The variation in the phytoplankton biomass over a decadal time scale, and its relationship with the Antarctic Circumpolar Wave (ACW) and climate change, has been poorly interpreted because of the limited satellite chlorophyll a (chl a) data compared with the physical parameters from satellite. We analyzed a long-term chl a data set along the Japanese Antarctic Research Expedition (JARE) cruise tracks since 1965 to investigate inter-annual variation of phytoplankton biomass. In the Southern Ocean, increasing trends of chl a and the spreading of higher chl a area to the north with 3--7 year cycles were found. Although relationships between the decadal change in chl a and climate change such as variation of sea ice extent and the El Ni¿o are still obscure, large variation of primary production in proportion to the chl a is implied.