Keywords:Ocean color, Phytoplankton, Bloom onset, Upwelling
Two-decade ocean color chlorophyll-a concentration (CHL, a metric of phytoplankton biomass) data from 1997 to 2017 were used to comprehend the impact of cyclonic eddies on phytoplankton biomass and phenology in the Kuroshio Extension region (KE) of the western North Pacific Ocean. The CHL data have 4-km and 8-day spatiotemporal resolution and were acquired from the ESA Ocean Colour-Climate Change Initiative (http://www.esa-oceancolour-cci.org/). Prior to defining spring bloom onset, spatiotemporal gaps in CHL data were filled by applying empirical orthogonal function-based interpolation. The analysis was mainly focused on the winter season (January-March) as oceanographic conditions during the winter largely control phytoplankton spring bloom in terms of both magnitude and onset. Based on winter sea surface height anomaly (SSHa), cyclonic eddies (negative SSHa) were dominant mainly within the periods from 1998 to 2001 and from 2005 to 2009, whereas anti-cyclonic eddies (positive SSHa) were dominant mainly within the periods from 2002 to 2004 and from 2011 to 2015. Based on datasets from all seasons, the concurrences of negative SSHa and positive phytoplankton chlorophyll anomaly (CHLa) were observed, albeit not always. This indicates an eddy-induced upwelling supplies nutrients to the near-surface layer. Upwelled nutrients eventually promote phytoplankton growth. Such a concurrence of low SSHa and high CHLa was less obvious during the winter indicating that another factor (such as intense winter vertical mixing) also non-trivially controlled nutrient input to the surface layer, and hence CHLa variability. Overall temporal mean indeed showed that the active cyclonic eddy area (approximately south of the Kuroshio Current main axis) was characterized by high CHLa, low SSHa, and shoaled mixed layer. In this study, a Cumulative Sum method was applied to find phytoplankton spring bloom onset. Correlation between SSHa and spring bloom onset time was investigated, and it was found that approximately over the active cyclonic eddy area, positive correlation between winter/late winter SSHa and time of spring bloom onset was observed. Such a positive correlation indicates that cyclonic eddies during the winter likely tended to advance phytoplankton spring bloom onset.