Almost all of the long distance Viking voyages (at least the ones done on purpose) weren’t done in the classic dragon-beaked “longship”. Erik the Red’s ship that he took to Greenland, Bjarni Herjolfsson’s ship in which he was blown to Labrador (which was also the one that Leif Eriksson took to Newfoundland) and the four ships that Thorvald Erifsson and Thorfinn Karlsefni used for the colonization attempt at L’Anse aux Meadows in Newfoundland, were a type called a knarr (or knorr). It was a cousin to the longship, clinker built, double ended, with a single mast and a single square sail and a side rudder. But they were longer, beamier, much greater displacement and a lot of freeboard. They had a foredeck and afterdeck but amidship it was open. Their cargo was laid in the open hold and covered with greased tarpaulins.
In Norse graves in Greenland, the men are clothed in hooded, ankle length woolen gowns. Mariners had big sleeping bags made of sheepskin or cowhide. These would have been handy since the partial deck and tarps would have provided little comfort from the spray and continual leaking.
A longship, being long and narrow with almost no freeboard would have broken or been swamped by any half way bad storm. It has been said that, while the longship was an excellent way to terrorize the coastal people of Europe, in the open sea, the only people it scared were its own passengers.
As mentioned above, the climate was better during the Viking Era. Prior to 3000 BC , the Climatic Optimum, the Arctic was a much nicer place to sail. It is possible that the Arctic ice completely melted during the summer. The weather then deteriorated until about the year 0, making it much worse then it is even now. Pytheas, sailing in these waters in 330BC, reported pack ice a day’s sail north of Iceland. About 400 AD, the weather started getting better. By 800 AD, it was much warmer and drier with fewer cyclonic storms than it is now. There was little sea ice in the summer, which probably was the source of the idea that one could sail the Arctic to China. This warmer period, called the Little Climatic Optimum, lasted until about 1200 AD. It was probably only warmer by 4 to 5 degrees but what would have helped the Vikings cope would have been the decrease in violent storms.
So the weather was better and the ships were better than one might have thought. I think the most important factor was that the Norse were some tough hombres. Edward Gibbon wrote that the North was not a nursery for weaklings.
When a bunch of Scandinavians were sailing a longship replica to America for the Chicago World Fair, they stopped in France to kick off one of their party who wanted to sleep under a blanket.