Chlorophyll blooms in the Antarctic Zone south of Australia and New Zealand in reference to the Antarctic Circumpolar Current fronts and sea ice forcing

Using new high-resolution maps of the frontal structure of the Antarctic Circumpolar Current (ACC) derived from satellite altimetry, the relationship between circulation, sea ice concentrations, availability of light, and surface chlorophyll in the Southern Ocean south of Australia and New Zealand is investigated. South of the Polar Front, in the Antarctic Zone (AZ), there is a distinct spring bloom, which occurs earlier, persists for longer, and reaches higher maximum values farther south near Antarctica. The seasonal changes in chlorophyll pattern in this zone are predominantly driven by the sea ice meltwater flux which improves the irradiance-mixing regime via formation of a very shallow mixed layer and provides an additional source of iron which triggers the chlorophyll blooms. There is a clear and consistent increase of about fivefold in both mean and maximum chlorophyll concentrations in regions of largest decrease in monthly mean sea ice concentrations. The seasonal signal in chlorophyll concentrations related to the ice melting is particularly strong between the southern ACC front and the southern boundary of the ACC. The changes in wind-driven mixed layer depth in the AZ in summer do not explain adequately the elevated surface chlorophyll concentrations observed between these two ACC fronts. Reduction in the cloud cover and consequent increase in photosynthetically active radiation near the Antarctic coast in the summer could also support higher phytoplankton concentrations within the seasonal sea ice zone and costal polynyas.