Variability of Surface Currents over Central Juan de Fuca Strait

The rapid expansion of urbanization along the world’s coastal areas requires a more comprehensive and accurate understanding of the coastal ocean. Over the past several decades, numerical ocean circulation models have tried to provide such insight, based on our developing understanding of physical ocean processes. The systematic establishment of coastal ocean observation systems adopting cutting-edge technology, such as high frequency (HF) radar, satellite sensing, and gliders, has put such ocean model predictions to the test, by providing comprehensive observational datasets for the validation of numerical model forecasts. The New York Harbor Observing and Prediction System (NYHOPS) is a comprehensive system for understanding coastal ocean processes on the continental shelf waters of New York and New Jersey. To increase confidence in the system’s ocean circulation predictions in that area, a detailed validation exercise was carried out using HF radar and Lagrangian drifter-derived surface currents from three drifters obtained between March and October 2010. During that period, the root mean square (RMS) differences of both the east–west and north–south currents between NYHOPS and HF radar were approximately 15 cm s−1. Harmonic analysis of NYHOPS and HF radar surface currents shows similar tidal ellipse parameters for the dominant M2 tide, with a mean difference of 2.4 cm s−1 in the semi-major axis and 1.4 cm s−1 in the semi-minor axis and 3° in orientation and 10° in phase. Surface currents derived independently from drifters along their trajectories showed that NYHOPS and HF radar yielded similarly accurate results. RMS errors when compared to currents derived along the trajectory of the three drifters were approximately 10 cm s−1. Overall, the analysis suggests that NYHOPS and HF radar had similar skill in estimating the currents over the continental shelf waters of the Middle Atlantic Bight during this time period. An ensemble-based set of particle tracking simulations using one drifter which was tracked for 11 days showed that the ensemble mean separation generally increases with time in a linear fashion. The separation distance is not dominated by high frequency or short spatial scale wavelengths suggesting that both the NYHOPS and HF radar currents are representing tidal and inertial time scales correctly and resolving some of the smaller scale eddies. The growing ensemble mean separation distance is dominated by errors in the mean flow causing the drifters to slowly diverge from their observed positions. The separation distance for both HF radar and NYHOPS stays below 30 km after 5 days, and the two technologies have similar tracking skill at the 95 % level. For comparison, the ensemble mean distance of a drifter from its initial release location (persistence assumption) is estimated to be greater than 70 km in 5 days.

The National Oceanographic and Atmospheric Association's (NOAA) 2006 Safe Seas Oil Spill Drill, conducted just outside the Golden Gate, was a prime opportunity to test the value and effectiveness of three recently deployed 13 MHz coastal radars that are part of the Central and Northern California Ocean Observing System. These Coastal Ocean Dynamic Application Radar (CODAR) systems were deployed by the Coastal Ocean Currents Monitoring Program (COCMP) to measure surface currents up to 85 km offshore from Point Reyes to just north of Half Moon Bay. CODAR systems measure surface currents by transmitting radio waves over the ocean and use the Doppler-shifted return sea echo to extract surface current velocities. Safe Seas 2006 was the first demonstrated use of High- Frequency Radar (HFR) to assist in oil-spill response in real time. Surface current maps were posted to the web hourly during the simulated oil spill to monitor surface current structure during the 48-hour exercise. NOAA's Quick Release Estuarine Buoy (QREB) was also deployed at the location of the simulated oil spill to obtain oceanographic environmental data in real time. The QREB is equipped with an Acoustic Doppler Current Profiler (ADCP), which provided a vertical profile of currents near the location of the simulated spill. In an effort to determine the reliability of the data produced from both measurement devices during the exercise, HFR total vector data from the three coastal systems were compared to the data acquired by the QREB surface bin located at approximately 3 m depth. Also, to verify individual HFR site performance, radial data from each site were compared with their respective radial components from the QREB data. Total-vector comparison results reveal strong correlation in both the cross-shore (R2 = 0.69) and along-shore (R2 = 0.90) components with RMS differences of less than 0.09 m/s. Both instruments also observed the same dramatic shift in along-shore current direction during the two-day exercise. Radial comparisons revealed strong correlation as well for the two HFR systems that acquired data at the QREB location. Endpoints of recovered drift cards released during the exercise also qualitatively correlate with trajectories produced from the HFR current maps. Further, tidal analyses were performed on the QREB and HFR data, utilizing Pawlowicz's widely accepted T-Tide algorithm, to further validate measurements made by these instruments. Despite the short data set, these analyses showed pronounced signals of K1 and M2 constituents in good agreement between the QREB and HFR instruments. The consistent comparison results described in this paper show that HFR can add significantly to the effectiveness of surface current mapping over a large area during oil spills and can complement the QREB measurements to enhance oil-spill response.

In this study, we investigated the impact of global warming on the variabilities of large-scale interannual and interdecadal climate modes and teleconnection patterns with two long-term integrations of the coupled general circulation model of ECHAM4/OPYC3 at the Max-Planck-Institute for Meteorology, Hamburg. One is the control (CTRL) run with fixed present-day concentrations of greenhouse gases. The other experiment is a simulation of transient greenhouse warming, named GHG run. In the GHG run the averaged geopotential height at 500 hPa is increased significantly, and a negative phase of the Pacific/North American (PNA) teleconnection-like distribution pattern is intensified. The standard deviation over the tropics (high latitudes) is enhanced (reduced) on the interdecadal time scales and reduced (enhanced) on the interannual time scales in the GHG run. Except for an interdecadal mode related to the Southern Oscillation (SO) in the GHG run, the spatial variation patterns are similar for different (interannual + interdecadal, interannual, and interdecadal) time scales in the GHG and CTRL runs. Spatial distributions of the teleconnection patterns on the interannual and interdecadal time scales in the GHG run are also similar to those in the CTRL run. But some teleconnection patterns show linear trends and changes of variances and frequencies in the GHG run. Apart from the positive linear trend of the SO, the interdecadal modulation to the El Nino/SO cycle is enhanced during the GHG 2040 ∼ 2099. This is the result of an enhancement of the Walker circulation during that period. La Nina events intensify and El Nino events relatively weaken during the GHG 2070 ∼ 2090. It is interesting to note that with increasing greenhouse gas concentrations the relation between the SO and the PNA pattern is reversed significantly from a negative to a positive correlation on the interdecadal time scales and weakened on the interannual time scales. This suggests that the increase of the greenhouse gas concentrations will trigger the nonstationary correlation between the SO and the PNA pattern both on the interdecadal and interannual time scales.

The systematic establishment of coastal ocean observation systems adopting cutting-edge technology, such as High Frequency (HF) radar, satellite sensing and gliders, has provided unprecedented opportunities for improving and assessing ocean model hindcasts and forecasts. We assess the impact of assimilating high quality HF radar observations covering the Middle Atlantic Bight (MAB) into an operational New York Harbor Observation and Prediction System (NYHOPS). A nudging or Newtonian damping scheme is developed to assimilate the HF radar surface currents during operational NYHOPS observation-based spin-up (-24 to 0 forecast h). The effectiveness of data assimilation (DA) is evaluated by comparisons to surface currents derived from Lagrangian drifters tracked during a total 67 days in 2010 and 2011. The impact of DA on NYHOPS's ability to simulate surface currents correctly during both the NYHOPS hindcast (-24 to 0 h) and free forecast (0 to 24 h) periods is analyzed quantitatively. In the MAB, DA decreased the root-mean-square-difference (RMSD) between the NYHOPS hindcast and the HF radar surface current components by approximately 3 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> between Jun 9 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> , 2011 and Jul 21 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> , 2011. Compared to historic current-meter-data estimates of the dominant semidiurnal tidal current constituent, harmonic analysis of the NYHOPS surface currents after DA shows an average 1.0 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> improvement in the semimajor axis, a 0.2 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> improvement in the semi-minor axis, a 10° decrease in the averaged phase, and a 13° decrease in the average ellipse orientation estimates. The average RMSD between NYHOPS hindcast surface currents and those derived along the observed drifter trajectories was 13.6 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , which, after DA, decreased by 1.1 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (8%). The benefit of DA during hindcast/spinup extended to the free forecast period: a decrease of approximately 5% from 21 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> RMSD without DA. The average mean-vector-velocity-difference (MVVD) between surface currents from the NYHOPS hindcast with DA and currents derived along drifter trajectories was 3.6 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , a 1.5 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> decrease compared with that of NYHOPS without DA. Again, the benefit extended to the free forecast period, with the MVVD error decreasing from 5 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> to 4 cm s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Finally, averaged over a set of 43 particle tracking runs, the separation distance between numerical particle trajectories based on the NYHOPS currents versus observed drifter trajectories also decreased after DA (by 7% per five hindcast days and 10% per one forecast day). These results indicate that DA of HF radar currents onto operational forecast models can improve the skill of such models and, in extension, their benefit to society through, for example, more accurate search and rescue path planning.

Quality control of surface current measurements from high frequency (HF) radar requires understanding of individual error sources and their contribution to the total error. Radial velocity error due to uncertainty of the bearing determination technique employed by HF radar is observed with both direction finding and phased array techniques. Surface current estimates utilizing Multiple Signal Classification (MUSIC) direction finding algorithm with a compact antenna design are particularly sensitive to the radiation pattern of the receive and transmit antennas. Measuring the antenna pattern is a common and straightforward task that is essential for accurate surface current measurements. Radial current error due to the a distorted antenna pattern is investigated by applying MUSIC to simulated HF radar backscatter for an idealized ocean surface current. A Monte Carlo type treatment of distorted antenna patterns is used to provide statistics of the differences between simulated and estimated surface current. RMS differences between the simulated currents and currents estimated using distorted antenna patterns are 3-12 cm/s greater than those using perfect antenna patterns given a simulated uniform current of 50 cm/s. This type of analysis can be used in conjunction with antenna modeling software to evaluate possible error due to the antenna patterns before installing a HF radar site.

To better understand the coastal sea level (SL) variations along the western boundary of the North Pacific, we quantitatively estimate the contributions of various forcing to the coastal SL variations on seasonal and longer time scales. Based on a western boundary SL theory and a linear least-squares regression, we obtain a polynomial equation to estimate the coastal SL variations from ocean interior information, atmospheric forcing, as well as local steric effects. The estimated results can explain about 91% (93%) of the SL variations at tide gauges south (north) of the Kuroshio extension jet. It is found that the local thermosteric effect is dominant on seasonal time scales. On interannual time scales, the signals from ocean interior and atmospheric forcing are dominant. For decadal SL trends, the coastal SL rise is mainly resulted from the signals from the open ocean. With the same polynomial equation, the SL variations at 6 new tide gauges were estimated and compared to the nearest satellite measurements. The newly estimated SL is generally in much better agreement with the tide gauge data than the satellite data. It is promising to apply the newly derived polynomial equation to estimate SL variations along the western boundary of the North Pacific where tide gauge data are not available. Particularly, the approach is promising to estimate the future SL change given the required oceanic and atmospheric conditions.

The relative influences of Indian and Pacific Ocean modes of variability on Australian rainfall and soil moisture are investigated for seasonal, interannual, and decadal time scales. For the period 1900–2006, observations, reanalysis products, and hindcasts of soil moisture during the cool season (June–October) are used to assess the impacts of El Niño–Southern Oscillation (ENSO) and the Indian Ocean dipole (IOD) on southeastern Australia and the Murray–Darling Basin, two regions that have recently suffered severe droughts. A distinct asymmetry is found in the impacts of the opposite phases of both ENSO and IOD on Australian rainfall and soil moisture. There are significant differences between the dominant drivers of drought at interannual and decadal time scales. On interannual time scales, both ENSO and the IOD modify southeastern Australian soil moisture, with the driest (wettest) conditions over the southeast and more broadly over large parts of Australia occurring during years when an El Niño and a positive IOD event (La Niña and a negative IOD event) co-occur. The atmospheric circulation associated with these responses is discussed. Lower-frequency variability over southeastern Australia, however, including multiyear drought periods, seems to be more robustly related to Indian Ocean temperatures than Pacific conditions. The frequencies of both positive and negative IOD events are significantly different during periods of prolonged drought compared to extended periods of “normal” rainfall. In contrast, the frequency of ENSO events remains largely unchanged during prolonged dry and wet periods. For the Murray–Darling Basin, there appears to be a significant influence by La Niña and both positive and negative IOD events. In particular, La Niña plays a much more prominent role than for more southern regions, especially on interannual time scales and during prolonged wet periods. For prolonged dry (wet) periods, positive IOD events also occur in unusually high (low) numbers.

Assessment of surface current measured by HF radars within a suspended kelp aquaculture zone in China

Estimation of 3-D current fields near the Rhine outflow from HF radar surface current data

High-Frequency (HF) Radar is an instrument using radio waves to measure ocean currents and waves remotely. This technology has many advantages, including has unprecedented spatial and temporal resolution, can operate in any weather condition, and is not dangerous for the environment. However, HF Radar's research is still limited in Indonesia. This research aimed to analyze the tidal and residual current in the Bali Strait in July 2020. Radial velocity from two HF Radar sites is combined to obtain the total currents. Current data from HF Radar were compared with Acoustic Doppler Current Profiler (ADCP) data to investigate its accuracy. Surface current data were analyzed using harmonic analysis to separate tidal and residual currents. Comparison between HF Radar and ADCP data are in good agreement for meridional current with a very high correlation of 0.813 and a small RMSE value of 0.22 m/s. Harmonic analysis shows that the dominant currents are tidal currents. The current direction was northward (southward) at flood (ebb), with maximum northward (southward) velocities are 2.17 m/s (2.97 m/s), respectively. The residual current has a random pattern, slightly faster northward than southward, and has similar spectral with the wind.

Cross-equatorial flow through an abyssal channel under the complete Coriolis force: Two-dimensional solutions

High frequency (HF) radars measure ocean surface currents by sending electromagnetic (EM) waves in the HF radio band (3–30 MHz) and recording the EM waves backscattered by ocean surface gravity waves. The recorded signal is dominated by EM waves backscattered from ocean surface waves with half the EM wavelength, called Bragg waves. Since their phase velocity is affected not only by wave-current interactions with mean Eulerian currents, but also by wave-wave interactions with all the other waves present at the sea surface, the question arises as to whether HF radars measure a quantity related to the wave-induced Stokes drift in addition to mean Eulerian currents. However, the literature is inconsistent on the expression and even on the existence of the contribution of the wave-induced Stokes drift to the HF radar measurements. Three different expressions have been proposed in the literature: (1) the filtered surface Stokes drift, (2) half of the surface Stokes drift, and (3) the weighted depth-averaged Stokes drift. We evaluate these expressions for directional wave spectra measured by a bottom-mounted Acoustic Wave and Current (AWAC) profiler in the Lower St. Lawrence estuary, Canada. We then compare the Eulerian surface currents measured by the AWAC with the radial currents measured by two HF radars: one Wellen Radar (WERA) and one Coastal Ocean Dynamics Applications Radar (CODAR). The AWAC and HF-radar currents are surprisingly not correlated, but when subtracting the wave-induced Stokes drift contributions from the radar measurements, moderate but significant correlations are obtained. The highest correlations are obtained when subtracting the filtered surface Stokes drift, suggesting that HF radars measure the latter in addition to mean Eulerian currents.

A mechanism for generating decadal sea surface temperature (SST) variability in the equatorial Pacific is investigated using an ocean general circulation model forced by observed wind stress. Equatorial SST variability is governed by different ocean dynamics on interannual and decadal time scales. At interannual time scales, equatorial SST anomalies are mainly driven by equatorial winds. At decadal time scales, on the other hand, they are forced both by equatorial winds and by off‐equatorial winds in the tropics, while the contribution from midlatitude winds poleward of 25° is negligible. Trade‐wind variations in the off‐equatorial tropics force equatorial SST variability by spinning up and down the subtropical cells that transport cold water into the equatorial upwelling zone. Equatorial SST anomalies induced by this mechanism lag behind the response to local equatorial winds by about two years.

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 (&lt;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.

The Mississippi Sound (MS Sound) is an elongated, shallow lagoon-type or bar-built estuary with its major axis parallel to the Gulf of Mexico that extends east-west along the southern coasts of Mississippi and Alabama, from Lighthouse Point, Mississippi to the Dauphin Island and on its southern side the MS Sound is partly separated from the Gulf of Mexico by the Mississippi-Alabama barrier islands [1]. Knowing the pattern and variability of the surface currents in the MS Sound is significant as they affect hypoxia development, oil spill contamination, storm surge-induced flooding of the coastal areas, advection of freshwater plumes and the variability of salinity, which impacts habitat suitability of organisms such as the eastern oyster. Hence, it is important to aim for understanding the MS Sound circulation, and how it affects the ecology of the estuary. This study addresses one objective towards that goal by conducting a validation study of a regional model by comparing its output to surface currents measured by High Frequency Radar (HFR) systems. Coastal-based HFR stations measure the surface currents that move towards or away from the station, in different azimuthal and radial distance bins. These movements are termed radial currents. HFR networks combine the overlapping radial currents from two or more stations to estimate total horizontal surface currents [2]. In this study, a comparison has been made between the radial currents from HFR stations and those computed from the output of the Coupled Ocean Atmosphere Wave Sediment Transport (COAWST) modeling system [3]. For this study, the comparability of HFR derived radial surface currents from two HFR sites have been analyzed against model data from an application of COAWST to Mississippi Sound for a time period in 2019 [4]. The two 25 MHz CODAR Ocean Sensor HFR systems, located in the western MS Sound, measure radial surface currents and provide an hourly average. The model provides hourly output at 400-m horizontal resolution. Similarities and differences between model and in-situ data have been identified in the context of wind and tidal forcing in the MS Sound during that time period. This study will help validate the model surface currents in accordance with the observed high frequency radar currents, it will lead to insights into the causes of periods of lower model fidelity, and ultimately support improvements of the model. Further study can also be done comparing the radial currents from high frequency radar station with the output of other regional ocean models.