Academic, government, and private organizations from around the globe have established High Frequency radar (hereinafter, HFR) networks at regional or national levels. Partnerships have been established to coordinate and collaborate on a single global HFR network (http://global-hfradar.org/). These partnerships were established in 2012 as part of the Group on Earth Observations (GEO) to promote HFR technology and increase data sharing among operators and users. The main product of HFR networks are continuous maps of ocean surface currents within 200 km of the coast at high spatial (1-6 km) and temporal resolution (hourly or higher). Cutting-edge remote sensing technologies are becoming a standard component for ocean observing systems, contributing to the paradigm shift toward ocean monitoring. In 2017 the Global HFR Network was recognized by the Joint Technical WMO-IOC Commission for Oceanography and Marine Meteorology (JCOMM) as an observing network of the Global Ocean Observing System (GOOS). In this paper we will discuss the development of the network as well as establishing goals for the future. The U.S. High Frequency Radar Network (HFRNet) has been in operation for over 13 years, with radar data being ingested from 31 organizations including measurements from Canada and Mexico. HFRNet currently holds a collection from over 150 radar installations totaling millions of records of surface ocean velocity measurements. During the past 10 years in Europe,
Linear array antennas and beamforming techniques offer some advantages compared to direction finding using squared arrays. The azimuthal resolution depends on the number of antenna elements and their spacing. Assuming an ideal beam pattern and no amplitude taper across the aperture, 16 antennas in a linear array spaced at half the electromagnetic wavelength theoretically provide a beam resolution of 3.58 normal to the array, and up to twice that when the beam is steered within an azimuthal range of 608 from the direction normal to the array. However, miscalibrated phases among antenna elements, cables, and receivers (e.g., caused by service activities without recalibration) can cause errors in the beam-steering direction and distortions of the beam pattern, resulting in unreliable ocean surface current and wave estimations. The present work uses opportunistic ship echoes randomly received by oceanographic highfrequency radars to correct an unusual case of severe phase differences between receiver channels, leading to a dramatic improvement of the surface current patterns. The method proposed allows for simplified calibrations of phases to account for hardware-related changes without the need to conduct the regular calibration procedure and can be applied during postprocessing of datasets acquired with insufficient calibration.
Using high-frequency radars, ocean surface currents were mapped every hour over an area of ≈5000 km2 in the inner Gulf of Tehuantepec (Mexico). The coastal circulation patterns (≈100 km offshore) were studied during spring, summer, and autumn 2006. The spring circulation was similar to the typical winter circulation, when the circulation is forced by outbursts of northerly winds (>8 m s–1) known locally as Tehuanos. Although Tehuano events are less common in spring than in winter, they are perfectly capable of modifying the sea surface by triggering cyclonic and anticyclonic eddies (≈50–200 km in diameter). Under moderate wind conditions, the ocean circulation showed a quasipermanent westward coastal current (≈50 cm s–1). Though the Tehuano winds were absent in summer, cyclonic eddies were observed and likely linked to the westward coastal current. Autumn was influenced by steady northerly winds with speeds of ≈12 m s–1 that remained over the region for almost 15 days. These conditions allowed us to study the competition between the wind-induced circulation and the more intense (≈100 cm s–1) westward coastal current during this period. The origin of this coastal current could be related to a warm coastal-trapped flow, composed of tropical low-salinity waters. The northwestward excursion of the observed coastal current is discussed, and the three-dimensional implications of surface current fields are studied by the Ekman theory and vorticity conservation.
Observations of coastal-trapped waves (CTW) are limited by instrumentation technologies and temporal and spatial resolutions; hence, their complete description is still limited. In the present work, we used measurements from high-frequency radio scatterometers (HFR) to analyze the subinertial dynamics of the Gulf of Tehuantepec in the Mexican Pacific, a region strongly influenced by offshore gap winds. The data showed subinertial oscillations that may be explained by poleward propagating CTWs. The oscillations showed higher coherence (95% confidence) with gap winds in the Gulfs of Papagayo and Panama than with local winds. Vertical thermocline oscillations, measured with a moored thermistor-chain, also showed subinertial oscillations coherent with Papagayo and Panama winds. The period of the observed oscillations was 4 days, which corresponds to the inertial period of the Gulf of Panama. This suggests that inertial oscillations generated by offshore wind outbursts over Panama may have traveled northward along the coastal shelf, and were detected as surface current pulses by the HFR installed approximately 2000 km further north in the Gulf of Tehuantepec. To further explore the presence of CTWs, the 4 day band-pass filtered currents measured by the HFR were analyzed using empirical orthogonal functions. We found that the first mode behaved like a CTW confined to the shelf break. Additionally, the observed oscillations were compared with baroclinic and barotropic CTW models. The results support the notion that nearly inertial baroclinic CTWs are generated in the Gulfs of Panama and Papagayo and then propagate toward the Gulf of Tehuantepec.
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