We used ocean color data of chlorophyll-a (CHL) from the period 1998 to 2017 to investigate phytoplankton phenology during winter–spring in association with the Kuroshio Extension (KE) instability. In the areas south of the KE, CHLs tended to be higher in winter during periods of unstable KEs (compared to stable KEs) which were attributed to the increases in nutrient and light availability. Nutrients were supplied from the deep layer due to physical processes indicated by negative sea surface height anomalies (SSHAs) and shallow mixed layer depths (MLDs). The increase in light availability could be attributed to greater exposure of phytoplankton to light in the shallower MLD. The same physical processes also explained higher CHLs in spring during unstable KEs. We also found that CHLs could possibly be lower during unstable KEs in spring which might be related to warmer SSTs in winter–spring. On average, the onset of the phytoplankton spring bloom south of the KE tended to be 1–3 weeks earlier during the period of unstable KEs than during the period of stable KEs. Whether this difference of 1–3 weeks impacts high-trophic-level organisms should be investigated in future studies.