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Observations of Surface Currents and Tidal Variability Off of Northeastern Taiwan from Shore-Based High Frequency Radar

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Observations of Surface Currents and Tidal Variability Off of Northeastern Taiwan from Shore-Based High Frequency Radar remote sensing Article Observations of Surface Currents and Tidal Variability Off of Northeastern Taiwan from Shore-Based High Frequency Radar Yu-Ru Chen 1 , Jeffrey D. Paduan 2 , Michael S. Cook 2, Laurence Zsu-Hsin Chuang 1 and Yu-Jen Chung 3,* 1 Institute of Ocean Technology and Marine Affairs, National Cheng Kung University, Tainan 701, Taiwan; [email protected] (Y.-R.C.); [email protected] (L.Z.-H.C.) 2 Department of Oceanography, Naval Postgraduate School—NPS, Monterey, CA 93943, USA; [email protected] (J.D.P.); [email protected] (M.S.C.) 3 Department of Marine Science, Naval Academy, Kaohsiung 813, Taiwan * Correspondence: [email protected] Abstract: A network of high-frequency radars (HFRs) has been deployed around Taiwan. The wide-area data coverage is dedicated to revealing near real-time sea-surface current information. This paper investigates three primary objectives: (1) describing the seasonal current synoptic variability; (2) determining the influence of wind forcing; (3) describing the tidal current field pattern and variability. Sea surface currents derived from HFR data include both geostrophic components and wind-driven components. This study explored vector complex correlations between the HFR time series and wind, which was sufficient to identify high-frequency components, including an Ekman balance among the surface currents and wind. Regarding the characteristics of mesoscale events and the tidal field, a year-long high-resolution surface dataset was utilized to observe the current–eddy– tide interactions over four seasons. The harmonic analysis results derived from surface currents off Citation: Chen, Y.-R.; Paduan, J.D.; Cook, M.S.; Chuang, L.Z.-H.; Chung, of northeastern Taiwan during 2013 are presented. The results agree well with the tidal parameters Y.-J. Observations of Surface Currents estimated from tide-gauge station observations. The analysis shows that this region features a strong, and Tidal Variability Off of mixed, mainly semidiurnal tide. Continued monitoring by a variety of sensors (e.g., satellite and Northeastern Taiwan from HFR) would improve the understanding of the circulation in the region. Shore-Based High Frequency Radar. Remote Sens. 2021, 13, 3438. https:// Keywords: high-frequency radar (HFR); surface currents; tidal variability; harmonic analysis doi.org/10.3390/rs13173438 Academic Editors: Miroslav Gaˇci´c, Milena Menna and Yukiharu Hisaki 1. Introduction Taiwan is situated between the Asian continent and the Pacific Ocean. It is a large Received: 6 June 2021 island with a total coastline length of approximately 1200 km. In terms of atmospheric Accepted: 24 August 2021 Published: 30 August 2021 forcing, synoptic scale systems occur frequently in the spring, typhoons are common in summer, and frontal systems with strong winds and heavy rainfall are common in winter Publisher’s Note: MDPI stays neutral due to Taiwan’s subtropical climate. These variable monsoon systems drive sea-surface with regard to jurisdictional claims in currents in the waters around the island. The interactions within the Western Boundary published maps and institutional affil- Current system between the warm Kuroshio Current and the rugged seafloor topography iations. create a complex ocean current system around Taiwan [1,2]. The Kuroshio Current moves rapidly off of northeastern Taiwan and is commonly associated with seasonal cyclones worthy of long-term observations. To routinely monitor the currents and sea state around Taiwan, a current measurement network consisting of 19 SeaSonde® high-frequency radar (HFR) systems was established in 2008. Copyright: © 2021 by the authors. Remote measurement and monitoring of the sea state using HFRs has become an Licensee MDPI, Basel, Switzerland. This article is an open access article indispensable field in marine science and technology. Such systems have the capability distributed under the terms and to measure near real-time surface currents with good temporal resolution with operating conditions of the Creative Commons frequencies ranging between 3 and 50 MHz. Regarding the general rule for electromagnetic Attribution (CC BY) license (https:// radiation (EM) in the high-frequency band, the EM signal coupled with the sea surface creativecommons.org/licenses/by/ can propagate via groundwaves over the horizon (OTH) [3]. In this study, two of the HFR 4.0/). network stations located off northeastern Taiwan were used as research instruments (the Remote Sens. 2021, 13, 3438. https://doi.org/10.3390/rs13173438 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, x FOR PEER REVIEW 2 of 19 Remote Sens. 2021, 13, 3438 HFR network stations located off northeastern Taiwan were used as research instruments2 of19 (the two green squares shown in Figure 1). HFRs have the capability to measure near real- time complete currents with good temporal and spatial resolutions. Figure 1 shows the areatwo of green maximum squares coverage shown inin Figure2013. Areas1). HFRs with have data the coverage capability of to75% measure or more near are real-time shown incomplete yellow, while currents areas with with good lower temporal data coverage and spatial are shown resolutions. in blue. The Figure data1 shows coverage the was area calculatedof maximum from coveragethe totals indata 2013. (TUV) Areas files, with which data are coverage generated of 75% by combining or more are the shown radial in datayellow, from while the two areas stations with lowerinto the data total coverage velocities are shownaccording in blue.to CODAR’s The data strict coverage quality was controlcalculated (QA) frommechanism. the totals This data study (TUV) exploits files, whichthe hourly are generated TUV files byfrom combining 2013 (8410 the in radial to- tal).data Each from circle the in two Figure stations 1 represents into the totalthe position velocities of a according current vector. to CODAR’s The data strict coverage quality overcontrol the course (QA) mechanism.of the year, counted This study based exploits on the number the hourly of observations, TUV files from is displayed 2013 (8410 as in a percentage.total). Each circleThese in radar Figure coverage1 represents areas the incl positionude the of islands a current of Yonaguni vector. The and data Iriomote, coverage whichover theare courseJapanese of theislands year, in counted the middle based and on on the the number eastern of observations,edge of the coverage is displayed area, as respectively.a percentage. The These red triangles radar coverage in Figure areas 1 repres includeent tide the islands(i.e., sea of level Yonaguni elevation) and stations Iriomote, onwhich the shoreline. are Japanese The data islands from in these the middle HFRs andand tide on the stations eastern are edge used of in the the coverage subsequent area, analysis.respectively. In addition The red to routinely triangles ininvestigatin Figure1 representg mesoscale tide oceanic (i.e., sea processes, level elevation) the HFRs stations also provideon the the shoreline. opportunity The data to resolve from thesethe tidal HFRs constituents and tide stations around areTaiwan. used HFRs in the have subsequent been usedanalysis. to map In tidal addition constituents to routinely and investigatinghighlight the mesoscaleeffects of tidal oceanic and processes, wind drivers the HFRson diur- also nalprovide movements the opportunity [4]. While tothese resolve direct the observat tidal constituentsions of tidal-period around Taiwan. current HFRs fluctuations have been existused for to some map tidalregions, constituents most tidal and studies highlight ha theve effectsbeen based of tidal on and cro windss-validation drivers on of diurnaltide- gaugemovements data and [4 ].models While these[5]. This direct study observations utilizes both of tidal-periodHFR and sea current level fluctuationsobservations existto describefor some the regions, tidal characteristics most tidal studies off northeastern have been based Taiwan. on cross-validation of tide-gauge data and models [5]. This study utilizes both HFR and sea level observations to describe the tidal characteristics off northeastern Taiwan. 90 00 -8 LD 25˚ 25°N N 80 WS 70 -800 40′ 40' Suao -800 SO 60 0 Habn -80 -8 0 50 20′ 20' 0 80 HP 0 - Latitude - Latitude 8 00 40 0 0 -8 -800 25˚ 24°N N HL (%) coverage Data 30 20 40′ 40' km 10 20′ 20' 0 102030 30′ 30' 122˚ 122°E E 30′ 30' 123˚ 123°E E 30′ 30' 124˚ 124°E E Longitude Longitude Figure 1. The surface current data coverage in percentages (%) in 2013. Green squares indicate the Suao and Habn HFR Figure 1. The surface current data coverage in percentages (%) in 2013. Green squares indicate the Suao and Habn HFR stations. The red triangles indicate the tide-gauge stations along the coastline. The island in the center and the island closest stations. The red triangles indicate the tide-gauge stations along the coastline. The island in the center and the island closestto the to eastern the eastern edge ofedge the of domain the domain are the are Japanese the Japanese islands islands of Yonaguni of Yonaguni and Iriomote, and Iriomo respectively.te, respectively. The black The box black inside box the insideinset the at the inset bottom at the left bottom represents left represents the study the area. study area. The main focus of this paper is to investigate the seasonal characteristics of the current patterns, the influence of wind forcing and the spatial distributions of tide constituents, Remote Sens. 2021, 13, 3438 3 of 19 taking advantage of the continuous time and space data from the coastal radar systems. There are some seasonal mesoscale events that appear in this area, but they have not been comprehensively studied. This paper is divided in five parts. In Section2, this study describes the dataset and methods used. Selected data are compared with the HFR data depending on spatial and temporal availability, with Table1 providing a summary of the datasets used and their temporal and spatial resolutions. The analysis methods used in this article to study the flow field, wind influence and tide are introduced in Section3.
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