knots), a hierarchy of larger-size Langmuir cells is developed. The maximum horizontal spacing between the larger cells is approximately the same as the depth of the mixing layer. The spa•cing of the convergence zones between the largest cells was 280 meters, and they were accompanied by medium and small-size cells of approximately 35 and 5 meters, respectively. Estimates of the apparent vertical diffusion associated with the large and medium Langmuir cells are thousands and hundreds of cm2/sec, respectively. (3) A stratified flow resembling an Ekman spiral is observed under moderate conditions. (4) There is coexistence between the small-size Langmuir cells and the Ekman spiral below. However, when the large Langmuir cells are developed, the flow pattern in the mixing layer does not resemble an Ekman spiral. The transition from the Ekman to the Langmuir regime occurs with turbulence Reynolds numbers of approximately 100. (5) Under calm conditions following a period of moderate winds and thermal instability, we observed convergence zones having anticyclonic rotation with inertial period.