Fig 1 - uploaded by Yasuhiro Hoshiba
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(a) Horizontal geography and (b) vertical topography used in the model. The width of the river mouth is 3 km. The width of the region shallower than 100 m with the angle alpha was changed. The area surrounded by the dashed line shows the area drawn in Figs. 5 and 9.
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Riverine input often leads to high biological productivity in coastal areas. In coastal areas termed as region of freshwater influence (ROFI), horizontal anticyclonic gyres and vertical circulation form by density differences between buoyant river water and sea water. Previous physical oceanography studies have shown that the horizontal pattern of...
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... gyres due to geostrophic adjustment (e.g., Isobe, 2005). There are previous studies related to the physics of the anticyclonic gyre enlarging with the bottom topography, for example, the horizontal gyre tends to be strongly trapped on the shelf and be suppressed toward the offshore direction in areas with a gently sloping bottom topography ( Fig. S1; Tanaka et al., ...
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... an OGCM with an ecosystem model to clarify how the bottom slope angle controls the NPP in phyto- plankton blooms. Model case studies varying the bottom slope angle show how physical processes of vertical circulation and horizontal gyre control various nutrient supplies from the river, b Width of the region shallower than 100 m depth (km) (see Fig. 1). c Slope angle is determined from the shallow area width. the sea-subsurface layer and regeneration. We used two sets of topographies: (1) idealized setting as the same in Hoshiba and Yamanaka (2013) except varying the bottom slope angle and (2) realistic topography, Ishikari Bay in Hokkaido, Japan with rea- listic topography and flat ...
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... version in Yoshikawa et al. (2005) was incorporated into an OGCM, COCO ver. 4.0 (Hasumi, 2002). The four prognostic variables in the ecosystem part are composed of nutrient, phytoplankton, zooplankton and detritus (Fig. S2). We dealt with fresh water and nutrients flowing from a river into the ocean which has a simplified rectangular domain ( Fig. 1(a) and (b)) on the f-plane (i.e., assuming the Coriolis force at a fixed latitude, about 43 °N). The model domain is large enough not to affect the river plume. The horizontal grid sizes are approximately 1 km (x- direction) and increasing from 1 km near the river mouth to 20 km far from the river mouth (y-direction). The model has 34 ...
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... are approximately 1 km (x- direction) and increasing from 1 km near the river mouth to 20 km far from the river mouth (y-direction). The model has 34 vertical layers with the layer thickness increasing from 2 m at the surface to 40 m at the bottom. The depth to the ocean floor varied from 6 m in the nearshore area to 200 m for the offshore area ( Fig. 1(b)). The horizontal diffusion coefficient and the diffusion coefficient along and across the isopycnals were set to typical values in the model 1.0 Â 10 5 cm 2 /s and 1.0 Â 10 6 cm 2 /s, respectively. The vertical diffusion coefficient was 0.5 cm 2 /s that is a somewhat large coefficient relative to the typical value (about 0.1 cm 2 /s; ...
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... idealized setting, we conducted ten experiments (Table 1) changing the bottom slope angle (α in Fig. 1(b)). We call Experi- ment 5 the control case, with a slope angle approximating the actual slope angle of Ishikari Bay. The sinking rate of detritus used is 20 m/day, except in Exp. 1 and 8 in which the sinking rate of detritus is changed (Table 2) referring to Syvitski et al. (1995) and Kriest ...
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... and decomposition of organic matter. However, RE-NPPs are large despite the small S-NPPs in the gentle slope cases, compared with those in the steep slope cases. This is because phytoplankton in the gentler slope easily use RE-nitrates concentrated near the coast due to the shallower depth and the horizontal gyre trapped along the coast (Fig. ...
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... in the sinking rate has much influence on RE-NPP (Fig. 11). Since a higher sinking rate of detritus leads to the con- version of detritus into RE-nitrate at deeper depths, RE-nitrate supply from the subsurface decreases and then RE-NPP ...
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... is easily used in the gentler slope due to the con- version in the shallower depth, as mentioned above. That is even if there is a high sinking rate, the conversion occurs on the shallow ocean floor. As a result, the dependency of the sinking rate on RE- NPPs in the gentle case (Green solid line in Fig. 11) is weaker than those in the steep case (Green broken line in Fig. ...
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... gentler slope due to the con- version in the shallower depth, as mentioned above. That is even if there is a high sinking rate, the conversion occurs on the shallow ocean floor. As a result, the dependency of the sinking rate on RE- NPPs in the gentle case (Green solid line in Fig. 11) is weaker than those in the steep case (Green broken line in Fig. ...
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... also conducted realistic simulations for Ishikari Bay. Tsushima Warm Current forced by geostrophic balance flows from the southwest to the northeast, offshore of Ishikari Bay (Fig. 12). The main current axis does not penetrate into the inside of Ishikari Bay. Anticyclonic gyre occurs in Ishikari Bay, and low salinity water (green-colored area in Fig. 12) from the Ishikari River is trans- ported in order of the upstream and offshore region with time, although the low salinity region toward the offshore direction ...
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... also conducted realistic simulations for Ishikari Bay. Tsushima Warm Current forced by geostrophic balance flows from the southwest to the northeast, offshore of Ishikari Bay (Fig. 12). The main current axis does not penetrate into the inside of Ishikari Bay. Anticyclonic gyre occurs in Ishikari Bay, and low salinity water (green-colored area in Fig. 12) from the Ishikari River is trans- ported in order of the upstream and offshore region with time, although the low salinity region toward the offshore direction shrinks after Day 13, due to the reduced discharge of the Ishikari River (Fig. 2). Low salinity region (orange to red region in Fig. 12; 29.2 $ 33.0) keeps to propagate to the ...
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... Bay, and low salinity water (green-colored area in Fig. 12) from the Ishikari River is trans- ported in order of the upstream and offshore region with time, although the low salinity region toward the offshore direction shrinks after Day 13, due to the reduced discharge of the Ishikari River (Fig. 2). Low salinity region (orange to red region in Fig. 12; 29.2 $ 33.0) keeps to propagate to the upstream from the river mouth, which is the similar transition pattern to that in the control case ( Fig. ...
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... estimated how much of the nitrates are consumed in Ish- ikari Bay case (solid lines in Fig. 13). Not only riverine nutrient input of the Ishikari River but also regeneration on the shelf causes phytoplankton blooms in Ishikari Bay due to flooding of the Ishi- kari River. That is, the time shift of the main nutrient source from RI-to RE-NPPs is similar to that in the control case (Fig. 6). S-NPP in Ishikari Bay case is much ...
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... compared RI-, S-and RE-NPPs in the cases with flat and realistic bottom topographies (Fig. 13). The relationships among RI-, S-and RE-NPPs with the flat and realistic bottoms are con- sistent relationships between the steep and gentle cases. That is, RE-NPP with the flat bottom has smaller values than with the realistic bottom, though S-NPP is very small with both bottom topographies due to the nutrient poor condition in the ...
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... gyres due to geostrophic adjustment (e.g., Isobe, 2005). There are previous studies related to the physics of the anticyclonic gyre enlarging with the bottom topography, for example, the horizontal gyre tends to be strongly trapped on the shelf and be suppressed toward the offshore direction in areas with a gently sloping bottom topography ( Fig. S1; Tanaka et al., ...
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... an OGCM with an ecosystem model to clarify how the bottom slope angle controls the NPP in phytoplankton blooms. Model case studies varying the bottom slope angle show how physical processes of vertical circulation and horizontal gyre control various nutrient supplies from the river, b Width of the region shallower than 100 m depth (km) (see Fig. 1). c Slope angle is determined from the shallow area width. the sea-subsurface layer and regeneration. We used two sets of topographies: (1) idealized setting as the same in Hoshiba and Yamanaka (2013) except varying the bottom slope angle and (2) realistic topography, Ishikari Bay in Hokkaido, Japan with realistic topography and flat ...
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... version in Yoshikawa et al. (2005) was incorporated into an OGCM, COCO ver. 4.0 (Hasumi, 2002). The four prognostic variables in the ecosystem part are composed of nutrient, phytoplankton, zooplankton and detritus (Fig. S2). We dealt with fresh water and nutrients flowing from a river into the ocean which has a simplified rectangular domain ( Fig. 1(a) and (b)) on the f-plane (i.e., assuming the Coriolis force at a fixed latitude, about 43 °N). The model domain is large enough not to affect the river plume. The horizontal grid sizes are approximately 1 km (xdirection) and increasing from 1 km near the river mouth to 20 km far from the river mouth (y-direction). The model has 34 ...
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... are approximately 1 km (xdirection) and increasing from 1 km near the river mouth to 20 km far from the river mouth (y-direction). The model has 34 vertical layers with the layer thickness increasing from 2 m at the surface to 40 m at the bottom. The depth to the ocean floor varied from 6 m in the nearshore area to 200 m for the offshore area ( Fig. 1(b)). The horizontal diffusion coefficient and the diffusion coefficient along and across the isopycnals were set to typical values in the model 1.0 Â 10 5 cm 2 /s and 1.0 Â 10 6 cm 2 /s, respectively. The vertical diffusion coefficient was 0.5 cm 2 /s that is a somewhat large coefficient relative to the typical value (about 0.1 cm 2 /s; ...
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... idealized setting, we conducted ten experiments (Table 1) changing the bottom slope angle (α in Fig. 1(b)). We call Experiment 5 the control case, with a slope angle approximating the actual slope angle of Ishikari Bay. The sinking rate of detritus used is 20 m/day, except in Exp. 1 and 8 in which the sinking rate of detritus is changed (Table 2) referring to Syvitski et al. (1995) and Kriest ...
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... and decomposition of organic matter. However, RE-NPPs are large despite the small S-NPPs in the gentle slope cases, compared with those in the steep slope cases. This is because phytoplankton in the gentler slope easily use RE-nitrates concentrated near the coast due to the shallower depth and the horizontal gyre trapped along the coast (Fig. ...
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... in the sinking rate has much influence on RE-NPP (Fig. 11). Since a higher sinking rate of detritus leads to the conversion of detritus into RE-nitrate at deeper depths, RE-nitrate supply from the subsurface decreases and then RE-NPP ...
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... is easily used in the gentler slope due to the conversion in the shallower depth, as mentioned above. That is even if there is a high sinking rate, the conversion occurs on the shallow ocean floor. As a result, the dependency of the sinking rate on RENPPs in the gentle case (Green solid line in Fig. 11) is weaker than those in the steep case (Green broken line in Fig. ...
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... the gentler slope due to the conversion in the shallower depth, as mentioned above. That is even if there is a high sinking rate, the conversion occurs on the shallow ocean floor. As a result, the dependency of the sinking rate on RENPPs in the gentle case (Green solid line in Fig. 11) is weaker than those in the steep case (Green broken line in Fig. ...
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... also conducted realistic simulations for Ishikari Bay. Tsushima Warm Current forced by geostrophic balance flows from the southwest to the northeast, offshore of Ishikari Bay (Fig. 12). The main current axis does not penetrate into the inside of Ishikari Bay. Anticyclonic gyre occurs in Ishikari Bay, and low salinity water (green-colored area in Fig. 12) from the Ishikari River is transported in order of the upstream and offshore region with time, although the low salinity region toward the offshore direction shrinks ...
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... also conducted realistic simulations for Ishikari Bay. Tsushima Warm Current forced by geostrophic balance flows from the southwest to the northeast, offshore of Ishikari Bay (Fig. 12). The main current axis does not penetrate into the inside of Ishikari Bay. Anticyclonic gyre occurs in Ishikari Bay, and low salinity water (green-colored area in Fig. 12) from the Ishikari River is transported in order of the upstream and offshore region with time, although the low salinity region toward the offshore direction shrinks after Day 13, due to the reduced discharge of the Ishikari River (Fig. 2). Low salinity region (orange to red region in Fig. 12; 29.2 $ 33.0) keeps to propagate to the ...
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... Bay, and low salinity water (green-colored area in Fig. 12) from the Ishikari River is transported in order of the upstream and offshore region with time, although the low salinity region toward the offshore direction shrinks after Day 13, due to the reduced discharge of the Ishikari River (Fig. 2). Low salinity region (orange to red region in Fig. 12; 29.2 $ 33.0) keeps to propagate to the upstream from the river mouth, which is the similar transition pattern to that in the control case ( Fig. ...
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... estimated how much of the nitrates are consumed in Ishikari Bay case (solid lines in Fig. 13). Not only riverine nutrient input of the Ishikari River but also regeneration on the shelf causes phytoplankton blooms in Ishikari Bay due to flooding of the Ishikari River. That is, the time shift of the main nutrient source from RI-to RE-NPPs is similar to that in the control case (Fig. 6). S-NPP in Ishikari Bay case is much smaller ...
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... compared RI-, S-and RE-NPPs in the cases with flat and realistic bottom topographies (Fig. 13). The relationships among RI-, S-and RE-NPPs with the flat and realistic bottoms are consistent relationships between the steep and gentle cases. That is, RE-NPP with the flat bottom has smaller values than with the realistic bottom, though S-NPP is very small with both bottom topographies due to the nutrient poor condition in the ...
Citations
... Coupling LTL marine ecosystem models to ocean general circulation models (OGCMs) and Earth system models enables three-dimensional (3-D) quantitative descriptions of the ecosystem and its temporally fine variability (e.g. Aumont and Bopp, 2006;Follows et al., 2007;Buitenhuis et al., 2010;Sumata et al., 2010;Hoshiba and Yamanaka, 2016). ...
Ecosystem models are used to understand ecosystem dynamics and ocean biogeochemical cycles and require optimum physiological parameters to best represent biological behaviours. These physiological parameters are often tuned up empirically, while ecosystem models have evolved to increase the number of physiological parameters. We developed a three-dimensional (3-D) lower-trophic-level marine ecosystem model known as the Nitrogen, Silicon and Iron regulated Marine Ecosystem Model (NSI-MEM) and employed biological data assimilation using a micro-genetic algorithm to estimate 23 physiological parameters for two phytoplankton functional types in the western North Pacific. The estimation of the parameters was based on a one-dimensional simulation that referenced satellite data for constraining the physiological parameters. The 3-D NSI-MEM optimized by the data assimilation improved the timing of a modelled plankton bloom in the subarctic and subtropical regions compared to the model without data assimilation. Furthermore, the model was able to improve not only surface concentrations of phytoplankton but also their subsurface maximum concentrations. Our results showed that surface data assimilation of physiological parameters from two contrasting observatory stations benefits the representation of vertical plankton distribution in the western North Pacific.
... Nutrients fuel phytoplankton productivity in coastal and shelf sea systems, once they are supplied by the buoyant outflows from rivers (e.g., Rabalais et al., 2002;Kudela et al., 2010;Schofield et al., 2013;Hoshiba and Yamanaka, 2016) and straits (Macias et al., 2010;Souvermezoglou et al., 2014;Sanchez-Garrido et al., 2015;Oguz et al., 2016). The anticyclonic bulges formed to the right of the outflow plumes may trap large amounts of fresh water and macronutrients to serve as sources of nutrients (Kudela et al., 2010). ...
Rivers transport freshwater and suspended sediment matter (SSM) from land to coastal seas. In coastal seas termed as regions of freshwater influence (ROFIs), SSM is not only passively transported but also changes the density of ambient water and influences the physical characteristics especially in flood events, when a lot of SSM is supplied to the sea. Although the influence of SSM on the physical field in ROFIs would be significant, interactive physical processes, such as dynamics of river plumes and estuarine circulations, have hardly been investigated for hypopycnal plumes (i.e., the riverine sediment-freshwater is not denser than the seawater). In order to quantitatively estimate the interactive effects of SSM, we employ a non-hydrostatic ocean model with Lagrangian particles, which represents SSM and affects the density and buoyancy of ambient water. We use two experimental settings: (1) realistic simulations of the Tango Bay, Japan under the flooding of September 2013 and (2) idealized simulations for an open-bay ROFI. The former is conducted to assess whether the simulations could reproduce an actual event to some extent. The realistic simulations demonstrate that the choice of parameters such as SSM-particle size and composition is important for coastal simulations of flood events. The latter is conducted to understand the basic physics and to study the quantitative sensitivities of the physical processes to the riverine flux, composition, and particle size of SSM. The idealized simulations demonstrate that a large amount of riverine SSM affects the physical field in ROFIs through the following process: 1) horizontal density differences between nearshore and offshore waters are reduced as apparent density is increased close to the river mouth by riverine SSM, 2) the strength of vertical circulation is weakened by the reduced horizontal density difference, and 3) vertical water exchange between the surface and the subsurface layers decreases. The process in the control case of this study increases the relative amounts of surface freshwater in the river plume by 0.8% of the total riverine freshwater input. Sensitivity experiments with changing the parameters of SSM flux into the river, SSM composition, and SSM-particle diameter show that the percentage can be raised up to 2%. Meanwhile, the above-mentioned processes do not apply to extreme cases of small particle size and enormous SSM input wherein homopycnal (i.e., a similar density between the riverine sediment-freshwater and the ambient seawater) and hyperpycnal (i.e., the density of the riverine sediment-freshwater mixture exceeds the ambient seawater) plumes take place.
