The transatlantic air corridors that aircraft utilize over Newfoundland, the tip of Greenland, Iceland, and Ireland are great circle routes. I understand that the volcanic ash from the Icelandic volcano's is spreading all over northern Europe, but for times when volcanic activity is imminent and there are no ash clouds in the North Atlantic, why couldn't aircraft use a more southerly route?

So, flying from Chicago to London, using a southerly route rather than the great circle route, one would actually fly from Chicago, south east towards the RDU area, leave the continent off the North Carolina coast and fly closer to Bermuda, the Central Azores and Brest/Normandy before flying over the English Channel to reach London.