Why are Some Boundary Currents Blocked by the Equator

摘要

Existing nonlinear theories for the effect of the equator on northward-flowingboundary jets adjacent to a finite depth upper layer suggest that such inertial currents can flow from the southern to the northern hemisphere with the equator playing no special role. This reprint demonstrates that, in contrast to these flows, nonlinear currents bounded by a front on their open ocean side (i.e. no adjacent upper layer) experience traumatic changes in their structure as they approach the equator. Within a deformation radius away from the equator, they separate from the coast and turn eastward to form a swift narrow jet. The equator blocks their advancement into the northern hemisphere. Consider a simple highly idealized 1 1/2 layer model of a (southern hemisphere) wedge-like boundary current flowing northward with a wall on its left and a front on its right (looking downstream). It is almost a trivial matter to demonstrate analytically that such a current (which is just a special case of the Anderson and Moore flow) cannot penetrate into the northern hemisphere. Instead, it must separate from the coast in the vicinity of the equator. Secondly, a more realistic and more complicated 2 1/2 layer model is considered. In this nonlinear analytical model the current consists of two active layers. As in the previous model, the core of the current is a wedge-like layer (i.e. it is bounded by a front on the oceanic side) but the remaining surrounding field (i.e. the second moving layer) extends to infinity. Possible application of this theory to the New Guinea Coastal Undercurrent is briefly discussed. This current turns offshore at the equator by our model. (JHD)