Although Lumulea is in fact split by the Sundersea and the Sea of Sorrows, people tend to regard it as one continent, comprised of northern and southern Lumulea. Thus, each hemisphere is dominated by one huge continent, with wide oceans in between. While it is possible to sail between the continents - and people occasionally do - it is far from common. Even wizards seldom cross the oceans, which means that the cultures of the two continents continue to evolve largely along their own lines.
As can be seen from the globes, Arth has no polar ice caps. The great oceans that occupy the northern and southern poles see few mariners, though. They are cold and storm-tossed, and contain few islands of any size. Moreover, mariners are often lost in these waters, led astray by their wandering compasses.