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, HFR networks have been showing steady growth with over 60 stations currently deployed and many in the planning stage. In Asia and Oceania countries, more than 110 radar stations are in operation. HFR technology can be found in a wide range of applications: for marine safety, oil spill response, tsunami warning, pollution assessment, coastal zone management, tracking environmental change, numerical model simulation of 3-dimensional circulation, and research to generate new understanding of coastal ocean dynamics, depending mainly on each country’s coastal sea characteristics. These radar networks are examples of national inter-agency and inter-institutional partnerships for improving oceanographic research and operations. As global partnerships grow, these collaborations and improved data sharing enhance our ability to respond to regional, national, and global environmental and management issues.

A decade (2007–2016) of hourly 6‐km‐resolution maps of the surface currents across the Mid‐Atlantic Bight (MAB) generated by a regional‐scale High Frequency Radar network are used to reveal new insights into the spatial patterns of the annual and seasonal mean surface flows. Across the 10‐year time series, temporal means and interannual and intra‐annual variability are used to quantify the variability of spatial surface current patterns. The 10‐year annual mean surface flows are weaker and mostly cross‐shelf near the coast, increasing in speed and rotating to more alongshore directions near the shelfbreak, and increasing in speed and rotating to flow off‐shelf in the southern MAB. The annual mean surface current pattern is relatively stable year to year compared to the hourly variations within a year. The 10‐year seasonal means exhibit similar current patterns, with winter and summer more cross‐shore while spring and fall transitions are more alongshore. Fall and winter mean speeds are larger and correspond to when mean winds are stronger and cross‐shore. Summer mean currents are weakest and correspond to a time when the mean wind opposes the alongshore flow. Again, intra‐annual variability is much greater than interannual, with the fall season exhibiting the most interseasonal variability in the surface current patterns. The extreme fall seasons of 2009 and 2011 are related to extremes in the wind and river discharge events caused by different persistent synoptic meteorological conditions, resulting in more or less rapid fall transitions from stratified summer to well‐mixed winter conditions.

This paper describes the development of the SeaSonde High Frequency Radar system into a dual-use application for the mapping of ocean surface currents and detection of ships at sea. This development entailed the creation of a new radar waveform that would permit this dual-use as well as a detection algorithm to identify the ships in the radar spectra. The detection algorithm utilizes two methods for calculating a background signal level: an infinite impulse response (IIR) filter and a two-dimensional median filter. These two methods are employed simultaneously with multiple length averaging times to maximize the number of detections. The initial phase of development focused on improving the radar waveform to maximize the results for ship detection while still retaining the ability to measure surface currents. The latter phase of the development concentrated on testing the detection algorithm on a known vessel in different environmental conditions.

The Middle Atlantic Bight (MAB) exhibits one of the largest seasonal cycles of temperature in the global ocean. During the summer, the water column is strongly stratified with warm surface waters separated from much colder bottom waters by a strong, shallow thermocline. The bottom water that remains cold in the summer months is referred to as the `cold pool'. The cold pool is formed as the stratification sets up in late spring and it remains until fall mixing breaks down the seasonal thermocline with each passing storm; it usually breaks down in September or October. The cold pool is an important feature of the MAB as it influences the surrounding waters, the intensity of storms, and fish assemblages over the MAB. We use various data sources to explore interannual variability and long-term climatic trends in summer stratification and cold pool source water. Annual trawls conducted by the National Oceanic and Atmospheric Administration National Marine Fisheries Services (NOAA NMFS) provide temperature and salinity measurements consistently from 1980 to the present. Slocum gliders have been regularly deployed along New Jersey shelf since 2003 and collect data for various organizations such as Rutgers University and the Environmental Protection Agency. NOAA's National Data Buoy Center provides several buoys in the region that have been collecting data since the early 1980's. A better understanding of the cold pool can lead to a better understanding of ocean and storm interactions in the MAB and of potential climate change impacts on the region.

Coastal ocean dynamics are complex with influences from freshwater input, alongshore flows, offshore slope water intrusions, and overlapping mixing and stratification processes. This complexity makes it difficult to elucidate patterns in observed coastal circulation important to our understanding of natural and anthropogenic marine ecosystem processes. This study used a Lagrangian diagnostic tool to characterize patterns of transport within the coastal Mid‐Atlantic Bight (MAB) off the eastern coast of the United States. The MAB is home to some of the largest seasonal and long‐term oceanographic variability that drives an equally dynamic ecosystem that supports many economically important fisheries. This region has also been continuously observed by a Mid‐Atlantic Regional Association Coastal Ocean Observing System high frequency radar (HFR) array for over 10 years, providing temporally resolved maps of surface currents. With these data as input, we developed a multi‐year seasonal analysis of Lagrangian coherent structures using Finite Time Lyapunov Exponent (FTLE) to map attracting features from 2013 to 2022. Spatial patterns showed areas of year‐round attracting features along the continental shelf break and south of Cape Cod where tidal amplitudes are large. Seasonal differences were found with more attracting features and higher within‐season variability in summer and spring stratified seasons compared to the winter and fall “mixed” seasons. A potential regime shift was also identified, with stronger FTLE features after 2017 especially in the northern half of MAB in fall and winter. Results from this study provide new insights to coastal circulation in the MAB and create opportunities for future work to investigate the ecological importance of these dynamics.