<P> Published estimates of the onset of the Antarctic Circumpolar Current vary, but it is commonly considered to have started at the Eocene / Oligocene boundary . The isolation of Antarctica and formation of the ACC occurred with the openings of the Tasmanian Seaway and the Drake Passage . The Tasmanian Seaway separates East Antarctica and Australia, and is reported to have opened to water circulation 33.5 Ma . The timing of the opening of the Drake Passage, between South America and the Antarctic Peninsula, is more disputed; tectonic and sediment evidence show that it could have been open as early as pre 34 Ma, estimates of the opening of the Drake passage are between 20 and 40 Ma . The isolation of Antarctica by the current is credited by many researchers with causing the glaciation of Antarctica and global cooling in the Eocene epoch . Oceanic models have shown that the opening of these two passages limited polar heat convergence and caused a cooling of sea surface temperatures by several degrees; other models have shown that CO levels also played a significant role in the glaciation of Antarctica . </P> <P> Antarctic sea ice cycles seasonally, in February--March the amount of sea ice is lowest, and in August--September the sea ice is at its greatest extent . Ice levels have been monitored by satellite since 1973 . Upwelling of deep water under the sea ice brings substantial amounts of nutrients . As the ice melts, the melt water provides stability and the critical depth is well below the mixing depth, which allows for a positive net primary production . As the sea ice recedes epontic algae dominate the first phase of the bloom, and a strong bloom dominate by diatoms follows the ice melt south . </P> <P> Another phytoplankton bloom occurs more to the north near the antarctic convergence, here nutrients are present from thermohaline circulation . Phytoplankton blooms are dominated by diatoms and grazed by copepods in the open ocean, and by krill closer to the continent . Diatom production continues through the summer, and populations of krill are sustained, bringing large numbers of cetaceans, cephalopods, seals, birds, and fish to the area . </P> <P> Phytoplankton blooms are believed to be limited by irradiance in the austral (southern hemisphere) spring, and by biologically available iron in the summer . Much of the biology in the area occurs along the major fronts of the current, the Subtropical, Subantarctic, and the Antarctic Polar fronts, these are areas associated with well defined temperature changes . Size and distribution of phytoplankton are also related to fronts . Microphytoplankton (> 20μm) are found at fronts and at sea ice boundaries, while nanophytoplankton (<20μm) are found between fronts . </P>

The west wind drift is an example of which of the following types of currents