[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-P06] Variability in phytoplankton Chlorophyll-a in the Kuroshio western boundary current: a result from 14-year observation by satellite remote sensing

*Takafumi Hirata¹, Keiko Sato¹, Satoshi Kitaoka (1.Arctic Research Center, Hokkaido University)

Keywords:phytoplankton, chlorophyll, Kuroshio, Remote Sensing

The Kuroshio western boundary current transports thermal energy and water mass from low latitudes to mid-latitudes in the Western Pacific and its water is characterized relatively oligotrophic by low Chlorophyll-a concentration (Chla). In spite of the oligotrophy, spawning and feeding grounds of fish are located along the Kuroshio, resulting in a higher fisheries production in the region. How the oligotrophic environment can support the high fishery production is unclear. To better understand the Kuroshio ecosystem, how a standing stock of primary producer varies and how higher trophic organisms follow it, spatio-temporal variability of Chla in and around the Kuroshio current between 125-140°E is investigated using 14 years (2006-2020) of monthly satellite remote sensing data. The surface water in the region was classified according to seasonality of Chla, to find out that a dominant mode of seasonality on the Kuroshio is characterized by both spring bloom and winter increase. The spring and winter increases in Chla were approximately 8-month away from each other within a specific year. They showed no correlation with each other, implying that there is no “biological memory” kept from one to the other. Anomalously-High Seasonal Amplitudes (AHSA, defined as a peak Chla value exceeds 2 or 3 times its standard deviation at an observation point of interest) were also found in the Kuroshio, most evidently between Year 2010 and 2012. One-by-one seasonal peak analysis using satellite images showed that the AHSA were found either when the Kuroshio passes through a very high Chla water or when turbulent water patches with high Chla horizontally intrudes into the Kuroshio. The latter implies a possibility of intermittent supply of high Chla into the Kuroshio by local turbulences even at smaller temporal scales not resolved by our satellite data. Time series analysis showed that there is seemingly a periodic interannual variability. We suspect that the seemingly periodic inter-annual oscillation of Chla may result from random turbulences of water mass in local scale in and along the Kuroshio, which implied that the variability may merely reflect a seasonal modulation of Chla rather than an existence of interannual variability caused by a mechanism independent of the seasonal modulation. The estimation of a 14-year trend using two low-pass filtered Chla time series (> 1.5 years variability included, and >7 years variability included) and a regression analysis did not show a statistically significant difference. Thus, there was no significant effect of the interannual variability on the 14-year trend.