The Drifting Confluence Zone

Abstract

Listen

Translate article

Alongshore migration of a western boundary current separation is investigated with a nonlinear inviscid reduced gravity model on an f plane. Separation due to a collision with an opposing current is considered. It is known that for such a case stationary collision and separation is possible only for boundary currents with “balanced” transports, that is, equal near-wall depths. The authors perturb this stationary solution with a small steplike variation of the opposing current transport and focus on the resulting time-dependent flow. Two different analytical methods to compute the migration rate are used. The first method involves integrated balances, and the second involves the path equation for the separated flow. Using the fist approach, it is found analytically that the flow consists of one current intruding into the area occupied by the other. After an initial adjustment period the intrusion becomes steadily propagating. The width of intrusion is much greater than the width of boundary currents, whereas the migration speed is much smaller than the speed of the currents. The speed of the opposing current intrusion into the main western boundary current is given approximately by the formula C ≈ (g′H)1/2(D21 − D22)2/4D3/22, where H is the undisturbed depth of the main poleward flowing current, D1 is its depth near the wall (nondimensionalized by H), and D2 is the opposing current near-wall (nondimensional) depth. Due to the nonlinearity of the process, the expression for the opposite speed (i.e., the main western boundary current intrusion into the opposing current) has a similar looking but different form. Using the second approach, the original initial value problem is reduced to a time-dependent path equation for the separated current. It is shown analytically that, as should be the case, in the limit t → ∞ the path equation solution is identical to the earlier solution for the steadily propagating intrusion. The migration process exhibits an hysteresis; that is, the progression of the separated currents differs from its corresponding regression. Application of the theory to the collision and separation of the Brazil and the Malvinas Currents is discussed. It is suggested that the observed migrations of the separation latitude may be caused by seasonal changes of the Malvinas Current transport.