Marine Ecosystem Model Team
Biochemical properties of warm eddies reproduced by western Arctic marine ecosystem model
Eiji Watanabe(1)
Michio J. Kishi(1, 2)
Akio Ishida(1, 3)
Maki Noguchi (Aita)(1, 2)
(1)JAMSTEC, Research Institute for Global Change
(2)Hokkaido University, School of Fisheries Sciences
(3)Fuji-Tokoha University, Department of Social Environment
Recent rapid sea ice retreat in the Arctic Ocean and its global/regional influence on atmospheric fields and ecosystem are a crucial topic. The Pacific water inflowing through the Bering Strait is a predominant source of heat, fresh water, and nutrients in the Arctic Ocean. Its transport has a potential to affect both the sea ice variation and marine ecosystem. Many previous studies have indicated that mesoscale baroclinic eddies play an important role in the Pacific water transport from the Chukchi shelf to the Canada Basin. The biogeochemical properties of an unusually large warm-core eddy observed by R/V Mirai in 2010 were reported in Nishino et al. [2011]. However, it is still unclear whether the eddy-like structures of biological signal attributed to lateral advection from the shelf region or were formed by primary production inside the eddies. In this study, the response of phytoplankton to the Beaufort shelf-break eddies in the western Arctic Ocean is examined using the satellite data analysis and the eddy-resolving coupled sea ice-ocean model [Hasumi, 2006] including marine ecosystem formulation of Kishi et al. [2007]. MODIS ocean-color scenes captured high chlorophyll-a distributions associated with warm-core eddies during the summer season in 2003 and 2010 (Figure 1). From the numerical experiment, the biogeochemical properties of shelf-break eddies are categorized into three stages. The sea ice margin is located north of the shelf-basin boundary, and several warm eddies are produced from July to September, as minutely described in Watanabe [2011]. The phytoplankton bloom depletes a great part of nitrate content over the shallow Chukchi shelf before the eddy-generation period, and consequently the initially generated warm eddies transport nutrient-poor shelf water to the Canada Basin interior (Figure 2a). The simulated lower nitrate concentration inside the warm eddies is consistent with the properties obtained by the Mirai cruise in 2010. At the same time, the large biomass of phytoplankton increased in the Chukchi shelf is taken into the warm eddies at the end of July. In the eddy-developing period, the phytoplankton biomass gradually decreases because of the limited primary production due to nutrient depletion and the grazing of zooplankton, which becomes a dominant component of phytoplankton biomass budget. On the other hand, in the eddy-maturity period, the local upwelling flow along the outer side of an individual eddy transports a significant amount of nitrate in the intermediate layer to the surface euphotic zone and enhances the primary productivity in the subsurface layer (Figure 2b). Thus, it is found that the shelf-break warm eddies contribute to biological activities in the western Arctic Ocean. The time lag between the phytoplankton bloom in the shelf region following the summertime sea ice retreat and the eddy generation along the Beaufort shelf break is an important index to determine biological regimes in the Canada Basin. When the phytoplankton bloom in the shelf occurs more later or continues more longer, the shelf-break eddies could enhance the primary productivity in the basin interior. 
Figure 1. MODIS 8-day level-3 composites of chlorophyll-a concentration on (a) 6-13 September 2003 and (b) 21-28 August 2010 [mg/m3]. Scene ID is (a) A20032492003256 and (b) A20102332010240. 
Figure 2. (a) Vertical profiles of (shade) phytoplankton and (contour) nitrate concentration across an individual warm shelf-break eddy on July 31 [μM]. (b) Total phytoplankton biomass in the top 60 m on September 25 [μM m]. Black contours show bottom bathymetry. 1μM≡1mmol-N/m3.
