Interannual variability in the magnitude and timing of the spring bloom in the Oyashio region

Inter-annual variability in the magnitude and timing of the spring bloom was investigated for the Oyashio region (40 °-48 °N, 143 °E-152 °E) using 10 years (from 1998 to 2007) of satellite ocean-color data. Geostrophic currents were examined using satellite altimeter data. Early spring blooms (>1.5 mg m−3) occurred in early April 2001 and late March 2002. The 2001 bloom continued for one month. Late blooms occurred from mid-May 1999, early June 2004 and late April 2006, continuing for about 1 month, 8 days and 16 days, respectively. A strong bloom (4.7 mg m−3) also occurred in mid-April 1998; however, it terminated in early May. We classified the Oyashio region based on the pattern of temporal variation of Chl-a concentration from March to June. The spatio-temporal variability in Chl-a concentration during spring was different among years. The area where Chl-a concentration was highest in April was more extensive in 2001, 2002 and 2006 than usual. In 1999, the area where Chl-a concentration was highest in May was the widest among the 10 years. Mesoscale eddies and currents with high velocity were frequently observed in the area of high Chl-a concentration east of Hokkaido, propagating Coastal Oyashio Water of low salinity and low density into the oceanic region. That strengthened stratification in the surface layer. We suggest that this seaward transfer of coastal water could be one of the important factors for phytoplankton distribution in two ways: (1) horizontal advection of water with high Chl-a concentration and (2) enhancement of stratification in the oceanic region.