Abstract
Previous investigations of upper ocean currents on a β-plane have shown that it is quite difficult for a parcel of fluid to cross the equator in the open ocean. Boundary currents sometimes can cross the equator, but even this crossing is not easily achieved. The main barrier for equatorial crossing of inviscid western boundary currents is the presence of a front on the open ocean side (
Nof, 1990,
Deep-Sea Research,
37, 853–875). One-and-a-half and
2
1
2
layer models are used to examine how this frontal blocking constraint is modified by bottom topography. Both models show that some topographic features, such as the Mid-Atlantic Ridge, may entirely relax the frontal blocking constraint.
The single layer crossing is modeled in terms of a heavy double-fronted inertial current (overlaid by a stagnant infinitely deep upper layer) flowing northward in a parabolic channel. Analytic solutions show that the current's position “flips” as it crosses the equator, it is situated next to the left flank of the channel (i.e. the western boundary) in the southern hemisphere and next to the right flank (i.e. the eastern part of the channel corresponding to the western side of the the mid-ocean ridge) in the northern hemisphere.
With the aid of the above model, a
2
1
2
layer model, which contains an additional intermediate current above the core, is considered. It is found that the nonfrontal southward (or northward) intermediate flow crosses the equator and remains adjacent to the western boundary. In contrast, the deep frontal flow underneath again “flips” from the left to the right boundary as it crosses the equator. Possible application of this theory to the dense Antarctic Bottom Water (AABW) and the Lower North Atlantic Deep Water (LNADW) is discussed.
