Guide Coastal Altimetry

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The second approach is the along-track comparison Sect. The HFR radial directions from both sites are represented by grey lines and the selected radial directions from the Matxitxako site for the pointwise comparison are plotted in red the central radial orthogonal to the track and in blue the adjacent radials. This method, previously applied in Liu et al.

This approximation allows us to directly use the radar radial currents, which are in the same direction as the across-track AC G. Then, the across-track AC HFR,R in our point was obtained by firstly averaging the values for each radial, so that only three values along the track were obtained. This permitted us to ensure a similar spatial smoothing for both data sets. For that purpose, AC HFR,T was interpolated into the along-track altimetry points, and it was rotated to the across-track direction.

Then, for each along-track AC G point, the average with its four adjacent points was calculated. As in the previous case, this permitted us to ensure a similar spatial smoothing for both data sets. The sensitivity to the number of adjacent points considered was tested, and this approach was the one that provided the best adjustment to the HFR data. Ekman currents were estimated to evaluate what their contribution to LF currents in the area was, and how this component contributed to part of the differences observed between HFR and altimetry.

These parameter values were taken from Caballero et al.


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Ekman currents initially computed in the locations of the WRF model nodes were interpolated and rotated from zonal and meridional directions to along-track and across-track directions. In the along-track current comparison, they were interpolated to the altimetry along-track points and, then, rotated to get the across-track component. All these parameters were computed for the study period.

Moreover, in Fig. Black arrows depict the slope current intensifications mentioned in the text. The RMSD also decreases between 0. The same effect was observed in Liu et al. Moreover, the first point in CTOH is removed because it is an outlier. Although in Fig.

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These spatial differences agree with what was observed in the area by Rubio et al. The lower current velocities and lower vertical coherence observed at Donostia buoy during winter could be linked to the complex bathymetry, which might force the IPC to flow over deeper grounds out of the point measured by the buoy. However, the differences are small and do not permit us to draw conclusions on their relative accuracy.

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However, the difference in the RMSD at each point i. The monthly values of the statistical parameters shown in the figure, have been computed considering all the available data for that month during all the study period. It can be observed that in terms of monthly mean currents, the three time series have the same tendency and that in general there is low discrepancy among them.

No significant differences in terms of monthly patterns are observed among the two altimetry products. The winter poleward current intensification is evident from October to January with a maximum in November ranging for all data sets from 7. The addition of AC E slightly strengthens the intensity of the slope currents for both poleward and equator-ward directions. This can be explained by the general wind patterns of the area, which are in agreement with the main local geostrophic regime, although winter south-westerlies are stronger than the summer north-easterlies Herbert et al.

These values are slightly increased by the addition of AC E , especially in winter when winds are stronger. In the last 3 months of the year, it is increased. Therefore, there is a higher variability in late autumn and winter, probably due to the slope current intensification and the stronger winds. This increase is coherent with the intensification of the slope current and the development of an anticyclonic structure in March and especially in April near Torrelavega canyon Caballero et al.

Afterwards, the variability is practically maintained, with small oscillations and an increase in CMEMS data. Temporal statistics considering all the study period for each point of the track are plotted as a function of the distance to the first point of the track. From there on, the mean is oriented equator-ward. The mean AC G is also close to zero; however, it shows larger variability, changing between positive and negative values along the track, with a lack of agreement between both altimetry products at some points.

The addition of AC E does not cause any spatial variation, and it barely changes the values. With regard to the variability, it is higher close to the coast.


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It can be once again observed that the addition of the AC E slightly increases the variability Fig. This can be linked to the absence of a strong and persistent geostrophic component and a higher signal-to-noise ratio for the HFR data which increases as we get away from the antennae. These values could be explained by the fact that those points are located in the middle of the slope, where the slope current is stronger and where they are out of the Capbreton canyon area.

At the same time, in that area, the slope current direction is nearly orthogonal to the track, so that the across-track component is stronger. For CTOH, the values around the maximum are relatively high, that is why the maximum is not a prominent peak.

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In order to provide a complementary insight into the synergies and differences between HFR and altimeter data, in this section the observability of different processes detected by HFR and altimetry is qualitatively analysed. Only CMEMS data are used for this analysis, since the statistical results are very similar for both altimetry products and CMEMS data have less data gaps in the period and study area, which is more suitable for monitoring ocean processes. This is coherent with the contribution of the main driving factor of the seasonal SLA variability in the area, the steric effect.

It was observed in Caballero et al. AC E shows a poleward seasonality with intensifications mainly in autumn and winter usually from November to February and weaker equator-ward currents in spring and summer usually from March to October Fig. This fact agrees with the general wind pattern in the area. In spring and summer the gradients are weaker and even suggest equator-ward currents along all the track. In spring and summer, although there are also several poleward current pulses, they are weaker. During this period, equator-ward current pulses are also observed. The black diamonds depict the IPC intensification signals whereas the black inverted triangles show the eddy events, all mentioned in the text.

More details on these events are provided in Fig.

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The poleward patterns observed by the HFR agree with the AC G observed along the altimetry track, which shows that poleward currents intensified over the slope. For the four events, the SST images show that the current intensifications along the slope are related to an increase of 0. In this event, the strongest agreement between AC G and HFR total currents is observed over the slope, while they disagree in the north-western area of the domain. These observations corroborate the results obtained in Sect.

The black line shows the altimeter's track. Although the effect of the presence of mesoscale eddies has not been explored in terms of statistical results, there is a qualitative agreement between AC G and HFR total currents when eddies are observed in the area covered by the two measuring systems even if the eddy core is not crossed by the altimeter track.

This happens when either the eddies cross the track of the altimeter or when the size of the eddies, whose centre is located off the track, is large enough to be observable by the altimetry. In this case the eddy is cyclonic, and though the HFR and altimetry currents in the area occupied by the structure agree with each other, this is not the case in the rest of the track. This anticyclone was analysed by Rubio et al. North of this eddy, the altimetry and the HFR detect a cyclonic circulation, but in this case, it is not clear from the HFR total current fields that the structure is an eddy.

Black arrows depict the HFR current fields. Note that the scale of each kind of arrow is not the same. In this study, we have investigated the synergies and differences between land-based HFR and satellite altimetry, two remote-sensing techniques that provide measurements of the ocean surface currents at different temporal and spatial scales.

The comparisons were carried out in terms of time anomaly of currents, following different approaches with radial and total OMA HFR data.


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    Email Address. First, we will focus on the general seasonal characteristics of the circulation. Then, we will analyze the boundary currents in more detail. Finally, we will propose a new schematic description of the seasonal circulation from altimetry. To do this, we applied the method already used in Birol et al. The results globally confirm the seasonal circulation features highlighted in previous studies based on shorter time series of in situ data, satellite imagery observations, or numerical modeling.

    However, its continuation on track is somewhat confusing as the latter clearly shows two current veins.

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    The separation, which occurs on the western flank of Adventure Bank, is likely due to bathymetric constraints. The northward Algerian Current branch would feed both the AIS and the northeast flow observed on track north of Sicily the Bifurcation Tyrrhenian Current, as stated by Sorgente et al. Near Cape Bon and the western Sicily coast, the amplitude of the seasonal cycle and the southeastern mean current vectors are also strong. This corresponds to the ATC along the track This matches the ATC path steered by the m isobath [ Alhammoud et al.

    The AIS related veins on tracks , , and show a maximum in summer. The northern part of the Sidra Gyre on track is maximum in early spring but the period of the maximum is unclear on its southern part. Over most of the remaining areas, the maximum amplitude is clearly reached in winter. However, the corresponding minimum phase displays a larger dispersion over the region ranging from June to September than the maximum. This does not allow full characterization of the corresponding seasonal cycles. Moreover, the seasonal variability of the circulation will also be discussed based on the climatological AGV patterns together with the corresponding SST fields Figure 11 over 4 months.

    In fact, the Atlantic Water signature is known to be associated with a shelf slope density front [ Hamad et al. Note that the resulting ATC branch is in phase with the Algerian Current seasonal cycle cyan and blue dashed lines, Figure This suggests that most of the variability in the area is strongly steered by the bathymetry, except where the stratification is maximal in summer.

    Moreover, this permanent South East flow on track green dashed line, Figure 10 is moderately in phase with the upper ATC branch near Cape Bon cyan dashed line, Figure This confirms the predominant South East direction of the ATC and its early winter intensification already observed in Gasparini et al. Another interesting feature highlighted by Figure 11 is the possible signature of the Sidra Gyre on track approximately The winter AGV also shows a coherent eastward flow over the southernmost part of tracks and Figure 11 along the m isobath.

    However, the opposite westward AGV on the southern end of track suggests a deviation offshore of the eastward ATC outflow, possibly twisting over the Sidra Gyre northern arm. Indeed, the phasing of the averaged AGV on the Atlantic Libyan Current path and the southern part of the Sidra Gyre red solid and blue squares line, Figure 10 confirms this finding. This joint seasonality and positions of the Atlantic Libyan Current and Sidra Gyre are consistent with recent works such as Sorgente et al.

    The southeastward AIS veins observed on the northern parts of tracks , , and show a clear seasonal cycle, with maximum values in summer and fall cyan, green and black solid lines, Figure 10 when the thermal front is well marked Figure As suggested by Figure 11 , the AIS summer intensification is mainly due to the Sicily upwelling on Adventure Bank induced by the dominant summer and fall northwesterly winds [ Lermusiaux and Robinson , ]. It shows a minimum in spring, consistent with the upward minimum on track black dashed and green solid lines, Figure Based on the above analysis Figures 9 - 11 , we can now propose a new regional circulation scheme Figure 12 for the seasonal surface circulation.

    Seasonal circulation schemes in the central Mediterranean. The pronounced suspected branches are in solid dashed lines. The new known branches are in green blue. The winter and spring patterns are quite similar. The summer and fall situations also show similar patterns with some differences. The fall season reveals two ways the AIS can be fed. This western extension of the ATC was revealed by numerical modeling in Sorgente et al. It was described as a stable and weak current flowing from November to April [ Sorgente et al.

    Both the SST and the AGV patterns along track suggest a vein that would first enter the Gulf of Gabes before again joining the m isobath advecting the Atlantic Water southeastward toward the Libyan coast. The vein appears to feed the narrow Atlantic Libyan Current that flows southeastward during winter when the Sidra Gyre extension is most contracted compared with the rest of the climatological year.

    The other seasonal schemes are very similar. This observation agrees with the findings of Sorgente et al. The study is based on recent releases of Level 3 altimetry products that include refined regional processing. Despite the relatively good agreement between in situ and altimetry data, several questions still remain, such as the ability of tide gauges in harbors to measure the same sea level variations as those observed by altimetry further offshore.

    The significant differences between both products especially appeared in qualitative comparisons with SST fields over the shallow water of the Gulf of Gabes Figure 7. These periods of maxima and minima correspond to those reported by Ciappa [ ], Sorgente et al. This new scheme is consistent with the recent works of Ciappa [ ] and Sorgente et al.

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    The persistence of the observed circulation features at a climatological scale indicates their recurrent character, especially the connection of the Atlantic Tunisian Current with the Atlantic Libyan Current in winter. This paper shows the ability of conventional altimetry to characterize the circulation seasonal variability in the study area.

    This is clearly beyond the scope of the present study and will be the subject of a future paper. As a next step, there is no doubt that the synergistic use of altimetry with regional modeling and complementary observing systems in situ data, HF radars, surface drifters, satellite remote sensing data should allow resolution of a wider spectrum of spatial and temporal scales of variability [ Birol et al. However, this study demonstrates that, when carefully analyzed, the derived geostrophic currents can be exploited to detect the regional circulation in the central Mediterranean.

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