19th Pacific Asian Marginal Seas Meeting (PAMS 2017), Jeju, KOREA, 11-13 April 2017

Meridional transition of upper-ocean primary productivity
due to diapycnal eddy mixing across the Kuroshio off Japan
Yusuke Uchiyama, Yota Suzue and Hidekatsu Yamazaki
Abstract
The Kuroshio is accompanied by intensive meso- and submesoscale eddies,
which substantially affect biogeochemical processes particularly in the upperocean. We examine primary production around the Kuroshio off Japan with aclimatological ocean modeling based on ROMS (Regional Oceanic ModelingSystem) coupled with a nitrogen-based pelagic NPZD (nutrient, phyto- andzooplankton, and detritus) biogeochemical model in a submesoscale eddypermitting configuration. The model results indicate significant differences in thebiogeochemical responses to eddy activities in the Kuroshio region (KR, offEnshu-nada Sea) and Kuroshio Extension region (KE, off Boso Peninsula). In theKR, persisting cyclonic eddies developed between the Kuroshio and the coastline
are responsible for upwelling-induced eutrophication in principle. However, theeddy-induced vertical nutrient flux counteracts to promote pronounced southand downward diapycnal nutrient transport from the mixed layer to beneath theKuroshio main body, which suppresses the near-surface productivity. In contrast,
the KE has higher productivity by 23.5% than the KR even at comparable eddy
intensity. Upward nutrient transport prevails extensively near the surface due topredominant cyclonic eddies particularly to the north of the KE, whereas thenutrient subduction barely occurs except at depth deeper than 400 m at muchlesser degree than that in the KR. An eddy energy conversion analysis reveals
that a combination of shear instability including topographic eddy shedding andprominent baroclinic instability near the Kuroshio front is essential togenerating eddies in the KR, leading to the increase of the eddy-induced verticalnitrate transport around the Kuroshio.

19th Pacific Asian Marginal Seas Meeting (PAMS 2017), Jeju, KOREA, 11-13 April 2017

On generation and propagation of oceanic mesoscale eddies
in the North Pacific analyzed with an eddy-tracking algorithm
Nobue Okada, Yusuke Uchiyama, Yu Liu, and Changming Dong
Abstract
Mesoscale eddies are ubiquitous in the world ocean that frequently collide withcurrents, leading to substantial alteration of the associated dynamics. For
instance, the Kuroshio plays a major role in preconditioning the coastal
environment around Japan through poleward heat and material transport fromthe subtropics, while its path is known to meander occasionally by eddy
collisions. To investigate the statistical nature of mesoscale eddies and theireffects on the oceanic currents, the velocity-based automated eddy detection andtracking algorism developed by Nencioli et al. (2010) is employed to a 20-year
long dataset derived from the satellite altimetry. It enables us to determine nearsurface geostrophic currents used for eddy detection in the North Pacific (NP) at
a lateral resolution of 0.25o and sampling intervals of a week.
In the NP, eddy kinetic energy (EKE) is highest in the Kuroshio Extension (KE)region off Boso Peninsula as a consequence of eastward release of the Kuroshioinduced eddies that interact with westward propagating eddies (WPEs). The
western-most region of the subtropical zonal band (SZB) around Luzon Strait
has secondarily energetic EKE due to accumulation of WPEs as baroclinic Rossby
waves. In contrast, the eastern NP (ENP) off the U.S. West Coast is a major
generation site where the equatorward seasonal wind causes persistent coastal
upwelling fronts and resultant WPEs. In the middle of the NP, topographies areessential to prompt generation of WPEs particularly around Hawaiian Ridge.
From 1993 to 2012, about 44,000 individual eddies with lifetimes longer than 8weeks are identified in the NP. About 20% of them are detected in the SZB, while
the contributions from the KE and ENP are 5% and 13%. In both the KE and SZB,
counterclockwise rotating, cyclonic eddies are more intensely distributed interms of vorticity magnitude yet with smaller sizes in their diameters than
anticyclonic eddies. On the other hand, the eddy statistics in the ENP exhibit
minor differences between cyclonic and anticyclonic eddies.

19th Pacific Asian Marginal Seas Meeting (PAMS 2017), Jeju, KOREA, 11-13 April 2017

Development of a coastal forecasting systemwith a 3DVAR assimilation for the Seto Inland Sea, Japan
Kenta Kurosawa, Yusuke Uchiyama and Takemasa Miyoshi
Abstract
By optimally combining ocean models with observation data, numerical oceanic
reanalysis and forecast systems allow us to predict the ocean more precisely. Ingeneral, data assimilation is exploited to prepare the initial condition for the forecast.
This technique has widely been employed in atmospheric prediction, whereas oceanic
prediction lags behind weather forecast. Accurate oceanic prediction systems havebeen demanded for operational purposes such as for fisheries, vessel navigation,
marine construction, offshore platform management, marine monitoring, etc.
The variational method is one of the major approaches in data assimilation. Based on
the maximum likelihood estimation theory, it attempts to obtain ocean state estimatesby minimizing a cost function defined as a sum of quadratic terms of errors in model
background states and observations. Three-dimensional variational (3DVAR) data
assimilation identifies the best estimate at a specific analysis time using observations
obtained around the time. Several research groups have developed realistic data
assimilation systems, some of which adopt the variational assimilation scheme.
In the present study, we develop an assimilation system toward coastal data prediction
around Japan based on a 3DVAR assimilation scheme for Regional Oceanic
Modeling Systems (ROMS). A high-resolution coastal circulation modelingencompassing the western part of the Japanese coastal region around the Seto Inland
Sea (SIS), Japan, is configured to improve the representation of coastal processes. In
crowded harbors and estuaries including the SIS, data assimilation has barely been
adapted because data from satellites and Argo floats essential to successful oceanic
predictions is desperately limited. Instead, there are a number of autonomous in-situ
monitoring systems for vertical profiles of temperature and salinity, tens of tidal gages,
along with continuous surface current measurement using HF radars in and around theSIS.
We first describe a theoretical framework of the 3DVAR algorithm optimal for thehigh resolution configuration. Subsequently, the developed 3DVAR is coupled with
the SIS ROMS model to compare the model outcomes against observation. The
comparison indicates that the 3DVAR ROMS model for the SIS outperforms theforward SIS model without assimilation with better model skill scores. We thendemonstrate that the assimilation system is capable of successfully reproducing thecomplex flow field in the SIS as for the complex topography and coastlines with morethan 3,000 islands.

19th Pacific Asian Marginal Seas Meeting (PAMS 2017), Jeju, KOREA, 11-13 April 2017

Model assessment of wastewater effluent
on a seaweed farm in Osaka Bay, JapanXu Zhang, Yusuke Uchiyama, and Yota Suzue
Abstract
The Seto Inland Sea (SIS), Japan, consists of several semi-enclosed basins withcomplex coastlines and thousands of islands with substantial effects from theKuroshio. Even though the water quality has been improved adequately becauseof enacted environmental preservation policies, Osaka Bay (OB) located at thenortheastern SIS still suffers from pollution due to dense population in thehinterland. Tarumi Sewage Treatment Plant (TSTP) is the major wastewater
treatment plants in OB near Akashi Strait where energetic and complex tidal flow
occurs. This area is quite famous for seaweed farming (cf., the second largest
prefectural product in the nation), while the local fishermen keep complaining
possible impacts from TSTP effluent on the farm. The local government has taken
an action to construct a new outfall away from the farm to mediate the impact by
seasonal diversion. Therefore, a numerical modeling is needed to assess thepresent situation as well as to further improve the outfall design.
In the present study, we develop a quadruple-nested high-resolution ocean
model based on ROMS. The outermost boundary conditions are provided by
JCOPE2 oceanic reanalysis and the capability of tracking sewage effluent isimplemented into the innermost ROMS-L4 model with horizontal grid spacing of
20 m as an additional Eulerian passive tracer model based on Uchiyama et al.
(2014). The sewage effluent is applied at the locations of the two outfalls as abottom-released freshwater plume at a constant volume rate of 180,000 m3/day.
Compared with normal sewage discharge, the western diversion works well asintended. The eastward mean and eddy-induced sewage effluent transport isdecreased by about 50 % on and around the eastern shore. The tracer
concentration fluctuates mostly at tidal frequency in the western area of thefarm, whereas subtidal variability dominates over tidal fluctuations in theeastern area.