Some case studies of vertical circulations associated with oceanic fronts

Conventional scale considerations provide a range of 10−3–10−2 cm sec−1 for the vertical motion in the oceans. However, either upward or downward motions could be much higher in uncommon situations, e.g., in the case of oceanic fronts. A simple numerical model was developed to calculate upwelling in these special situations. According to the model the circulation underneath a front is a result of the action of three forces: (1) differential hydrostatic pressure gradient force, a function of the slope of the front and the density contrast between the two water masses, (2) ageostrophic acceleration, a function of the Coriolis parameter and the ageostrophic velocity, and (3) viscous force. Case studies of fronts, two in the Atlantic and three in the Pacific, disclosed that the typical values of the vertical velocities are usually about 0.1 cm sec−1. For these cases the slope of the front varied considerably in the horizontal, and owing to this factor the horizontal current field in a vertical column differed appreciably from that in the adjacent region. This difference induced horizontal convergence or divergence and was most pronounced where the frontal slope reached a sharp maximum. Further analyses of the results revealed that in the region under the front, as it approached the surface, mainly viscous force and the differential hydrostatic pressure gradient force counteract each other. In the upper and middle layers the hydrostatic pressure gradient force and the ageostrophic acceleration balance each other.