Chlorophyll interannual variability and its relation with mixed layer depth changes in the East Sea

Abstract

We investigated year-to-year variability of chlorouhyll-a (Chl-a) in the East Sea and its relation with mixed layer depth (MLD) changes 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, CHL-a concentration in the Ulleung basin in April reaches a maximum during the period of 2004-2010. This increase in CHL-a concentration 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 in 2004 did not intensified although the winter MLD was considerably deep. A deeper spring MLD in 2004 than normal years appeared 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.ulated from 1/12° Global HYbrid Coordinate Ocean Model (HYCOM) for the period (2004-2010). In 2008, CHL-a concentration in the Ulleung basin in April reaches a maximum during the period of 2004-2010. This increase in CHL-a concentration 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 in 2004 did not intensified although the winter MLD was considerably deep. A deeper spring MLD in 2004 than normal years appeared 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.