Spring chlorophyll changes in relation with mixed layer variability in the East Sea (Japan Sea)

Abstract

Year-to-year variability of chlorouhyll-a (Chl-a) in the East Sea and its relation with mixed layer depth (MLD) changes are investigated by using Chl-a concentration data estimated from satellite (SeaWiFS and MODIS) measurement. MLD was calculated from 1/12° Global HYbrid Coordinate Ocean Model (HYCOM) for the period (2004&#8722 2010). In 2008, spring CHL-a concentration in the Ulleung Basin reaches its maximum. This anomalous bloom can be attributed to relatively deep winter mixed layer that can entrain more nutrients into the upper ocean. Comparison of MLD with surface atmospheric forcing (wind and surface heat flux) suggests that the deep MLD was probably caused by a strong wind due to a strengthened Siberian high and Aleutian low, and the associated intensified surface cooling. On the other hand, spring CHL-a concentration in 2004 was not increased although the winter MLD was considerably deep. A deeper spring MLD in 2004 than normal years appears to contribute to an unfavorable light condition for spring bloom, thus resulting in the relatively low Chl-a concentration. Our finding suggests that, in addition to winter MLD, spring MLD also plays a crucial role in interannual variability of CHL-a in the East Sea. The low correlation between Asian dust and CHL-a concentrations suggest that Asian dusts did not likely influence on the 2008 spring bloom in the East Sea.culated from 1/12° Global HYbrid Coordinate Ocean Model (HYCOM) for the period (2004&#8722 2010). In 2008, spring CHL-a concentration in the Ulleung Basin reaches its maximum. This anomalous bloom can be attributed to relatively deep winter mixed layer that can entrain more nutrients into the upper ocean. Comparison of MLD with surface atmospheric forcing (wind and surface heat flux) suggests that the deep MLD was probably caused by a strong wind due to a strengthened Siberian high and Aleutian low, and the associated intensified surface cooling. On the other hand, spring CHL-a concentration in 2004 was not increased although the winter MLD was considerably deep. A deeper spring MLD in 2004 than normal years appears to contribute to an unfavorable light condition for spring bloom, thus resulting in the relatively low Chl-a concentration. Our finding suggests that, in addition to winter MLD, spring MLD also plays a crucial role in interannual variability of CHL-a in the East Sea. The low correlation between Asian dust and CHL-a concentrations suggest that Asian dusts did not likely influence on the 2008 spring bloom in the East Sea.