The Antarctic Convergence or Antarctic Polar Front is a curve continuously encircling Antarctica, varying in latitude seasonally, where cold, northward-flowing Antarctic waters meet the relatively warmer waters of the subantarctic. Antarctic waters predominantly sink beneath subantarctic waters, while associated zones of mixing and upwelling create a zone very high in marine productivity, especially for Antarctic krill. This line, like the arctic tree line, is a natural boundary rather than an artificial one like a line of latitude. It not only separates two hydrological regions, but also separates areas of distinctive marine life associations and of different climates. There is no Arctic equivalent, due to the amount of land contiguous with the northern polar region.