YEAR-TO-YEAR VARIABILITY OF CHLOROPHYLL-A IN RELATION WITH MIXED LAYER DEPTH CHANGES IN THE EAST SEA (JAPAN SEA)

Abstract

We examine year-to-year variability of springtime chlorophyll-a (CHL-a) concentration in association with ocean mixed layer depth (MLD) in the East Sea (Japan Sea) by analysing monthly-mean data of MLD (1/12° Global HYbrid Coordinate Ocean Model (HYCOM)), and of satellite (SeaWiFS and MODIS) CHL-a for the period (2004-2010). The spring CHL-a concentration shows substantial year-to-year variability. In 2008, CHL-a concentration in the Ulleung basin in April reaches a maximum (8 times larger than the other years) for the period (2004-2010). The increase of the spring CHL-a concentration in 2008 is attributed to considerable deeper winter mixed-layer (about 2 times larger than normal years) that probably entrains more deep-ocean nutrients into the upper ocean, making favorable spring bloom condition. The larger winter MLD in 2008 is caused by intensified wind and ocean surface cooling that are associated with strengthened Siberian high and Aleutian low. On the other hand, spring CHL-a in 2004 is not elevated significantly, although the winder MLD deepens considerably. We will present possible mechanisms (different springtime stratification, circulation, and available nutrients etc.) to explain the difference responses of the phytoplankton to MLD changes. Model (HYCOM)), and of satellite (SeaWiFS and MODIS) CHL-a for the period (2004-2010). The spring CHL-a concentration shows substantial year-to-year variability. In 2008, CHL-a concentration in the Ulleung basin in April reaches a maximum (8 times larger than the other years) for the period (2004-2010). The increase of the spring CHL-a concentration in 2008 is attributed to considerable deeper winter mixed-layer (about 2 times larger than normal years) that probably entrains more deep-ocean nutrients into the upper ocean, making favorable spring bloom condition. The larger winter MLD in 2008 is caused by intensified wind and ocean surface cooling that are associated with strengthened Siberian high and Aleutian low. On the other hand, spring CHL-a in 2004 is not elevated significantly, although the winder MLD deepens considerably. We will present possible mechanisms (different springtime stratification, circulation, and available nutrients etc.) to explain the difference responses of the phytoplankton to MLD changes.