WHY ARE SOME BOUNDARY CURRENTS BLOCKED BY THE EQUATOR

Existing nonlinear theories for the effect of the equator on northward-flowing boundary 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 (Anderson and Moore, 1979, Deep-Sea Research, 26, 1-22). It is demonstrated in this paper 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. The above behavior is demonstrated by examining two nonlinear analytical models. First, we 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. It is shown that the flow in the second layer car cross the equator without any traumatic changes (as suggested by Anderson and Moore, 1979), but the core must again separate from the coast in the vicinity of the equator. As in the previous model, the current associated with the core is blocked by the equator. The general picture of the 2 1 2 layers is then of an initial two-layer inertial boundary current flowing toward the equator in the southern hemisphere. In the immediate vicinity of the equator the current splits into two branches. One separates from the coast and turns eastward whereas the other continues flowing northward until it ultimately reaches its own mid-latitude separation point. Possible application of this theory to the New Guinea Coastal Undercurrent is briefly discussed. This current turns offshore at the equator as predicted by our model. © 1990.