Not an expert, but I'm going to point out something obvious (maybe not obvious enough?): Icebergs.

Southeast Greenland, unlike the southwest coast, is mostly covered in glaciers and has no ports, settlements, or airports other than temporary military bases. Those glaciers exit into the sea, of course, and the currents along the coast carry them mostly south. We're not talking about the gulf stream but the land-hugging currents that run in the opposite direction. The result is usually a big blob of icebergs off the southeastern tip of the land.

Remember, the Titanic sank at the southern part of those convoy routes. You're talking about shifting them a few hundred miles straight north.