that is driven by global density gradients created by surface heat and evaporation. Definition of subantarctic: political versus scientific Several distinct water masses converge in the immediate vicinity of the APF or Antarctic Convergence (in particular the Subantarctic Surface Water (Subantarctic Mode Water or SAMW), Antarctic Surface Water, and the Antarctic Intermediate Water). This convergence creates a unique environment, noted for its very high marine productivity, especially for antarctic krill. Because of this, all lands and waters situated south of the Antarctic Convergence are considered to belong to the Antarctic from a climatological, biological and hydrological standpoint. However, the text of the Antarctic Treaty, article VI ("Area covered by Treaty") states: "The provisions of the present Treaty shall apply to the area south of 60° South latitude". Therefore, Antarctica is defined from a political standpoint as all land and ice shelves south of 60°S latitude. Subantarctic islands The Tristan da Cunha Group, Gough Island, Amsterdam Island, and Saint Paul Island are all isolated volcanic islands situated at between 37°–40° south of the Equator, just south of the southern horse latitudes. Because they are located far to the north of the Antarctic Convergence and have a relatively temperate climate, they are not typically considered to be subantarctic islands. At between about 46°–50° south of the Equator, in the region often referred to as the Roaring Forties, are the Crozet Islands, Prince Edward Islands, Bounty Islands, Snares Islands, Kerguelen Islands, Antipodes Islands, and Auckland Islands. The geography of these islands is characterized by tundra, with some trees on Snares and Auckland Islands. These islands are all located near the Antarctic Convergence (with Kerguelen south of the Convergence) and are properly considered to be subantarctic islands. At between 51°–56° south of the Equator, the Falkland Islands, Isla de los Estados, Ildefonso Islands, Diego Ramírez Islands, and other islands associated with Tierra del Fuego and Cape Horn, lie north of the Antarctic Convergence in the region often referred to as the Furious Fifties. Unlike other subantarctic islands, these islands have trees, temperate grasslands (mostly tussac grass), and even arable land. They also lack tundra and permanent snow and ice at their lowest elevations. Despite their more southerly location, it is debatable whether these islands should be considered as such because their climate and geography differs significantly from other subantarctic islands. At between 52°–57° south of the Equator, the Campbell Island Group, Heard Island and