Fig. 2

Time series of key variables measured or derived from float 6901516 from September 2013 to July 2014. a Daily average surface heat flux (black continuous lines) and the maximum surface heat flux at 15 h GMT, close to the local noon (black dashed lines). b Mixed layer (H, black circles) and euphotic layer depths (H_e, gray circles). A zoomed view of H and H_e for depths shallower than 100 m is included as an inset. c Mixed layer averaged phytoplankton carbon biomass concentration (P_ml, blue circles), and depth-integrated phytoplankton carbon biomass (\({\langle P \rangle}\), yellow circles). The continuous lines in panels a–c represent 24-day running averages that remove short-term fluctuations. d Net population growth rate, r_p. e Rate of change of mixed layer depth, \(\frac{1}{H}\frac{\partial H}{\partial t}\), computed from the 24-day average mixed layer depth. The first vertical line marks the initiation of the weak winter accumulation phase, computed as the time when r_p becomes positive for at least 24 days. The second vertical line marks the initiation of the spring bloom, computed as the time when \(\frac{1}{H}\frac{\partial H}{\partial t}\) becomes negative for at least 24 days