In the subtropical gyre of the northwest Pacific Ocean, low nutrients and low chlorophyll condition is observed throughout the year. Recent study indicates that high chlorophyll patches are regionally or temporally observed, which are induced by disturbances such as typhoon, cyclonic eddy. But very little is known about the physical processes causing high chlorophyll condition. In the present study, physical and biogeochemical data are obtained by autonomous profiling float “Argo floats” to clarify temporal and spatial variations of each process.

We used Argo float data from 23 February 2008 to 23 May 2010. We obtained vertical profiles of temperature, salinity, and chlorophyll-a concentration from 200 m to 10 m depth. Mixed layer depth was defined as the first depth where density was 0.125 kg m^-3 higher than surface water.

We obtained 80 profiles which cover the four seasons. The seasonal variation of temperature, chlorophyll-a concentration and mixed layer were not so intense compared to subarctic oceans but detected. Additionally, there is a negative correlation between chlorophyll-a concentration and mixed layer depth. We obtained 4 anomaly profiles showing high chlorophyll-a condition (Smirnov test P<0.05). Finally, we discussed causes of high chlorophyll condition from these anomaly profiles.