Derived Surface Currents Research Articles

4D-Var data assimilation and observation impact on surface transport of HF-Radar derived surface currents in the North-Western Mediterranean Sea

We investigated the effect of the assimilation of surface velocities from two HF-Radars (HFRs) and satellite sea surface temperature (SST) into an ocean circulation model, implemented on the North-Western Mediterranean Sea, using the ROMS 4D-Var Data Assimilation (DA) System. A three months period from September to November 2020 is analysed. A model free run is used as reference simulation to quantify the impact of the data assimilation procedure on the surface circulation. The DA framework consists of a sequence of 3-day windows during which the observations are assimilated into the model, followed by forecast simulations, starting from the last time step of the analysis run. The comparison of simulated velocities and SST against the assimilated observations shows that both forecast and analysis runs improve the solution as compared to the freerun, with error reduction up to the 47% and doubling of correlation values. If SST is not assimilated, the simulated sea surface temperature slightly degrades as a result of the DA procedure. Furthermore, we used surface velocities from Lagrangian drifters to test the model against independent observations. The results show that both the forecast and the analysis simulate surface circulation better than the free run within the areas covered by HFR observations, with error reduction ranging from 10% to 50%, and increase in correlation from 5% to more than doubled. Nevertheless, farther afield, outside the area covered by HFRs, improvements and degradations of the solution due to the DA procedure balance each other. The impact of assimilated observation on the alongshore transport of the upper 50 m is analysed along three transects adjacent to the two HFR areas and along a section in between. The transport increment is significant where the surface velocities are assimilated whereas it is negligible elsewhere. The effect of SST is to modify the velocity distribution along the transects, while keeping the transport increment unchanged. Modifications to the initial conditions have the most significant effect on the alongshore transport, whereas the corrections to boundary conditions and atmospheric forcing have different importance according to the analysed transects. The assimilation of surface velocities and SST affects more the ageostrophic component of the surface velocity field, rather than the geostrophic one.

A diagnosis of surface currents and sea surface heights in a coastal region

Upcoming satellite missions will observe the sea surface height (SSH) fields at a very high spatial resolution, which has generated an urgent need to better understand how well geostrophy can represent the ocean current field at finer scales, particularly in coastal regions characterized by complex flow geometry. We conduct statistical and spectral analyses of high-resolution surface currents and SSHs off the Oregon coast to examine the relative contribution of geostrophy and ageostrophy in coastal ocean currents. We analyze forward numerical simulations based primarily on a regional ocean model (ROMS) and use regional observations of high-frequency radar (HFR)-derived surface currents and altimeter-derived geostrophic currents and a subset of global domain numerical simulations (MITgcm) as secondary resources. Regional submesoscale ageostrophic currents account for up to 50% of the total variance and are primarily associated with near-inertial currents and internal tides. Geostrophy becomes dominant at time scales longer than 3 to 10 days and at spatial scales longer than 50 km, and is dependent on the depth and distance from the coast in the cross-shore direction. Ageostrophy dominates in the near-inertial and super-inertial frequency bands, which correspond to near-inertial motions (Coriolis force dominates) and high-frequency internal waves/tides (pressure gradient dominates), respectively. Because of ageostrophy, it may not be possible to estimate submesoscale currents from SSHs obtained from upcoming satellite missions using the geostrophic relationship. Thus, other concurrent high-resolution in-situ observations such as HFR-derived surface currents, together with data assimilation techniques, should be used for constructive data integration to resolve submesoscale currents.

Tidally-induced submesoscale features in the atlantic jet and Western Alboran Gyre. A study based on HF radar and satellite images

An analysis is presented of the submesoscale tidally-induced anticyclonic eddies formed at the eastern mouth of the Strait of Gibraltar and their role in explaining mesoscale features of the Atlantic Jet – Western Alboran Gyre (AJ-WAG) system. These eddies have been identified for the first time using an observational approach based on the analysis of High Frequency Radar (HFR) derived surface currents on the eastern mouth of the Strait which has been complemented with the analysis of Sea Surface Temperature (SST) satellite images. It is found that the major source of positive vorticity to the AJ is provided by its interaction with these anticyclonic eddies which in turn help maintain the water feeding of the upper branch of the WAG. This new role in maintaining the WAG is added to others previously recognized by other authors based on the supply of negative vorticity that these eddies provide to the whole WAG. In addition, these tidally-induced eddies may contribute to the destabilization of the AJ-WAG system when strong and persistent easterly winds coincide with periods of high tidal current amplitude. In these cases, meteorological forcing produces a weak and southward deflected AJ almost disconnected from a displaced eastward WAG. The WAG in turn is being flattened by an overdeveloped North Western Cyclonic Gyre (NWCG) that may be favored by the action of the mentioned eddies.

Prediction of ocean surface trajectories using satellite derived vs. modeled ocean currents

We have assessed the use of satellite derived currents to calculate oceanic surface drift, as an alternative to using currents from ocean models. Predicted trajectories are compared to observed trajectories of two types of drifting buoys, which are subject to different degree of wind and wave forcing, in addition to the ocean surface current. This variable degree of ocean-, wind- and wave forcing is highly relevant for the practical problem of predicting the drift of e.g. oil to aid cleanup operations, and the drift of floating objects to aid search and rescue operations. The calculated trajectories are evaluated in terms of separation distance, and a calculated skill score. For the submerged drifters (CODE/DAVIS), we find that a high resolution non-assimilated ocean model gives the best results, whereas a coarser scale assimilated model performs similarly to satellite derived surface currents from the GlobCurrent project (geostrophic + Ekman components). For the wind-exposed drifters (iSphere), the wind-drift contribution is found to be dominating, and there is only a small impact of using any of the available information about ocean currents. The wind drift contribution to the iSphere drift is estimated to be 3% of the wind if Stokes drift is included in the calculations, and 4% if the Stokes drift is not included. Including Stokes drift in the calculations gives improvement for the submerged drifters, but shows no improvement for the wind exposed drifters.

Quantifying predictability of sea ice around the Indian Antarctic stations using coupled ocean sea ice model with shelf ice

The importance of ice shelf in realistic simulation of the ocean sea ice variability is demonstrated in the Southern Ocean region [9 E - 78 E; 58 S - 71 S] covering the Indian Antarctic Stations Maitri [11.7 E; 70.7 S] and Bharati [76.1 E; 69.4 S] during 2002–2012. A coupled ocean sea-ice modeling with ice shelf is carried out for this purpose. It is shown that the inclusion of shelf ice in the coupled model setup leads to better simulation of the sea surface temperature (SST), sea surface salinity (SSS), and sea ice concentration (SIC) of the region. The effect of ice shelf on the sub-surface temperature and salinity is also examined. The model derived surface currents are used to compute the Finite Time Lyapunov Exponent (FTLE) as a measure of spatio-temporal local predictability in the domain of study. The regions of high and low predictability and their seasonal evolution are clearly demonstrated using the FTLE. The chaotic time series analysis of sea ice volume (SIV) around the Maitri and Bharati stations is performed to examine the predictability of SIV. The largest Lyapunov exponent and Correlation dimension are used as the measures of predictability for this purpose. It is found that there exists a low-dimensional chaotic attractor around the Maitri and Bharati with the correlation dimension of 2.95 and 1.9, respectively. The FTLE as well as the largest Lyapunov exponent analysis suggests that the Maitri region is less predictable as compared to the Bharati region. The analysis reveals that the limit of predictability of the SIV around the Maitri and Bharati regions is 22 days and 25 days, respectively.

Tropical cyclone sensitivity to ocean coupling in the ECMWF coupled model

AbstractWe present an investigation of the performance of the ECMWF coupled atmosphere‐waves‐ocean model for different ocean and atmosphere resolutions on a series of tropical cyclones in the Western Pacific with the aim to better understand the coupled feedback mechanisms in these extreme conditions. For some of the test cases, we only find little impact of coupling the atmosphere to the ocean, while in others, we observe a very large impact. To further understand these differences, we have selected two tropical cyclones (TCs) as case studies: TC Haiyan (with small impact of coupling) and TC Neoguri (with large impact of coupling). The comparison between these two cases suggests that the upper ocean stratification is the key in determining the strength of the coupled feedback. A strong coupled feedback is found whenever the ocean heat content of the upper layer is low while a very weak coupled feedback is found whenever the ocean has a thick warm mixed layer. The oceanographic response to tropical cyclones for the two storms has been compared to sea surface temperature and derived surface currents from drifting buoys and to subsurface observations from Argo and ship launched XBT's. These comparisons show that we are able to realistically reproduce the atmospheric and oceanographic interaction during tropical cyclone conditions which gives us confidence that the coupled modeling system is physically sound.

The MEDESS-GIB database: tracking the Atlantic water inflow

Abstract. On 9 September 2014, an intensive drifter deployment was carried out in the Strait of Gibraltar. In the frame of the MEDESS-4MS Project (EU MED Program), the MEDESS-GIB experiment consisted of the deployment of 35 satellite tracked drifters, mostly of CODE-type, equipped with temperature sensor sampling at a rate of 30 min. Drifters were distributed along and on both sides of the Strait of Gibraltar. The MEDESS-GIB deployment plan was designed as to ensure quasi-synoptic spatial coverage. To this end, four boats covering an area of about 680 NM2 in 6 h were coordinated. As far as these authors know, this experiment is the most important exercise in the area in terms of number of drifters released. Collected satellite-tracked data along drifter trajectories have been quality controlled and processed to build the presented MEDESS-GIB database. This paper reports the MEDESS-GIB data set that comprises drifter trajectories, derived surface currents and in situ SST measurements collected along the buoys tracks. This series of data is available through the PANGAEA (Data Publisher for Earth and Environmental Science) repository, with the following doi:10.1594/PANGAEA.853701. Likewise, the MEDESS-GIB data will be incorporated as part of the Copernicus Marine historical products. The MEDESS-GIB data set provides a complete Lagrangian view of the surface inflow of Atlantic waters through the Strait of Gibraltar and thus, very useful data for further studies on the surface circulation patterns in the Alboran Sea, and their links with one of the most energetic Mediterranean Sea flows: the Algerian Current.

Quality Assessment of HF Radar–Derived Surface Currents Using Optimal Interpolation

AbstractThis study investigates the applicability of the optimal interpolation (OI) method proposed by Kim et al. for estimating ocean surface currents from high-frequency radar (HFR) in the northeastern Chukchi Sea, where HFR siting is dictated by power availability rather than optimal locations. Although the OI technique improves data coverage when compared to the conventional unweighted least squares fit (UWLS) method, biased solutions can emerge. The quality of the HFR velocity estimates derived by OI is controlled by three factors: 1) the number of available incorporating radials (AR), 2) the ratio of the incorporating radials from multiple contributing site locations [ratio of overlapping radial velocities (ROR) or radar geometry], and 3) the positive definiteness [condition number (CN)] of the correlation matrix. Operationally, ROR does not require knowledge of the angle covariance matrix used to compute the geometric dilution of precision (GDOP) in the UWLS method and can be computed before site selection to optimize coverage or after data processing to assess data quality when applying the OI method. The Kim et al. method is extended to examine sensitivities to data gaps in the radial distribution and the effects on OI estimates.

COASTAL CURRENTS MONITORING USING RADAR SATELLITES BASED ON WAVE TRACKING APPROACH

Abstract. Use of high resolution radar imagery for coastal currents monitoring has been known as an active topic in several research institutions. In this study, we demonstrate a new method for coastal currents monitoring based on current tracking by deformed wave reconstruction. The magnitude and shape of the echoes (or waveforms) hold information regarding the surface characteristics that can be described analytically. Accordingly from this shape, several parameters can be construed, by comparing the real waveform with the theoretical curve. Safety in coastal zones containing coastal engineering for construction of ports, marine structures, beach erosion, natural hazards, and environmental problems are the issues related to hydrodynamic studies of currents, waves, and winds. Therefore, monitoring of coastal currents is very important .The ability of the internet to accompany our results to track past and near real-time movements of every coastal waters for monitoring is appropriate. Surface Current Mapping is utilized using an interface to radar altimetry data derived surface currents. Then Data access is available in a number of formats and protocols such as Google Earth KML and so on. In this study Jason1 satellite data has been used as input data for assessing the coastal environment.

Interpretation of Coastal HF Radar–Derived Surface Currents with High-Resolution Drifter Data

Abstract Dense arrays of surface drifters are used to quantify the flow field on time and space scales over which high-frequency (HF) radar observations are measured. Up to 13 drifters were repetitively deployed off the Santa Barbara and San Diego coasts on 7 days during 18 months. Each day a regularly spaced grid overlaid on a 1-km2 (San Diego) or 4-km2 (Santa Barbara) square, located where HF radar radial data are nearly orthogonal, was seeded with drifters. As drifters moved from the square, they were retrieved and replaced to maintain a spatially uniform distribution of observations within the sampling area during the day. This sampling scheme resulted in up to 56 velocity observations distributed over the time (1 h) and space (1 and 4 km2) scales implicit in typical surface current maps from HF radar. Root-mean-square (RMS) differences between HF radar radial velocities obtained using measured antenna patterns, and average drifter velocities, are mostly 3–5 cm s−1. Smaller RMS differences compared with past validation studies that employ current meters are due to drifter resolution of subgrid-scale velocity variance included in time and space average HF radar fields. Roughly 5 cm s−1 can be attributed to sampling on disparate time and space scales. Despite generally good agreement, differences can change dramatically with time. In one instance, the difference increases from near zero to more than 20 cm s−1 within 2 h. The RMS difference and bias (mean absolute difference) for that day exceed 7 and 12 cm s−1, respectively.

Error Characteristics Estimated from Champ, GRACE and GOCE Derived Geoids and from Satellite Altimetry Derived Mean Dynamic Topography

This paper presents a review of geoid error characteristics of three satellite gravity missions in view of the general problem of separating scientifically interesting signals from various noise sources. The problem is reviewed from the point of view of two proposed applications of gravity missions, one is the observation of the mean oceanic circulation whereby an improved geoid model is used as a reference surface against the long term mean sea level observed by altimetry. In this case we consider the presence of mesoscale variability during assimilation of derived surface currents in inverse models. The other experiment deals with temporal changes in the gravity field observed by GRACE in which case a proposed experiment is to monitor changes in the geoid in order to detect geophysical interesting signals such as variations in the continental hydrology and non-steric ocean processes. For this experiment we will address the problem of geophysical signal contamination and the way it potentially affects monthly geoid solutions of GRACE.

A Modeling Study of the Three-Dimensional Continental Shelf Circulation off Oregon. Part II: Dynamical Analysis

Abstract Sixty-day simulations of the subinertial continental shelf circulation off Oregon are performed for a hindcast study of summer 1999. In Part I, the model results are shown to compare favorably with in situ currents and hydrographic measurements obtained from an array of moored instruments and field surveys and high-frequency radar–derived surface currents. In this paper, the modeled three-dimensional, time-varying circulation and dynamical balances are analyzed, providing a detailed synoptic description of the continental shelf circulation off Oregon for summer 1999. The circulation is clearly wind driven and strongly influenced by alongshore variations in shelf topography. In the region of the coast where the alongshore topographic variations are small the upwelling circulation is consistent with standard conceptual models for two-dimensional across-shore circulation. In the regions where the alongshore topographic variations are greater, the upwelling circulation is highly three-dimensional. Ov...

Ocean and climate: A quantitative answer, TOPEX/POSEIDON

Since B. Franklin and F. Arago, the Oceans are known for their regulator role in the climate evolution. Beyond this simple ascertainment, it is now necessary and possible to quantify more precisely the impact of the Oceans in climate global changes. Accurate observations at a global scale and over several decades, jointly used with dynamic models, are needed to solve this problem. One dedicated technique is radar altimetry on-board satellites. Sea surface slopes and derived surface currents can thus be measured with a remarkable accuracy. The TOPEX/POSEIDON altimetry mission was optimized to fulfil this objective, this project was jointly conducted by NASA and CNES with support of the associated scientific community involved in the design, the evaluation and the analysis of the system. Two years after the launch (10 August 1992), the results are even better than expected. Sea surface heights and variations are measured at a basin-scale, with an accuracy of 1–2 centimeters over 10 days. Such performances give access to the seasonal changes due to the thermal expansion between the two hemispheres, the interannual variability and the detection and propagation of Kelvin and Rossby waves in the tropical regions. These observations can then be assimilated in numerical models to describe and predict the ocean circulation in four dimensions.