Editorial

4 pages, 181 KiB

Open AccessEditorial

Editorial for Special Issue “Radar Technology for Coastal Areas and Open Sea Monitoring”

by Giovanni Ludeno and Marco Uttieri

J. Mar. Sci. Eng. 2020, 8(8), 560; https://doi.org/10.3390/jmse8080560 - 25 Jul 2020

Cited by 4 | Viewed by 1458

The sea has always played a fundamental role in the social and economic development, as well as in the shaping and functioning of natural ecosystems and services [...] Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

Research

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13 pages, 7892 KiB

Open AccessArticle

Wave Orbital Velocity Effects on Radar Doppler Altimeter for Sea Monitoring

by Ferdinando Reale, Eugenio Pugliese Carratelli, Angela Di Leo and Fabio Dentale

J. Mar. Sci. Eng. 2020, 8(6), 447; https://doi.org/10.3390/jmse8060447 - 19 Jun 2020

Cited by 13 | Viewed by 2469

Abstract

The orbital velocity of sea wave particles affects the value of sea surface parameters as measured by radar Doppler altimeters (also known as delay Doppler altimeter (DDA)). In DDA systems, the along-track resolution is attained by algorithms that take into account the Doppler [...] Read more.

The orbital velocity of sea wave particles affects the value of sea surface parameters as measured by radar Doppler altimeters (also known as delay Doppler altimeter (DDA)). In DDA systems, the along-track resolution is attained by algorithms that take into account the Doppler shift induced by the component along the Earth/antenna direction of the satellite velocity, V_S. Since the vertical component of the wave particle orbital velocity also induces an additional Doppler effect (in the following R-effect), an error arises on the positioning of the target on the sea surface. A numerical investigation shows that when the wavelength of sea waves is of the same order of magnitude of the altimeter resolution, the shape of the waveform might be significantly influenced by the R-effect. The phenomenon can be particularly important for the monitoring of long swells, such as those that often take place in the oceans. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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19 pages, 2752 KiB

Open AccessFeature PaperArticle

An Integrated Reconstruction of the Multiannual Wave Pattern in the Gulf of Naples (South-Eastern Tyrrhenian Sea, Western Mediterranean Sea)

by Simona Saviano, Daniela Cianelli, Enrico Zambianchi, Fabio Conversano and Marco Uttieri

J. Mar. Sci. Eng. 2020, 8(5), 372; https://doi.org/10.3390/jmse8050372 - 23 May 2020

Cited by 13 | Viewed by 2802

Abstract

Surface gravity waves retrieved by a network of HF (High Frequency) radars and measured in situ by an ADCP (Acoustic Doppler Current Profiler) current meter connected to an elastic beacon were used to carry out a multiple-year characterization of the wave field of [...] Read more.

Surface gravity waves retrieved by a network of HF (High Frequency) radars and measured in situ by an ADCP (Acoustic Doppler Current Profiler) current meter connected to an elastic beacon were used to carry out a multiple-year characterization of the wave field of the Gulf of Naples (south-eastern Tyrrhenian Sea, western Mediterranean). The aim of the work was to create a climatology of the study area and to demonstrate the potential of an integrated platform for coastal studies. The patterns recorded by the different instruments were in agreement with the wave climatology of the southern Tyrrhenian Sea as well as with previous scores for the same area. The results presented in this work also highlight seasonal and interannual consistency in the wave patterns for each site. In a wider context, this study demonstrates the potential of HF radars as long-term monitoring tools of the wave field in coastal basins, and supports the development of integrated observatories to address large-scale scientific challenges such as coastal ocean dynamics and the impact of global change on the local dynamics. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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25 pages, 7288 KiB

Open AccessArticle

Surface Currents Derived from SAR Doppler Processing: An Analysis over the Naples Coastal Region in South Italy

by Virginia Zamparelli, Francesca De Santi, Andrea Cucco, Stefano Zecchetto, Giacomo De Carolis and Gianfranco Fornaro

J. Mar. Sci. Eng. 2020, 8(3), 203; https://doi.org/10.3390/jmse8030203 - 15 Mar 2020

Cited by 11 | Viewed by 2813

Abstract

Several studies have shown the capabilities of Synthetic Aperture Radar to map sea currents in ocean regions mainly characterized by large flows. We consider the well known method based on the analysis of the Doppler Centroid. The Doppler, as, in general, the scattering [...] Read more.

Several studies have shown the capabilities of Synthetic Aperture Radar to map sea currents in ocean regions mainly characterized by large flows. We consider the well known method based on the analysis of the Doppler Centroid. The Doppler, as, in general, the scattering from the sea, is sensitive to several phenomena, occurring between the upper ocean and atmospheric boundary layers. To investigate such phenomena, we considered the combined use of both ENVISAT calibrated amplitude and Doppler data in conjunction with hindcast wind information provided by atmospheric models as well as Wind and Doppler Geophysical Model Functions (W/D-GMF) developed, in the literature, for C-Band systems. This integrated analysis for the interpretation of the Doppler surface currents measurements was carried out on a case study located in the Mediterranean Sea which is characterized by a general low circulation regime: specifically, the coastal region around the city of Naples. In this case study, we show that generally, wind plays a direct significant role in the observed Doppler surface current. The availability of an oceanographic numerical model for one of the analyzed cases also allowed us to attempt to interpret the effect of the typical thermohaline circulation pattern on the Doppler anomaly. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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21 pages, 17301 KiB

Open AccessArticle

High-Frequency Radar Observations of Surface Circulation Features along the South-Western Australian Coast

by Simone Cosoli, Charitha Pattiaratchi and Yasha Hetzel

J. Mar. Sci. Eng. 2020, 8(2), 97; https://doi.org/10.3390/jmse8020097 - 05 Feb 2020

Cited by 10 | Viewed by 3032

Abstract

A new merged high-frequency radar (HFR) data set collected using SeaSonde and WERA (WEllen RAdar) systems was used to examine the ocean surface circulation at diurnal, seasonal and inter-annual time scales along the south-west coast of Australia (SWWA), between 29°–32° S. Merging was [...] Read more.

A new merged high-frequency radar (HFR) data set collected using SeaSonde and WERA (WEllen RAdar) systems was used to examine the ocean surface circulation at diurnal, seasonal and inter-annual time scales along the south-west coast of Australia (SWWA), between 29°–32° S. Merging was performed after resampling WERA data on the coarser SeaSonde HFR grid and averaging data from the two HFR systems in the area of common overlap. Direct comparisons between WERA and SeaSonde vectors in their overlapping areas provided scalar and vector correlation values in the range Ru = [0.24, 0.76]; Rv = [0.39, 0.83]; ρ = [0.44, 0.75], with mean bias between velocity components in the range [−0.02, 0.28] ms⁻¹, [−0.16, 0.16] ms⁻¹ for the U, V components, respectively. The lower agreement between vectors was obtained in general at the boundaries of the HFR domains, where the combined effects of the bearing errors, geometrical constraints, and the limited angular field of view were predominant. The combined data set allowed for a novel characterization of the dominant features in the region, such as the warmer poleward-flowing Leeuwin Current (LC), the colder Capes Current (CC) and its northward extensions, the presence of sub-mesoscale to mesoscale eddies and their generation and aggregation areas, along with the extent offshore of the inertial-diurnal signal. The contribution of tides was weak within the entire HFR domain (<10% total variance), whilst signatures of significant inertial- and diurnal-period currents were present due to diurnal–inertial resonance. A clear discontinuity in energy and variance distribution occurred at the shelf break, which separates the continental shelf and deeper offshore regions, and defined the core of the LC. Confined between the LC and the coastline, the narrower and colder CC current was a feature during the summer months. Persistent (lifespan greater than 1 day) sub-mesoscale eddies (Rossby number O (1)) were observed at two main regions, north and south of 31.5° S, offshore of the 200 m depth contour. The majority of these eddies had diameters in the range 10–20 km with 50% more counter clockwise rotating (CCW) eddies compared to clockwise (CW) rotating eddies. The northern region was dominated by CCW eddies that were present throughout the year whilst CW eddies were prevalent in the south with lower numbers during the summer months. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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14 pages, 2877 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Implementation of the Listen-Before-Talk Mode for SeaSonde High-Frequency Ocean Radars

by Simone Cosoli

J. Mar. Sci. Eng. 2020, 8(1), 57; https://doi.org/10.3390/jmse8010057 - 19 Jan 2020

Cited by 5 | Viewed by 2430

Abstract

The International Telecommunication Union (ITU) Resolution 612, in combination with Report ITU-R M2.234 (11/2011) and Recommendation ITU-R M.1874-1 (02/2013), regulates the use of the radiolocation services between 3 and 50 MHz to support high frequency oceanographic radar (HFR) operations. The operational frame for [...] Read more.

The International Telecommunication Union (ITU) Resolution 612, in combination with Report ITU-R M2.234 (11/2011) and Recommendation ITU-R M.1874-1 (02/2013), regulates the use of the radiolocation services between 3 and 50 MHz to support high frequency oceanographic radar (HFR) operations. The operational frame for HFR systems include: band sharing capabilities, such as synchronization of the signal modulation; pulse shaping and multiple levels of filtering, to reduce out-of-band interferences; low radiated power; directional transmission antenna, to reduce emission over land. Resolution 612 also aims at reducing the use of spectral bands, either through the application of existing band-sharing capabilities, the reduction of the spectral leakage to neighboring frequency bands, or the development and implementation of listen-before-talk (LBT) capabilities. While the LBT mode is operational and commonly used at several phased-array HFR installations, the implementation to commercial direction-finding systems does not appear to be available yet. In this paper, a proof-of-concept is provided for the implementation of the LBT mode for commercial SeaSonde HFRs deployed in Australia, with potential for applications in other networks and installations elsewhere. Potential critical aspects for systems operated under this configuration are also pointed out. Both the receiver and the transmitter antennas may lose efficiency if the frequency offset from the resonant frequency or calibration pattern are too large. Radial resolution clearly degrades when a dynamical adaptation of the bandwidth is performed, which results in non-homogeneous spatial resolution and reduction of the quality of the data. A recommendation would be to perform the LBT-adapt scans after a full measurement cycle (1-h or 3-h, depending on the system configuration) is concluded. Mutual cross-interference from clock offsets between two HFR systems may bias the frequency scans when the site computers controlling data acquisitions are not properly time-synchronized. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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14 pages, 4310 KiB

Open AccessArticle

Synergistic Use of Synthetic Aperture Radar and Optical Imagery to Monitor Surface Accumulation of Cyanobacteria in the Curonian Lagoon

by Francesca De Santi, Giulia Luciani, Mariano Bresciani, Claudia Giardino, Francesco Paolo Lovergine, Guido Pasquariello, Diana Vaiciute and Giacomo De Carolis

J. Mar. Sci. Eng. 2019, 7(12), 461; https://doi.org/10.3390/jmse7120461 - 14 Dec 2019

Cited by 12 | Viewed by 2899

Abstract

Phytoplankton blooms in internal water bodies are an unpleasant sight that often emerges on top like a layer of foam containing high concentrations of toxins (scum event). Monitoring the concentration of algae and the occurrence of scum in lakes and lagoons has become [...] Read more.

Phytoplankton blooms in internal water bodies are an unpleasant sight that often emerges on top like a layer of foam containing high concentrations of toxins (scum event). Monitoring the concentration of algae and the occurrence of scum in lakes and lagoons has become a topic of interest for management and science. Optical remote sensing is a validated tool but unfortunately it is highly hindered by clouds. For regions with frequent cloud cover, such as the Baltic region, this means loss of data, which limits the purpose of sensing to spatially and temporally characterize any scum for a comprehensive ecological analysis. The aim of this paper is to investigate whether the use of synthetic aperture radar (SAR) images can compensate for the weaknesses of optical images for cyanobacteria bloom monitoring purposes in the event of cloudy skies. A “ready to use” approach to detect cyanobacteria bloom in the Curonian Lagoon based on the level 2 ocean product of Sentinel-1 images is proposed. This method is empirically validated for the images of summer/autumn 2018 of the Curonian Lagoon. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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12 pages, 6949 KiB

Open AccessArticle

Estimation of the Significant Wave Height from Marine Radar Images without External Reference

by Giovanni Ludeno and Francesco Serafino

J. Mar. Sci. Eng. 2019, 7(12), 432; https://doi.org/10.3390/jmse7120432 - 27 Nov 2019

Cited by 19 | Viewed by 2850

Abstract

In the context of the sea state monitoring by means of the X-band marine radar, the estimation of a significant wave height (

H_{s})

is, currently, one of the most challenging tasks. For its estimation, a calibration is usually required using [...] Read more.

In the context of the sea state monitoring by means of the X-band marine radar, the estimation of a significant wave height (

H_{s})

is, currently, one of the most challenging tasks. For its estimation, a calibration is usually required using an external reference, such as in situ sensors, and mainly buoys. In this paper, a method that allows us to avoid the need for an external reference for

H_{s}

estimation is presented. This strategy is, mainly, based on the correlation between a raw radar image and the corresponding non-calibrated wave elevation image to which varying its amplitude by using a scale factor creates a mathematical model for the radar imaging. The proposed strategy has been validated by considering a simulated waves field, generated at varying sea state conditions. The results show a good estimation of the significant wave height, confirmed by a squared correlation coefficient greater than 0.70 for each considered sea state. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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17 pages, 6795 KiB

Open AccessArticle

Comparison of HF Radar Fields of Directional Wave Spectra Against In Situ Measurements at Multiple Locations

by Guiomar Lopez and Daniel C. Conley

J. Mar. Sci. Eng. 2019, 7(8), 271; https://doi.org/10.3390/jmse7080271 - 14 Aug 2019

Cited by 23 | Viewed by 3776

Abstract

The coastal zone hosts a great number of activities that require knowledge of the spatial characteristics of the wave field, which in coastal seas can be highly heterogeneous. Information of this type can be obtained from HF radars, which offer attractive performance characteristics [...] Read more.

The coastal zone hosts a great number of activities that require knowledge of the spatial characteristics of the wave field, which in coastal seas can be highly heterogeneous. Information of this type can be obtained from HF radars, which offer attractive performance characteristics in terms of temporal and spatial resolution. This paper presents the validation of radar-derived fields of directional wave spectra. These were retrieved from measurements collected with an HF radar system specifically deployed for wave measurement, using an established inversion algorithm. Overall, the algorithm reported accurate estimates of directional spectra, whose main distinctive characteristic was that the spectral energy was typically spread over a slightly broader range of frequencies and directions than in their in situ-measured counterparts. Two errors commonly reported in previous studies, namely the overestimation of wave heights and noise related to short measurement periods, were not observed in our results. The maximum wave height recorded by two in situ devices differed by 30 cm on average from the radar-measured values, and with the exception of the wave spreading, the standard deviations of the radar wave parameters were between 3% and 20% of those obtained with the in situ datasets, indicating the two were similarly grouped around their means. At present, the main drawback of the method is the high quality signal required to perform the inversion. This is in part responsible for a reduced data return, which did not exceed 55% at any grid cell over the eight-month period studied here. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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18 pages, 2111 KiB

Open AccessFeature PaperArticle

Inversion of HF Radar Doppler Spectra Using a Neural Network

by Rachael L. Hardman and Lucy R. Wyatt

J. Mar. Sci. Eng. 2019, 7(8), 255; https://doi.org/10.3390/jmse7080255 - 06 Aug 2019

Cited by 13 | Viewed by 3253

Abstract

For a number of decades, coastal HF radar has been used to remotely measure ocean surface parameters, including waves, at distances exceeding 100 km. The information, which has value in many ocean engineering applications, is obtained using the HF radar cross-section, which relates [...] Read more.

For a number of decades, coastal HF radar has been used to remotely measure ocean surface parameters, including waves, at distances exceeding 100 km. The information, which has value in many ocean engineering applications, is obtained using the HF radar cross-section, which relates the directional ocean spectrum to the received radar signal, through a nonlinear integral equation. The equation is impossible to solve analytically, for the ocean spectrum, and a number of numerical methods are currently used. In this study, a neural network is trained to infer the directional ocean spectrum from HF radar Doppler spectra. The neural network is trained and tested on simulated radar data and then validated with data collected off the coast of Cornwall, where there are two HF radars and a wave buoy to provide the sea-truth. Key ocean parameters are derived from the estimated directional spectra and then compared with the values measured by both the wave buoy and an existing inversion method. The results are encouraging; for example, the RMSE of the obtained mean wave direction decreases from 20.6° to 15.7°. The positive results show that neural networks may be a viable solution in certain situations, where existing methods struggle. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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17 pages, 4268 KiB

Open AccessArticle

Estimation of Coastal Currents Using a Soft Computing Method: A Case Study in Galway Bay, Ireland

by Lei Ren, Jianming Miao, Yulong Li, Xiangxin Luo, Junxue Li and Michael Hartnett

J. Mar. Sci. Eng. 2019, 7(5), 157; https://doi.org/10.3390/jmse7050157 - 20 May 2019

Cited by 5 | Viewed by 2935

Abstract

In order to obtain forward states of coastal currents, numerical models are a commonly used approach. However, the accurate definition of initial conditions, boundary conditions and other model parameters are challenging. In this paper, a novel application of a soft computing approach, random [...] Read more.

In order to obtain forward states of coastal currents, numerical models are a commonly used approach. However, the accurate definition of initial conditions, boundary conditions and other model parameters are challenging. In this paper, a novel application of a soft computing approach, random forests (RF), was adopted to estimate surface currents for three analysis points in Galway Bay, Ireland. Outputs from a numerical model and observations from a high frequency radar system were used as inputs to develop soft computing models. The input variable structure of soft computing models was examined in detail through sensitivity experiments. High correlation of surface currents between predictions from RF models and radar data indicated that the RF algorithm is a most promising means of generating satisfactory surface currents over a long prediction period. Furthermore, training dataset lengths were examined to investigate influences on prediction accuracy. The largest improvement for zonal and meridional surface velocity components over a 59-h forecasting period was 14% and 37% of root mean square error (RMSE) values separately. Results indicate that the combination of RF models with a numerical model can significantly improve forecasting accuracy for surface currents, especially for the meridional surface velocity component. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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13 pages, 5442 KiB

Open AccessArticle

A Quality Control Method for Broad-Beam HF Radar Current Velocity Measurements

by Belinda Lipa, Donald Barrick and Chad Whelan

J. Mar. Sci. Eng. 2019, 7(4), 112; https://doi.org/10.3390/jmse7040112 - 19 Apr 2019

Cited by 8 | Viewed by 3155

Abstract

This paper describes a method to provide quality control for radial velocity maps derived from radar echo voltage cross spectra measured by broad-beam high frequency radars. The method involves the comparison of voltage cross spectra measured at Doppler frequencies in the Bragg region [...] Read more.

This paper describes a method to provide quality control for radial velocity maps derived from radar echo voltage cross spectra measured by broad-beam high frequency radars. The method involves the comparison of voltage cross spectra measured at Doppler frequencies in the Bragg region with values predicted from basic equations defining the complex voltage cross spectra in terms of the measured antenna patterns and the radar cross section. Poor agreement at a given Doppler frequency indicates contamination of the spectra, usually due to interference; velocity results from that Doppler frequency are then eliminated. Examples are given of its application to broad-beam radars operating at four sites. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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28 pages, 16089 KiB

Open AccessArticle

Automatic Shoreline Position and Intertidal Foreshore Slope Detection from X-Band Radar Images Using Modified Temporal Waterline Method with Corrected Wave Run-up

by Dipankar Kumar and Satoshi Takewaka

J. Mar. Sci. Eng. 2019, 7(2), 45; https://doi.org/10.3390/jmse7020045 - 12 Feb 2019

Cited by 8 | Viewed by 3850

Abstract

Automatic and accurate shoreline position and intertidal foreshore slope detection are challenging and significantly important for coastal dynamics. In the present study, a time series shoreline position and intertidal foreshore slope have been automatically detected using modified Temporal Waterline Method (mTWM) from time-averaged [...] Read more.

Automatic and accurate shoreline position and intertidal foreshore slope detection are challenging and significantly important for coastal dynamics. In the present study, a time series shoreline position and intertidal foreshore slope have been automatically detected using modified Temporal Waterline Method (mTWM) from time-averaged X-band radar images captured throughout the course of two-week tidal cycle variation over an area spanning 5.6 km on the Hasaki coast between 12 April 2005 and 31 December 2008. The methodology is based on the correlation map between the pixel intensity variation of the time-averaged X-band radar images and the binary signal of the tide level ranging from −0.8 m to 0.8 m. In order to ensure the binary signal represented each of the water levels in the intertidal shore profile, determining the water level direction-wise bottom elevation is considered as the modification. Random gaps were detected in the captured images owing to the unclear or oversaturation of the waterline signal. A horizontal shift in the detected shoreline positions was observed compared to the survey data previously collected at Hasaki Oceanographical Research Station (HORS). This horizontal shift can be attributed to wave breaking and high wave conditions. Wave set-up and run-up are the effects of wave breaking and high wave conditions, respectively. The correction of the wave set-up and run-up is considered to allow the upward shift of the water level position, as well as shoreline position, to the landward direction. The findings indicate that the shoreline positions derived by mTWM with the corrected wave run-up reasonably agree with the survey data. The mean absolute bias (MAB) between the survey data and the shoreline positions detected using mTWM with the corrected wave run-up is approximately 5.9 m, which is theoretically smaller than the spatial resolution of the radar measurements. The random gaps in the mTWM-derived shoreline positions are filled by Garcia’s data filling algorithm which is a Penalized Least Squares regression method by means of the Discrete Cosine Transform (PLS-DCT). The MAB between survey data and the gap filled shoreline positions detected using TWM with corrected wave run-up is approximately 5.9 m. The obtained results indicate the accuracy of the mTWM with corrected wave run-up integrated with Garcia’s method compared to the survey observations. The executed approach in this study is considered as an efficient and robust tool to automatically detect shoreline positions and intertidal foreshore slopes extracted from X-band radar images with the consideration of wave run-up correction. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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16 pages, 3682 KiB

Open AccessArticle

Long-Term Monitoring of the Atlantic Jet through the Strait of Gibraltar with HF Radar Observations

by Pablo Lorente, Silvia Piedracoba, Marcos G. Sotillo and Enrique Álvarez-Fanjul

J. Mar. Sci. Eng. 2019, 7(1), 3; https://doi.org/10.3390/jmse7010003 - 02 Jan 2019

Cited by 14 | Viewed by 3349

Abstract

The present work focuses on the long-term coastal monitoring of the Atlantic surface inflow into the Mediterranean basin through the Strait of Gibraltar. Hourly current maps provided during 2016–2017 by a High Frequency radar (HFR) system were used to characterize the Atlantic Jet [...] Read more.

The present work focuses on the long-term coastal monitoring of the Atlantic surface inflow into the Mediterranean basin through the Strait of Gibraltar. Hourly current maps provided during 2016–2017 by a High Frequency radar (HFR) system were used to characterize the Atlantic Jet (AJ) since changes in its speed and direction modulate the upper-layer circulation of the Western Alboran Gyre (WAG). The AJ pattern was observed to follow a marked seasonal cycle. A stronger AJ flowed north-eastwards during autumn and winter, while a weaker AJ was directed more southwardly during the middle of the year, reaching a minimum of intensity during summertime. A strong relationship between AJ speeds and angles was evidenced: the AJ appeared to be frequently locked at an angle around 63°, measured clockwise from the North. The AJ speed usually fluctuated between 50 cm·s⁻¹ and 170 cm·s⁻¹, with occasional drops below 50 cm·s⁻¹ which were coincident with abrupt modifications in AJ orientation. Peaks of current speed clearly reached values up to 250 cm·s⁻¹, regardless of the season. A number of persistent full reversal episodes of the surface inflow were analyzed in terms of triggering synoptic conditions and the related wind-driven circulation patterns. High sea level pressures and intense (above 10 m·s⁻¹), permanent and spatially-uniform easterlies prevailed over the study domain during the AJ collapse events analyzed. By contrast, tides seemed to play a secondary role by partially speeding up or slowing down the westward currents, depending on the phase of the tide. A detailed characterization of this unusual phenomenon in the Strait of Gibraltar is relevant from diverse aspects, encompassing search and rescue operations, the management of accidental marine pollution episodes or efficient ship routing. Full article

(This article belongs to the Special Issue Radar Technology for Coastal Areas and Open Sea Monitoring)

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