The Justification for narrowing the search zone...
I've been watching the Fox and CNN coverage this morning, where all the theorists are bemoaning the lack of debris and whether or not the black-box pinger batteries are almost spent. It would appear that they are totally disregarding any scientific nexus between the lack of a continuous pinger signal and the total absence of debris.
To me the answer is self-evident:
a. The pinger detections are 2nd or third convergence zone (which in deep water puts the impact area some 100's of miles away from the pinger detections locus - in any direction. And the debris field may be further on in the opposite direction (to the pinger-ensonified area). i.e. debris has floated one way in its wind and current-affected splaying migration, and the pinger detection location is anything up to 180 degrees away directionally (i.e. presently being detected somewhere in the opposite hemisphere of this very largish encirclement of the actual impact area).
b. Because of the I.O.'s sea-floor topography and depth, and the fact that the acoustic sound source for MH370 pingers is ON the bottom, the usual assumptions about bottom bounce propagation over great distances will be to some extent inapplicable. ASW experience has always dealt with a sound source "hiding" beneath a layer (i.e. a steep thermocline) at some intermediate depth - not emanating from a "bottomed" target. In the case of MH370, if it has settled within a sea-floor canyon, anywhere near its vertex, you will have a focused (and thus strengthened) pinger signal compounding the convergence zone effect. There is also an attenuation effect due to any silt and irregularities in the bottom contours (both vertically and horizontally). That can lead to the acoustics displaying a rippling, frequency modified and highly directional characteristic. Those "ripples of sound" have (in my opinion) been passing through the present search area - as they wax and wane. That is a distinct deviation from the toroidal annulus that is the normal ASW experience (the afore-mentioned distorted and incomplete doughnut of sound that reaches up to the towed arrays and passive listening air-deployed sonobuoys).
c. To sum up, it's apparent that the vagaries of sound propagation from a bottomed pinging source in deep water has seduced the searchers into a misbegotten belief that their target is proximate. I doubt very much that it is.