Mesoscale Eddies Control the Timing of Spring Phytoplankton Blooms: A Case Study in the Japan Sea

Satellite Chlorophyll a (CHL) data were used to investigate the influence of mesoscale anticyclonic eddies (AEs) and cyclonic eddies (CEs) on the timing of spring phytoplankton bloom initiation around the Yamato Basin (133–139°E and 35–39.5°N) in the Japan Sea, for the period 2002–2011. The results showed significant differences between AEs and CEs in the timing and initiation mechanism of the spring phytoplankton bloom. Blooms were initiated earlier in CEs which were characterized by shallow mixed‐layer depths (< 100 m). The early blooming preceded the end of winter cooling (i.e., while net heat flux (Q0) is still negative) and is initiated by the increased average light within the shallow mixed‐layer depth. Conversely, blooms appeared in the AEs despite deeper mixed‐layer depth (> 100 m) but close to the commencement of positive Q0. This suggests that the relaxation of turbulent mixing is crucial for the bloom initiation in AEs. Key Points Timing of bloom initiation is early in cyclonic eddies (CEs) compared to anticyclonic eddies (AEs) The difference in the timing of bloom initiation is mainly caused by the difference in the mixed‐layer depth Bloom triggers are increased average light in shallow mixed‐layer CEs and relaxation of convective mixing in deep mixed‐layer AEs