High-frequency Radar Measurements Research Articles

Analysis of Nearshore Near-Inertial Oscillations Using Numerical Simulation with Data Assimilation in the Pearl River Estuary of the South China Sea

The High-Frequency (HF) radar network has become an effective method for detecting coastal currents. In this study, we confirmed the effectiveness of the HF radar measurements by comparing with the Acoustic Doppler Current Profiler (ADCP) and explore the possibility of assimilating radar data into a regional coastal ocean model. A regional high-resolution model with resolution of 10 m was first built in the Pearl River Estuary (PRE). However, analysis of the Hovmöller diagrams from the model simulations in this study indicated a significant deficiency in representing Near-Inertial Oscillations (NIOs) in the PRE, particularly in the east–west direction, despite including wind fields in the input data, during the week from 3 to 8 August 2022. To overcome the model deficiency, we conducted a set of assimilation experiments and performed sensitivity analyses. The results of sensitivity experiments indicate that the model exhibits a sufficient capacity to replicate NIOs after assimilation, lasting approximately 5–6 days. To further analyze the reasons for the decay in the magnitude of the NIOs, data from the three ADCP stations were compared with model results by applying the momentum equation. The assimilated vertical diffusion term outperforms the unassimilated model in representing NIOs. These findings highlight the importance of the vertical diffusion term for simulating NIOs and the data assimilation in improving the model’s representation of physical processes.

Surface circulation in Todos Santos Bay, Baja California, México

Depending on dimensions, orientation and topographic features, the circulation of semi-enclosed seas adjacent to regions of coastal upwelling are strongly influenced by their interaction with a shelf upwelling jet of adjacent waters. A special case are square bays, where opening is about the same size as length, and are bordered by headlands at the entrance. This study analyzed surface currents measured between 2009 and 2020 with high-frequency (HF) radar in Todos Santos Bay, a square bay located in northwestern México, to obtain mean and seasonal surface circulation patterns. HF radar measurements indicate that the average circulation pattern within the bay is cyclonic, with water of the California Current (CC) entering primarily as a coastal jet through the northern mouth of the bay. The similarity of monthly average maps with the long-term average suggests the cyclonic circulation persists year-round. Scale analysis demonstrates that given the size of the bay, only one eddy, primarily controlled by inertia, is dominantly formed inside. The mean cyclonic circulation defines the bay as an upwelling shadow.

Kinematics of surface currents at the northern margin of the Gulf of Cádiz

Abstract. The subtidal surface water circulation at the northern margin of the Gulf of Cádiz, at the southern extremity of the Iberian upwelling system, is described based on validated hourly high-frequency radar measurements from 2016 to 2020. Statistical analyses (mean, standard deviation, eccentricity and empirical orthogonal functions) are applied to the dataset, which is completed with ADCP time series from multiple moorings at five inner-shelf stations and ERA5 wind. Off the shelf, the main circulation pattern consists of a slope current, best developed in summer when north-westerlies dominate, in particular at the most exposed western region. Mechanisms other than upwelling must contribute to this flow in order to explain its seasonal persistence. The slope circulation reverses for regional wind events with an east component > 10 m s−1, approximately. On the shelf, currents are mainly alongshore and balanced. The circulation is generally continuous along the coast, except for weak (< 0.1 m s−1, broadly) poleward flows. In the latter case, the flow tends to remain equatorward near Cape Santa Maria. In winter, coastal poleward flows often extend over the entire margin and are mainly wind-driven. In summer, these flows generally consist of coastal counter currents (CCCs) with the poleward direction opposed to that of the slope current. The CCCs are associated with significant cyclonic recirculation, strongest to the west, where a transient eddy is shortly observed for weak wind stress. This circulation develops after periods of strong north-westerlies, supporting that CCCs result from the imbalance of a regional alongshore pressure gradient.

High-Frequency Radar Measurements with CODAR in the Region of Nice: Improved Calibration and Performance

AbstractWe report on the installation and first results of one compact oceanographic radar in the region of Nice for a long-term observation of the coastal surface currents in the northwest Mediterranean Sea. We describe the specific processing and calibration techniques that were developed at the laboratory to produce high-quality radial surface current maps. In particular, we propose an original self-calibration technique of the antenna patterns, which is based on the sole analysis of the database and does not require any shipborne transponder or other external transmitters. The relevance of the self-calibration technique and the accuracy of inverted surface currents have been assessed with the launch of 40 drifters that remained under the radar coverage for about 10 days.

Wind Direction Data from a Coastal HF Radar System in the Gulf of Naples (Central Mediterranean Sea)

Results on the accuracy of SeaSonde High Frequency (HF) radar wind direction measurements in the Gulf of Naples (Southern Tyrrhenian Sea, Central Mediterranean Sea) are here presented. The investigation was carried out for a winter period (2 February–6 March) and for one summer month (August) of the reference year 2009. HF radar measurements were compared with in situ recordings from a weather station and with model data, with the aim of resolving both small scale and large scale dynamics. The analysis of the overall performance of the HF radar system in the Gulf of Naples shows that the data are reliable when the wind speed exceeds a 5 m/s threshold. Despite such a limitation, this study confirms the potentialities of these systems as monitoring platforms in coastal areas and suggests further efforts towards their improvement.

Assessment of near-shore currents from CryoSat-2 satellite in the Gulf of Cádiz using HF radar-derived current observations

This study evaluated the possibility of studying mesoscale surface circulation in coastal areas, as is the Gulf of Cádiz, Spain, using high-resolution altimetry data (20-Hz of posting rate) along with the use of wind and bottom friction ageostrophic corrections. Absolute cross-track surface zonal current velocities, derived from filtered along-track CryoSat-2 SIRAL-SARM Absolute Dynamic Topography (ADT) measurements, are compared with high-frequency radar (HFR) data in the coastal area of the Gulf of Cádiz. The filter strategy followed in this study for the altimetry data is dependent on the HFR measurements for each track. Absolute surface geostrophic velocities obtained from 20-Hz altimetry data agree well HFR further than 25 km from the coast. Close to land ([3–25] km) the ageostrophic component of the surface current (due to the wind drag and the bottom friction) needs to be considered in the altimeter data. On average, the correlation between altimetry and HFR improved from 0.61 (no correction) to 0.72 (correcting these effects). The root mean square error (RMSE) lowered from 12.54 cm·s−1 to 7.35 cm·s−1. Furthermore, it has also been demonstrated that corrected altimeter measurements are useful for the study of dynamics and patterns of coastal areas.

Dynamically Unstable Strong Wind Shears Observed in the Polar Mesosphere Summer Echo Layer Associated With Geomagnetic Disturbances

AbstractWe report observations of dynamically unstable strong wind shear (Richardson number < 0.25) capable of inducing Kelvin‐Helmholtz instability in the polar mesospheric summer echoes (PMSE) layer (80–90 km) using very high frequency radar measurements in Kiruna (67.8°N, 20.4°E), Sweden, in 2006. The unstable strong wind shear can play an important role in producing PMSE by inducing turbulence and adiabatic cooling. We find that the strong shears take up 64% of the observed wind profiles and are frequently composed of systematically single‐shear/multishear (layer) structures, which gradually or abruptly vary in wind directions, so‐called wind shifts, through heights at intervals of 4–8 km. The strong shear rate normalized by PMSE counts has a good correlation (R = 0.7) in day‐to‐day variation with energetic electron (>30 keV) precipitations that were related to high‐speed solar wind streams. The observations of strong wind shear can be supported by satellite‐measured temperature modulations matching with the peaks of the first three high‐speed solar wind stream events. This study suggests that PMSE production is closely associated with the strong shear that is in turn linked to the effects of energetic electron precipitation.

Joint analysis of coastal altimetry and high-frequency (HF) radar data: observability of seasonal and mesoscale ocean dynamics in the Bay of Biscay

Abstract. Land-based coastal high-frequency (HF) radar systems provide operational measurements of coastal surface currents (within 1–3 m depth) with high spatial (300 m–10 km) and temporal (≤1 h) sampling resolutions, while the near-continuous altimetry missions provide information, from 1993 until today, on geostrophic currents in the global ocean with typical along-track and temporal sampling resolutions of >7 km and >9 days, respectively. During the last years, the altimetry community has made a step forward in improving these data in the coastal area, where the data present lower quality than in the open ocean. The combination of HF radar and altimetry measurements arises as a promising strategy to improve the continuous monitoring of the coastal area (e.g. by expanding the measurements made by HF radars to adjacent areas covered by the altimetry or by validating/confirming improvements brought by specific coastal algorithms or new altimeter missions). A first step towards this combination is the comparison of both data sets in overlapping areas. In this study, a HF radar system and two Jason-2 satellite altimetry products with different processing are compared over the period from 1 January 2009 to 24 July 2015. The results provide an evaluation of the performance of different coastal altimetry data sets within the study area and a better understanding of the ocean variability contained in the HF radar and altimetry data sets. Both observing systems detect the main mesoscale processes within the study area (the Iberian Poleward Current and mesoscale eddies), and the highest correlations between radar and altimetry (up to 0.64) occur in the slope where the Iberian Poleward Current represents a significant part of the variability in the circulation. Besides, the use of an Ekman model, to add the wind-induced current component to the altimetry-derived geostrophic currents, increases the agreement between both data sets (increasing the correlation by around 10 %).

Assessment of Surface Currents Measured With High-Frequency Phased-Array Radars in Two Regions of Complex Circulation

Surface velocity data from two WERA high-frequency (HF) ocean radar systems, deployed as part of the Australian Integrated Marine Observing System (IMOS), are compared with near-surface currents obtained from drifters and acoustic Doppler current profilers (ADCPs). We evaluate data from two contrasting locations in the first detailed evaluation of the IMOS HF radar surface velocities. HF radar measurements are generally robust but demand quality-control procedures to eliminate obvious errors and outliers that appear temporarily or systematically in the data. A number of different quality control procedures and filters are applied and assessed including Taylor diagrams and Hampel and Savitzky–Golay filters. In addition, the need for and effect of averaging are discussed. The radar measurements of surface current agreed better with the near-surface drifter currents than with the subsurface ADCP currents. Nonetheless, the ADCP comparisons are consistent with those previously reported in other regions. The value of the Taylor diagram for comparing different surface current data sets and processing approaches is demonstrated. Noise levels in the radar current spectra are used to estimate the error in the measurements and in some cases, these errors were found to approach the precision of the radar estimates. Our results give guidance on the most useful temporal sampling resolution. In particular we show that at these sites and these operating frequencies, using 10-min sampling without further averaging does not provide additional information because the higher frequencies are dominated by noise. Averaging the radials over 30 min may be sufficient for many applications.

Hindcasting and Forecasting of Surface Flow Fields through Assimilating High Frequency Remotely Sensing Radar Data

In order to improve the forecasting ability of numerical models, a sequential data assimilation scheme, nudging, was applied to blend remotely sensing high-frequency (HF) radar surface currents with results from a three-dimensional numerical, EFDC (Environmental Fluid Dynamics Code) model. For the first time, this research presents the most appropriate nudging parameters, which were determined from sensitivity experiments. To examine the influence of data assimilation cycle lengths on forecasts and to extend forecasting improvements, the duration of data assimilation cycles was studied through assimilating linearly interpolated temporal radar data. Data assimilation nudging parameters have not been previously analyzed. Assimilation of HF radar measurements at each model computational timestep outperformed those assimilation models using longer data assimilation cycle lengths; root-mean-square error (RMSE) values of both surface velocity components during a 12 h model forecasting period indicated that surface flow fields were significantly improved when implementing nudging assimilation at each model computational timestep. The Data Assimilation Skill Score (DASS) technique was used to quantitatively evaluate forecast improvements. The averaged values of DASS over the data assimilation domain were 26% and 33% for east–west and north–south velocity components, respectively, over the half-day forecasting period. Correlation of Averaged Kinetic Energy (AKE) was improved by more than 10% in the best data assimilation model. Time series of velocity components and surface flow fields were presented to illustrate the improvement resulting from data assimilation application over time.

Surface current dynamics under sea breeze conditions observed by simultaneous HF radar, ADCP and drifter measurements

Ocean surface boundary layer dynamics off the southern coast of France in the NW Mediterranean is investigated by using velocity observations by high-frequency (HF) radars, surface drifting buoys and a downward-looking drifting acoustic Doppler current profiler (ADCP). The analysis confirms that velocities measured by HF radars correspond to those observed by an ADCP at the effective depth z f = k −1, where k is wavenumber of the radio wave emitted by the radar. The radials provided by the radars were in a very good agreement with in situ measurements, with the relative errors of 1 and 9 % and root mean square (RMS) differences of 0.02 and 0.04 m/s for monostatic and bistatic radar, respectively. The total radar-based velocities appeared to be slightly underestimated in magnitude and somewhat biased in direction. At the end of the survey period, the difference in the surface current direction, based on HF radar and ADCP data, attained 10°. It was demonstrated that the surface boundary layer dynamics cannot be reconstructed successfully without taking into the account velocity variation with depth. A significant misalignment of ∼30° caused by the sea breeze was documented between the HF radar (HFR-derived) surface current and the background current. It was also found that the ocean response to a moderate wind forcing was confined to the 4-m-thick upper layer. The respective Ekman current attained the maximum value of 0.15 m/s, and the current rotation was found to be lagging the wind by approximately 40 min, with the current vector direction being 15–20° to the left of the wind. The range of velocity variability due to wind forcing was found comparable with the magnitude of the background current variability.

Forecasting of Surface Currents via Correcting Wind Stress with Assimilation of High-Frequency Radar Data in a Three-Dimensional Model

This paper details work in assessing the capability of a hydrodynamic model to forecast surface currents and in applying data assimilation techniques to improve model forecasts. A three-dimensional model Environment Fluid Dynamics Code (EFDC) was forced with tidal boundary data and onshore wind data, and so forth. Surface current data from a high-frequency (HF) radar system in Galway Bay were used for model intercomparisons and as a source for data assimilation. The impact of bottom roughness was also investigated. Having developed a “good” water circulation model the authors sought to improve its forecasting ability through correcting wind shear stress boundary conditions. The differences in surface velocity components between HF radar measurements and model output were calculated and used to correct surface shear stresses. Moreover, data assimilation cycle lengths were examined to extend the improvements of surface current’s patterns during forecasting period, especially for north-south velocity component. The influence of data assimilation in model forecasting was assessed using a Data Assimilation Skill Score (DASS). Positive magnitude of DASS indicated that both velocity components were considerably improved during forecasting period. Additionally, the improvements of RMSE for vector direction over domain were significant compared with the “free run.”

Sensitivity of HF radar-derived surface current self-organizing maps to various processing procedures and mesoscale wind forcing

We performed a number of sensitivity experiments by applying a mapping technique, self-organizing maps (SOM) method, to the surface current data measured by high-frequency (HF) radars in the northern Adriatic and surface winds modelled by two state-of-the-art mesoscale meteorological models, the Aladin (Aire Limitee Adaptation Dynamique Developpement InterNational) and the Weather and Research Forecasting models. Surface current data used for the SOM training were collected during a period in which radar coverage was the highest: between February and November 2008. Different pre-processing techniques, such as removal of tides and low-pass filtering, were applied to the data in order to test the sensitivity of characteristic patterns and the connectivity between different SOM solutions. Topographic error did not exceed 15 %, indicating the applicability of the SOM method to the data. The largest difference has been obtained when comparing SOM patterns originating from unprocessed and low-pass filtered data. Introduction of modelled winds in joint SOM analyses stabilized the solutions, while sensitivity to wind forcing coming from the two different meteorological models was found to be small. Such a low sensitivity is considered to be favourable for creation of an operational ocean forecasting system based on neural networks, HF radar measurements and numerical weather prediction mesoscale models.

Comparison of HF radar measurements with Eulerian and Lagrangian surface currents

High-frequency (HF) radar-derived ocean currents are compared with in situ measurements to conclude if the radar observations include effects of surface waves that are of second order in the wave amplitude. Eulerian current measurements from a high-resolution acoustic Doppler current profiler and Lagrangian measurements from surface drifters are used as references. Directional wave spectra are obtained from a combination of pressure sensor data and a wave model. Our analysis shows that the wave-induced Stokes drift is not included in the HF radar-derived currents, that is, HF radars measure the Eulerian current. A disputed nonlinear correction to the phase velocity of surface gravity waves, which may affect HF radar signals, has a magnitude of about half the Stokes drift at the surface. In our case, this contribution by nonlinear dispersion would be smaller than the accuracy of the HF radar currents, hence no conclusion can be made. Finally, the analysis confirms that the HF radar data represent an exponentially weighted vertical average where the decay scale is proportional to the wavelength of the transmitted signal.

Surface transport in the Northeastern Adriatic Sea from FSLE analysis of HF radar measurements

This study focuses on the surface transport in the Northeastern Adriatic Sea and the related hydrodynamic connectivity with the Gulf of Trieste (GoT) under calm or typical wind conditions: Bora (from the NE) and Sirocco (from the SE). The surface transport in the area has been investigated by evaluating the Finite-Size Lyapunov Exponents (FSLE) on the current field measured by the High Frequency (HF) coastal radar network. FSLE allow us to estimate Lagrangian Coherent Structures (LCSs), which provide information on the transport patterns associated with the flow and identify regions characterized by different dynamics. This work includes the development and set-up of the FSLE algorithm applied for the first time to the specific Adriatic area considered. The FSLE analysis during calm wind reveals an attractive LCS crossing the GoT entrance, marking the convergence between the Northern Adriatic and the outflowing gulf waters. During Bora episodes this attractive LCS is displaced westward with respect to the calm wind case, indicating that Bora drives an extended coherent outflow from the GoT. On the other hand, Sirocco wind piles up the water along the northern end of the basin. In this area an attractive LCS is found, extending mainly in the SW–NE direction. The sirocco-induced inflow of Adriatic waters in the GoT is mainly driven along its northern (Italian) side, as evidenced by the orientation of the LCS. Under Sirocco condition, as in the Bora case, there is no barrier in front of the gulf. No relevant LCSs are observed in the southern radar coverage area except for Bora cases, when a repulsive LCS develops in front of the Istrian coast separating water masses to the North and the South of it.

Inverse estimation of wind from the waves measured by high-frequency radar

In this work, an attempt was made to derive wind speeds from the wave parameters recorded by a high-frequency (HF) radar by resorting to the techniques of an artificial neural network (ANN) and a model tree (MT) and by considering it as an inverse-modelling problem. The time series of significant wave height, average wave period, wave direction and wind direction collected over the years 2007 and 2008 by the Bodega Marine Laboratory (BML) at the Bodega Bay, California, were used along with the corresponding wind speeds measured by a floating buoy in the vicinity. The ANN and MT models were trained and tested using alternative data splits to assess their performance over varying sample sizes. Both these methods worked very well in this application, with the ANN showing better flexibility in model fitting. This study thus indicates that data-driven methods can be effectively used to derive unobserved wind speed values in HF radar measurements.

Simulation and detection of tsunami signatures in ocean surface currents measured by HF radar

High-frequency (HF) surface wave radars provide the unique capability to continuously monitor the coastal environment far beyond the range of conventional microwave radars. Bragg-resonant backscattering by ocean waves with half the electromagnetic radar wavelength allows ocean surface currents to be measured at distances up to 200 km. When a tsunami propagates from the deep ocean to shallow water, a specific ocean current signature is generated throughout the water column. Due to the long range of an HF radar, it is possible to detect this current signature at the shelf edge. When the shelf edge is about 100 km in front of the coastline, the radar can detect the tsunami about 45 min before it hits the coast, leaving enough time to issue an early warning. As up to now no HF radar measurements of an approaching tsunami exist, a simulation study has been done to fix parameters like the required spatial resolution or the maximum coherent integration time allowed. The simulation involves several steps, starting with the Hamburg Shelf Ocean Model (HAMSOM) which is used to estimate the tsunami-induced current velocity at 1 km spatial resolution and 1 s time step. This ocean current signal is then superimposed to modelled and measured HF radar backscatter signals using a new modulation technique. After applying conventional HF radar signal processing techniques, the surface current maps contain the rapidly changing tsunami-induced current features, which can be compared to the HAMSOM data. The specific radial tsunami current signatures can clearly be observed in these maps, if appropriate spatial and temporal resolution is used. Based on the entropy of the ocean current maps, a tsunami detection algorithm is described which can be used to issue an automated tsunami warning message.

Study on the impact of sudden stratosphere warming in the upper mesosphere-lower thermosphere regions using satellite and HF radar measurements

Abstract. The occurrence of a sudden stratospheric warming (SSW) excites disturbances in the mesosphere-lower thermospheric (MLT) wind and temperature. Here, we have examined the high frequency (HF) radar wind data from the South African National Antarctic Expedition, SANAE (72° S, 3° W), a radar which is part of the Super Dual Auroral Radar Network (SuperDARN). Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) on board the Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (TIMED) satellite temperature data and National Centre for Environmental Prediction (NCEP) temperature and wind data are used to investigate the dynamical effects of the unprecedented September 2002 SSW in the Antarctica stratosphere and MLT. The mean zonal wind (from SANAE HF radar) at the MLT shows reversal approximately 7 days before the reversal at 10 hPa (from NCEP). This indicates that there was a downwards propagation of circulation disturbance. Westerly zonal winds dominate the winter MLT, but during the 2002 winter there are many periods of westward winds observed compared to other years. The normalised power spectrums of both meridional and zonal winds show presence of planetary waves (of ~14-day period) before the occurrence of the SSW. The SABER vertical temperature profiles indicated the cooling of the MLT region before the SSW event.

Methodology for a regional tidal model evaluation, with application to central California

Abstract Observations from disparate observational assets, including tide gauges, moorings, and high-frequency (HF) radars, were used to depict the tidal variability, and to evaluate model tidal simulations, for a region off central California, including the Monterey Bay. For this study, the hydrodynamic model was forced only with tides derived from a large-scale model for the northeast Pacific Ocean. Homogeneous density, and initially horizontally uniform density stratification, cases were considered. The model successfully reproduced tidal sea-surface height variations within the model domain, as determined by comparisons with sea level or bottom pressure measured at six locations. To achieve tidal currents with realistic amplitudes, as determined from HF radar and moored measurements, it was found that barotropic velocity, as well as sea level, from the large-scale regional tidal model must be included in specifying the open-boundary condition. However, even with such forcing, the model with homogeneous density field under-predicted the semidiurnal and diurnal barotropic currents as estimated from depth-averaged currents measured at 11 locations. In the diurnal frequency band, the observed surface and nearshore depth-averaged currents are likely influenced by meteorological forcing, which was not included in the model. The HF radar-measured surface tidal currents, both semidiurnal and diurnal, are consistent from year to year and between the winter season and the entire year. Semidiurnal surface tidal currents derived from year-long HF radar measurements do not resemble either the modeled or measured barotropic current fields. Rather, they exhibit amplitudes and small-scale spatial variability indicative of the presence of internal tides, thus indicating that model-derived barotropic tidal currents cannot be validated over large spatial extents using long time series of HF radar-derived surface currents. With initially horizontally uniform vertical density stratification, the model produced surface currents with spatial variability and amplitude range comparable to what was derived from HF radar surface current measurements, but the point by point comparisons are not impressive for this region of complex topography. Likewise, the subsurface current comparisons, performed at four deepwater locations, show considerable model-data differences. Possible reasons for these disparities include the effects of atmospheric forcing, spatially and temporally varying stratification, remotely generated coastally trapped waves, and remotely generated internal tides.

Observations of divergence and upwelling around Point Loma, California

Historical records of near‐surface water temperatures in the Southern Californian Bight often show a preferential cooling in the lee of headlands such as Point Dume, Palos Verdes, and Point Loma. At times, this cooler water is associated with an increase in chlorophyll‐a as is evident in satellite images of ocean color from the region. Here we combine hydrographic data from a 1 day cruise aboard the RV Roger Revelle (a precursor to the 0304 California Cooperative Oceanic Fisheries Investigations (CalCOFI) cruise) with high‐frequency (HF) radar (CODAR) measurements, satellite images, and long‐term thermistor records of near‐surface temperature to identify a small‐scale, isolated upwelling in the lee of Point Loma (32.5°N). Associated with the more saline water downstream of the headland are higher nutrient concentrations, an increase in chlorophyll‐a concentration, and a bloom of chain‐forming diatoms, indicative of a mature upwelling system. It is suggested that this upwelling is not primarily due to local or remote wind forcing but rather to the divergence of the prevailing southerly flow as it passes the Point Loma headland. Time series of surface vorticity calculated from HF radar measurements of sea surface velocity show that as the flow separates from the headland, relative vorticity increases offshore of the cape. Inshore, the time series of divergence/convergence shows a tendency toward divergence at the surface, indicating a preferential upwelling which appears to raise the thermocline, thus resulting in a flux of cold nutrient‐rich water to the surface. In the presence of high nutrients and light, photosynthetic organisms bloom in these upwelled waters as they are advected away from the headland and offshore by the prevailing surface currents.