There's a possibility of topographical noise stripping there, as 60hz is right in the sweet spot of distant shipping on a Wenz curve. The depths look plenty deep for CZ propagation, too. But again like I said earlier, I don't know what the ambient noise modelling is like in the game (I would suspect it's based on Wenz curves though).
In other words, the sub in the shallower water is seeing ambient noise from distant shipping stripped off, while the sub in deeper water is still seeing it, so the signal to noise for him is significantly poorer.
In other words I don't think that upslope/downslope propagation, like you guys are postulating, is particularly relevant here. The water is quite deep, whereas that phenomenon is more likely to occur in slope/shelf areas as far as I can recall.
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