Climatic forcing and phytoplankton phenology over the subarctic North Pacific from 1998 to 2006, as observed from ocean color data

We investigated phenological changes in phytoplankton in the subarctic North Pacific and the relationship to climatic forcing variability from 1998 to 2006, using ocean color satellite data combined with climatological data. The interannual to 9‐year mean variability in the timing, magnitude, and duration of phytoplankton blooms were examined using SeaWiFS data. Based on 10‐day composites of SeaWiFS data, the date of bloom initiation, peak chlorophyll a concentration, and bloom duration were estimated each year for every 2 × 2 degree grid within the range of 40–60°N and 130°E–120°W. The peak chlorophyll a concentration, date of bloom initiation, and bloom duration were derived by a Gaussian curve fitting technique. In addition, to investigate the geographic pattern of phytoplankton phenology, we classified the oceanic regions of the subarctic North Pacific into three groups based on K‐means clustering: group A was distributed in the coastal regions of marginal seas, waters around the Kamchatka Peninsula, and along the Aleutian Islands; group B was mainly distributed offshore in western and central regions; and group C was in southeastern and central regions. The timing of spring blooms of group A was earlier in El Niño phase and later in La Niña phase, whereas the opposite pattern was seen in group B. Our study clearly revealed regional differences in phytoplankton seasonality, phenology patterns, and relationships between interannual variability of phytoplankton phenology and El Niño Southern Oscillation‐scale variation.