Characteristic Analysis Of Submesoscale Signals In Coastal Regions Of Lianyungang

Submesoscale processes(SPs)are hot research topics in physical oceanography,which significantly affect ocean circulations,nutrient distributions and are closely linked to vertical transport of heat and carbon,energy cascade and biogeochemical processes in coastal regions.There are some limitations in observations and analysis of coastal SPs,including observations of low temporal-spatial resolution and small spatial coverage.So analyzing the characteristics of coastal SPs and obtaining their structural and evolutionary characteristics can enhance the understandings on coastal SPs and play an important role in coastal port planning and fishery resource management.In this study,submesoscale signals that extracted from MODIS color index(CI)imagery and ocean current data that derived from high-frequency radar(HFR)and between June 2017 and March 2019 are used to investigate the structural characteristics of coastal submesoscale eddies.The temporal and spatial resolution of HFR-derived surface currents are 20 min and 3 km respectively,which are good enough to capture submesoscale eddies.MODIS CI imagery have higher spatial resolution(?500 m)and larger spatial coverages,which can fill a gap left by HFR.For surface currents derived from HFR,the centers and boundaries of submesoscale eddies are identified based on a vector geometry(VG)method.For MODIS CI data,CI is used to compute CI gradient parameters,from which submesoscale features are extracted by a modified information-extraction approach.In addition,the effects of topography,wind field,and tide on the characteristics of submesoscale eddies along the coast of Lianyungang were analyzed through the Regional Ocean Modeling System(ROMS)numerical simulation.The spatial-temporal variations of submesoscale eddies in the coastal regions of Lianyungang were analyzed from June 2017 to March 2019 with high-resolution observations of HFR-derived surface currents and CI-derived gradient parameters.The results show that HFR and CI imagery both have the ability to capture submesoscale signals.The typical radius of the detected eddies is 2-4 km and the eddies are mostly located around the seamount and off the cape in the southwest part of HFR observation regions.Although no significant difference in eddy properties is observed between HFR and CI imageries,CI-derived gradient parameters are able to show more detailed submesoscale eddies because of their higher resolution and spatial coverage.By combing the two observation products,coastal submesoscale signals can be effectively studied.In general,HFR products can capture submesoscale eddy features and evolutions of eddy structures,while CI-derived gradient parameters can be used to extract submesoscale signals in larger regions and fill a gap left by HFR.Through ROMS numerical simulations,two ideal sensitivity experiments were performed.One is the sensitivity experiment on topography and wind fields.The other is the sensitivity experiment on tides that is forced by real external forcing.The model results suggest that,the topography induction in the coastal region of Lianyungang plays an important role in submesoscale processes.The most frequent region where submesoscale eddies exist is the area around the seamount in the center of the study regions.Wind and tidal forcing significantly affect the movement of eddies.The input of wind forcing has a certain positive effect on the frequency of eddy generations,while it may have different positive effect on anticyclonic and cyclonic eddies.