Interannual variability of chlorophyll associated with mixed layer depth changes in the East Sea (Japan Sea)

Abstract

Year-to-year variability of chlorophyll-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 (2004-2010). In 2008, spring CHL-a concentration in the Ulleung basin reaches a maximum during the period of 2004-2010. This CHL-a increase can be attributed to relatively deep winter mixed layer that can entrain more deep 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 intensified 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. In addition to roles of mixed layer depth, Asian dust effects on spring CHL-a concentration variability also will be discussed in the presentation.culated from 1/12° Global HYbrid Coordinate Ocean Model (HYCOM) for the period (2004-2010). In 2008, spring CHL-a concentration in the Ulleung basin reaches a maximum during the period of 2004-2010. This CHL-a increase can be attributed to relatively deep winter mixed layer that can entrain more deep 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 intensified 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. In addition to roles of mixed layer depth, Asian dust effects on spring CHL-a concentration variability also will be discussed in the presentation.