Spatial distribution of seasonality of SeaWiFS chlorophyll-a concentrations in the East/Japan Sea

•Three seasonality patterns of ten-year chl-a time series were identified in the EJS.•The patterns are spatially distinct corresponding to the EJS geographical regions.•The patterns are associated with both climatology and local physical conditions.

To investigate the spatial characteristics of seasonality in phytoplankton productivity over the entire East/Japan Sea (EJS), a wavelet analysis was applied to SeaWiFS chlorophyll-a (chl-a) data over the period from 1998 to 2007. The wavelet analysis revealed that a 6-month period of change in chl-a concentration, explained by spring and fall blooms, was persistent throughout EJS during the ten years examined. The time series seasonality could be further classified into three patterns: (1) the highest variance peak at the 6-month period, (2) double variance peaks with similar values at the periods of 6- and 12-month, and (3) the highest variance at the 12-month period. A spatial map of the ratios between variances at the 6- and 12-month periods illustrated geographical distribution of the chl-a seasonality patterns. The Japan Basin and Ulleung Warm Eddy area were characterized by pattern 1 associated with clear spring and fall blooms. The region near the sub-polar front had seasonality pattern 2 characterized by two similar, weak blooms, and the area of East Koran Bay and the Yamato Basin are distinguished by pattern 3 with strong spring bloom, but irregular and unnoticeable fall bloom due to relatively high chl-a concentrations in wintertime. Distinct regional distribution of the seasonality patterns may suggest that phytoplankton productivity and its seasonal cycles respond not only to basin-scale or mesoscale climate forcing but also to changes in local physical properties characterizing each region.