Search

Over the last 3 decades, satellite altimetry has observed sea surface height variations, providing a regular monitoring of the surface ocean circulation. Altimetry measurements have an intrinsic signal-to-noise ratio that limits the spatial scales of the currents that can be captured. However, the recent progress made on both altimetry sensors and data processing allows us to observe smaller geophysical signals, offering new perspectives in coastal areas where these structures are important. In this methodological study, we assess the ability of three altimeter missions with three different technologies to capture the Northern Current (northwestern Mediterranean Sea) and its variability, namely Jason-2 (Ku-band low-resolution-mode altimeter, launched in 2008), SARAL/AltiKa (Ka-band low-resolution-mode altimeter, launched in 2013) and Sentinel-3A (synthetic aperture radar altimeter, launched in 2016). Therefore, we use a high-resolution regional model as a reference. We focus along the French coast of Provence, where we first show that the model is very close to the observations of high-frequency radars and gliders in terms of surface current estimates. In the model, the Northern Current is observed 15–20 km from the coast on average, with a mean core velocity of 0.39 m s -1. Its signature in terms of sea level consists of a drop whose mean value at 6.14 ∘ E is 6.9 cm, extending over 20 km. These variations show a clear seasonal pattern, but high-frequency signals are also present most of the time. In comparison, in 1 Hz altimetry data, the mean sea level drop associated with the Northern Current is overestimated by 3.0 cm for Jason-2, but this overestimation is significantly less with SARAL/AltiKa and Sentinel-3A (0.3 and 1.4 cm respectively). In terms of corresponding sea level variability, Jason-2 and SARAL altimetry estimates are larger than the model reference (+1.3 and +1 cm respectively), whereas Sentinel-3A shows closer values (-0.4 cm). When we derive geostrophic surface currents from the satellite sea level variations without any data filtering, in comparison to the model, the standard deviations of the velocity values are also very different from one mission to the other (3.7 times too large for Jason-2 but 2.4 and 2.9 times too large for SARAL and Sentinel-3A respectively). When low-pass filtering altimetry sea level data with different cutoff wavelengths, the best agreement between the model and the altimetry distributions of velocity values are obtained with a 60, 30 and 40–50 km cutoff wavelength for Jason-2, SARAL and Sentinel-3A data respectively. This study shows that using a high-resolution model as a reference for altimetry data allows us not only to illustrate how the advances in the performances of altimeters and in the data processing improve the observation of coastal currents but also to quantify the corresponding gain. [ABSTRACT FROM AUTHOR]

Geostrophic currents derived from altimetry are used to investigate the surface circulation in the Western Tropical Atlantic over the 1998-2017 period. Using six horizontal sections defined to capture the current branches of the study area, we investigate their respective variations at both seasonal and interannual time-scales as well as the spatial distribution of these variations. Our results show that the central branch of the South Equatorial Current, the North Brazil Current component located south of the equator, the Guyana Current and the northern branch of the South Equatorial Current at 42° W have similar annual cycles, with maxima/minima during boreal winter-spring/October-November. In contrast, the seasonal cycles of the North Brazil Current branch located between the equator and 7-8° N, the North Brazil Current retroflected branch and the North Equatorial Countercurrent show maxima/minima during boreal fall/May. West of 42° W, an eastward current is observed between 0°-2° N, identified as the equatorial extension of the retroflected branch of the North Brazil Current. It is part of a large cyclonic circulation observed between 0°-6° N and 35°-45° W during boreal spring. The North Equatorial Countercurrent shows a two-core structure during the second half of the year, when we also observe the two regions where the North Brazil Current retroflects. The latter can be related to the wind stress curl seasonal changes. At interannual scales, depending on which side of the equator, the North Brazil Current exhibits two opposite scenarios related to the tropical Atlantic Meridional Mode phases. The interannual variability of the North Equatorial Countercurrent and of the northern branch of the South Equatorial Current (in terms of both strength and/or latitudinal shift) at 42° W are also associated to the Atlantic Meridional Mode, while they are associated to the zonal mode phases at 32° W. [ABSTRACT FROM AUTHOR]

Coastal HF radar systems provide operational measurements of coastal surface currents with high spatial (~ 1-5 km) and temporal (~ hourly) sampling resolution while the near continuous altimetry missions afford, from 1993 to nowadays, information of 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 datasets in the overlapping areas. In this study, a HF radar system and two Jason-2 satellite altimetry products with different processing (CTOH and CMEMS) 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 datasets within the study area and a better understanding of the ocean variability contained in the HF radar and both altimetry data sets. The variability of the radar and altimetry measurements is higher near the coast, and both observing systems detect the Iberian Poleward Current and eddies, which are the main mesoscale processes within the study area. The highest correlations between radar and altimetry (up to 0.64) take place in the slope, where the Iberian Poleward Current represents a significant part of the mesoscale variability. Besides, the use of a simple Ekman model to add the wind-induced current component to the to the altimetry-derived geostrophic currents increases the agreement between both data sets (increasing the correlation in around 10 %). [ABSTRACT FROM AUTHOR]

Abstract: Altimetry has become a powerful tool to understand the dynamics of the deep-sea ocean circulation. Despite the technical problems encountered in the coastal zone by this observational technique, resulting in large data gaps in those areas, solutions already exist to mitigate this issue and to allow the retrieval of coastal information from existing altimetric data. Using some of these solutions, we have reprocessed a new set of 14.5years of the TOPEX/Poseidon and Jason-1 satellite altimeter data over the Northwestern Mediterranean Sea, leading to a significant increase in the quantity of available data near coastlines. Time series of geostrophic surface velocity anomalies have been computed from the along-track altimeter sea level anomalies. In this paper, we evaluate the ability of these altimeter-derived currents to capture the main surface circulation features and the associated seasonal variability in the area of interest. In-situ ADCP current measurements are used to estimate the accuracy of altimeter geostrophic surface velocity anomalies at different locations on the shelf edge. The results indicate good qualitative altimeter performances at seasonal time scales, confirming that altimetry is reliable to observe synoptic variations of the Liguro–Provençal–Catalan Current System. The seasonal evolution of the shelf edge flow is then documented using results from satellite altimetry and from sea surface temperature (SST). The regional picture of the shelf edge circulation that emerges agrees fairly well with previous knowledge (the flow is much stronger during winter than during summer) but also reveals interesting aspects of the coastal current system: (1) the characteristics of the seasonal cycle observed appear highly consistent along the Northwestern Mediterranean shelf break, suggesting a continuous current from the Tyrrhenian to the Balearic Seas, (2) the relationship with the Balearic Current appears somewhat more complex and suggests that its evolution is controlled by another inflow contribution, at least in spring, (3) the seasonal variations of the shelf edge flow over a particular year can show large discrepancies with the averaged picture presented in this study, since large year to year differences are observed. [Copyright &y& Elsevier]

We present a methodology to derive surface geostrophic current from a newly released altimetric sea-level data set. TOPEX/Poseidon data were first completely reprocessed from Geophysical Data Records using new algorithms accommodating marginal seas and coastal conditions. The methodology applied to the reprocessed data essentially consists of a smoothing of the raw along-track coastal altimetric data at scales at which the geostrophic equilibrium holds. This was reduced to a computational procedure using a set of objective criteria. We have applied the method to the East India Coastal Current (EICC) at the western boundary of the Bay of Bengal. This paper first examines the quality of the new data set, which compares well with tide-gauge data; the current we derived is consistent with independent estimates. Our methodology reveals the full spectrum of the along-shore current, ranging from intra-seasonal to inter-annual time scales, from the deep ocean to the shelf-break area where the EICC exists. The algorithm can be applied to any coastal region where an order of the Rossby radius can be defined, and it therefore opens up bright prospects for mapping the variability of other boundary-current systems in the world ocean from altimetry. [ABSTRACT FROM AUTHOR]

Satellite altimetry enables to obtain the Sea Surface Height (SSH) variations along the satellite tracks. More and more accurate observations are available near the coasts thanks to advances in the data processing and to technological innovations. As a consequence, fine-scale coastal structures (eg down to 30-50 km, depending on their signature in SSH) can be regularly monitored. However, in terms of geostrophic current derived from the SSH altimetry gradients, intercomparisons with in situ observations show regularly that their amplitude is underestimated. This is due to the signal-over-noise ratio associated to altimetry measurements, which results in a loss of information. If we want to optimize the use of altimetry in coastal studies, it is important to quantify what current component can/can't be observed. This is the objective of this work.The North-Western Mediterranean Sea (NWMed) has become a pilot area for coastal altimetry studies. It is a particularly interesting area in terms of coastal ocean dynamics, associated to a large number of fine scale structures. It also benefits from a large number of in situ current observations (ADCP, gliders, HF radars). In this study, the objectives are to quantify the SSH signature of dedicated coastal processes in the NWMed and to analyse the spatio-temporal scales associated. We focus on the Northern Current (or NC, less than 60 km wide) which flows cyclonally along the Italian, French and Spanish coasts. We use a high resolution numerical model (Symphonie) which is first validated against independent in situ datasets. Then it is used to quantify the SSH signature of the NC at two distinct locations along the French coast. We then compared the results obtained with the spatio-temporal variations observed in satellite altimetry data and quantify and study the differences. We also analyze the origin of the differences observed between different types of in situ current observations and altimetry data : their respective spatial/temporal resolution, track orientation, differences in location, geostrophic vs total current component,... Finally, this study also allows us to understand the intrinsic differences obtained from different types of ocean current observations. [ABSTRACT FROM AUTHOR]