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We have developed an assimilation system toward coastal prediction around Japan. The system consists of a four-dimensional variational (4DVAR) assimilation scheme with an eddy-resolving model in the western North Pacific (MOVE-4DVAR-WNP) and fine-resolution coastal models covering western part of the Japanese coastal region around the Seto Inland Sea (MOVE-Seto) and covering all Japan coastal region (SICAT02). An initialization scheme of Incremental Analysis Update (IAU) is incorporated into MOVE-4DVAR-WNP to filter out high-frequency noises. During the backward integration of the adjoint model, it works as an Incremental Digital Filtering. Coastal models MOVE-Seto and SICAT02 are nested within the eddy-resolving model of MOVE-4DVAR-WNP. Temperature and Salinity fields of the two coastal models are forced by MOVE-4DVAR-WNP analysis fields using the IAU scheme.
The MOVE-4DVAR-WNP improves mesoscale variability compared to the 3DVAR version. In particular, short-term variability such as small-scale Kuroshio fluctuations is much enhanced. Using MOVE-Seto with MOVE-4DVAR-WNP we also performed a case study focused on an unusual tide event that occurred at the south coast of Japan in September 2011. MOVE-Seto succeeds in reproducing a significant sea-level rise associated with this event, indicating the effectiveness of the system for coastal sea-level variability. Newest version of the coastal model SICAT02 will be adopted in experiments of sea level variability such as rapid tide (Kyucho) under climate change in a national project of the adaptation to global warming (SICAT). Preliminary result for the analysis of Kyucho shows a relationship to the Kuroshio variability.
Impact of 4D-Var Assimilation of Coastal Altimetry Data in the Sicily Channel Model
The present study focuses on the impact of the assimilation of coastal products (Altika), in respect to their oceanic counterpart (Jason), in the reproduction of the circulation of the Sicily Channel (Central Mediterranean) by an assimilative sub-regional model.
The Sicily Channel Model is a regional implementation of ROMS (Regional Ocean Modeling System). The assimilation system is a 4D-Var assimilation (Moore et al. 2011) developed for ROMS. The experimental setup simulated the year 2014. We performed three experiment: NA) control run with no assimilation; AC): assimilation of coarse resolution data ; AH) Assimilation of high resolution data. All the three simulation have in common the same parameterizations and surface and boundary conditions. Surface forcing is provided from ECMWF (ERA-interim) while boundaries are provided by the Mediterranean Forecasting System (MFS) thorugh the Copernicus service.
Outputs of the three setup are intercompared and evaluated vs an independent set of data including CTD and optical/infrared satellite products able to provide the signature of the circulation features we focus on.
One of the main goal is to correctly reproduce the behavior and path of the Atlantic Ionian Stream (AIS) a meandering current flowing eastward south of Sicily towards the Eastern Mediterranean.
Intercomparison of Sea Level Variation Across the Tsushima Strait Among Tide Gauge Data, a Coastal Altimetry Product and an Ocean Reanalysis FORA-WNP30.
1Meteorological Research Institute, Japan; 2Japan Agency for Marine-Earth Science and Technology, Japan
The Japan Sea is a semi-enclosed marginal sea of the North Pacific Ocean connected to the adjacent seas with shallow and narrow straits. Warm saline water enters the Japan Sea through the Tsushima Strait (TSM) and flows mostly out through the Tsugaru and Soya Straits. The surface circulation of the Japan Sea is largely affected by the variations of heat, salt, and moment through the straits.
Recently, an ocean reanalysis FORA-WNP30 (Four-dimensional variational Ocean ReAnalysis for the Western North Pacific over 30 years) has been produced using a 4-dimensional variational ocean data assimilation system, MOVE-4DVAR (Usui et al., 2015) with an eddy-resolving ocean general circulation model. FORA-WNP30 successfully reproduces not only the major ocean currents such as the Kuroshio and Oyashio but also the associated meso-scale phenomena such as eddies, fronts and meanders. We found that low-frequency variability of volume transport through the TSM is largely in phase with that of the ocean heat content (OHC) in the Japan Sea on interannual to decadal time scales, implying that the variation in volume transport through the TSM is a major cause for the OHC variations in the Japan Sea.
In order to evaluate the reproduction of volume transport through the TSM in FORA-WNP30, we compare the sea levels in FORA-WNP30 with independent data such as tide gauge data and altimetry data around coastal areas. Since sea level difference across the TSM is strongly related to the volume transport through the TSM, tide gauge data are useful for evaluation the volume transport through the TSM. Sea level variation in FORA-WNP30 is largely consistent with tide gauge data around the Japan coast. In addition, we plan to intercompare the sea levels across the TSM among tide gauge data, a coastal altimetry product and FORA-WNP30.
Inter-Annual Variation of the Tsugaru Warm Current Revealed from the Long-Term Coastal Ocean Reanalysis
1Japan Agency for Marine-Earth Scinece and Technology, Japan; 2Meteorological Research Institute, Japan Meteorological Agency
The Tsugaru warm current (TWC) is a buoyant outflow from the Tsugaru Strait in northern Japan. TWC carries warm, saline subtropical water which goes through the Sea of Japan as part of the northward Tsushima Warm Current and spreads over cold, less saline subarctic water at the eastern mouth of the Tsugaru Strait. The path of the TWC outflow is known to exhibit bimodal patterns, the coastal mode and the gyre mode (e.g., Conlon 1982, Mizuno 1984). The transition between two modes occurs regularly in seasonal cycle with inter-annual variations. Dynamics behind its typical seasonal transitions between the two outflow modes are relatively well understood and documented in the previous studies. However, its long term inter-annual variation and dynamics are yet poorly known due to a lack of long term observational record that can resolve the coastal circulation scale of order 10km. Since the variation of TWC has a strong connection with a formation of fishing ground of the pelagic fish downstream along the Sanriku coast, to understand and predict the variation of TWC outflow has important value for the local fishery communities.
We have compiled an eddy-resolving, long-term ocean reanalysis data, FORA-WNP30 (Four-dimensional variational Ocean ReAnalysis for the Western North Pacific over 30 years), based on variational ocean data assimilation system, MOVE-4DVAR (Usui et al., 2015) recently and its coastal downscaled product has been started to be produced. In the preliminary studies conducted for the period from 2009 to 2011, the coastal ocean reanalysis data reveals a realistic inter-annual variation of TWC outflow which is inferred from high resolution satellite sea surface temperature data. In this presentation, we will make a report on analysis of long-term inter-annual variation of TWC and its validation against independent coastal observation data from tide gauge, high-frequency radar and coastal altimetry.
Multi-Scale Analysis of Coastal Altimetry Data, Multi-Sensor Observations and Numerical Modelling Over the North Western Mediterranean Sea
Marco Meloni1, Jerome Bouffard2, Andrea Doglioli3, Anne Petrenko3
1Serco, Italy; 2Rhea, Belgium c/o ESA/ESRIN; 3MIO (Mediterranean Institute of Oceanography), France
The proposed paper addresses the issue of exploiting and cross-comparing coastal altimetry, in situ data and model-based approaches to monitor the positioning and intensity of the Mediterranean Northern Current (NC). The approach is based on the combination of several tracks from different altimetric missions (lasting more than 12 years), in order to reduce the residual noise and detect the NC position. To achieve this goal, several multi sensor platforms (MVP (Moving Vessel Profiler), ADCP (Acoustic Doppler Current Profiler) and CTD) from marine observatories and cruise campaign (OSCAHR: Observing Submesoscale Coupling At High Resolution) as well as a numerical simulation (SYMPHONIE model), have been conjointly used in order to validate and optimize the altimetry processing methods . Across-track geostrophic currents derived from several altimetric datasets (PEACHI (Experimental Products), X-Track, AVISO) have been cross-compared to the SYMPHONIE model in order to statistically characterize the NC position and intensity over the Ligurian Sea and the Gulf of Lion. MVP , CTD and ADCP measurements from the OSCAHR cruise campaign as well as the JULIO (Judicious Location for Intrusion Observation) current meter have also been processed and compared with geo-located altimetric tracks. The obtained results show a good agreement and pave the way for the definition of new science oriented diagnostics particularly relevant for next altimetric satellite missions (Sentinel-3, Sentinel-6).