Jérôme Bouffard

Exploiting the potential of an improved multimission altimetric data set over the coastal ocean

Until now, most satellite altimetry studies of the coastal ocean have been based on along-track data from a single mission, whereas up to four missions were operative in 2002–2005. Here, to monitor the coastal ocean we have applied specialized corrections and dedicated processing strategies to compute a multimission data set at a mean distance of 32 km of the coast. The resulting altimetric data set is compared with sea level data from three in situ stations over a coastal zone of the northwestern Mediterranean. The mean rms difference between this data set and the sea level stations is 2.9 cm against 3.7 cm when using the AVISO altimetric product. Comparison of altimeter-derived geostrophic velocities with a mooring also shows that the spatial and temporal variability of the surface current field is well reproduced. The agreement with in situ measurements extends to intraseasonal time scales showing a significant improvement compared to previous studies in the 50 km coastal-band.

Post-processing Altimeter Data Towards Coastal Applications and Integration into Coastal Models

Altimetry missions in the last 16 years (TOPEX/Poseidon, ERS-1/2, GFO, Jason-1 and ENVISAT) and the recently-launched Jason-2 mission have resulted in great advances in deep ocean research and operational oceanography. However, oceanographic applications using satellite altimeter data have become very challenging over regions extending from near-shore to the continental shelf and slope (Cipollini et al. 2008). In these regions, intrinsic difficulties in the corrections (e.g., the high frequency ocean response to tidal and atmospheric loading, the mean sea level, etc.) and issues of land contamination in the radar altimeter and radiometer footprints result in systematic flagging and rejection of these data. Forthcoming altimeter missions (SARAL/AltiKa, SWOT, Sentinel-3, etc.) are designed to be better-suited for use in the coastal ocean. However, a number of studies have dealt with the problem of re-analysing, improving and exploiting the existing archive to monitor coastal dynamics. The early encouraging results (Vignudelli et al. 2005; Bouffard et al. 2008, Birol et al. submitted J Mar Syst 2009) support the need for continued research in coastal altimetry, with the opportunity of providing input and recommendations for future missions.

Introduction and Assessment of Improved Coastal Altimetry Strategies: Case Study over the Northwestern Mediterranean Sea

Improved coastal altimetry strategies are validated over the northwestern Mediterranean Sea with tide gauge (TG) records. Cross-comparisons made with a standard altimetric product highlight significant qualitative and quantitative improvements. The data processed by improved methods are able to detect smaller dynamical processes compared to the standard altimetric products. Lastly, the improved datasets allow us to recover additional coastal data, principally closer to the coast. The improved altimetric data have been used to monitor the Liguro-Provencal-Catalan current coastal dynamics in the Gulf of Lion. For the first time, we have both altimetric and sea surface temperature (SST) observations of intrusions impinging on the continental shelf following a strong southeasterly wind event, which are consistent with past studies based on numerical simulations.

Monitoring open‐ocean deep convection from space

Deep convection (DC) is a key-process of the oceanic circulation, costly to monitor in situ and under the influence of climate change. Our study is a first step toward monitoring DC from space: we investigate the feasibility of observing its variability using improved satellite altimetry. An oceanic simulation of the Mediterranean circulation was performed for the 1999-2007 period. DC interannual variability is realistically modelled, and the sea surface elevation (SSE) is in agreement with altimetry data. Numerical results show a strong correlation between the annual DC characteristics and the winter SSE. From that, we propose a method to monitor DC interannual variability and long term evolution using altimetry data. Our method, applied to the longest available altimetry series, represents correctly the interannual variability of DC in the Northwestern Mediterranean between 1994 and 2007. Citation: Herrmann, M., J. Bouffard, and K. Beranger ( 2009), Monitoring open-ocean deep convection from space, Geophys. Res. Lett., 36, L03606, doi:10.1029/2008GL036422.

Exploiting satellite altimetry in coastal ocean through the ALTICORE project

Altimeter-derived information on sea level and sea state could be extremely important for resolving the complex dynamics of the coastal ocean. Satellite altimetry was not originally conceived with coastal ocean in mind, but future missions (AltiKa and CryoSat-2) promise much improved nearshore capabilities. A current priority is to analyze the existing, under-exploited, 15-year global archive of coastal altimeter data to draw recommendations for these missions. There are intrinsic difficulties in processing and interpretation of the data, e.g.: the proximity of land, control by the seabed, and rapid variations due to tides and atmospheric effects. But there are also unexploited possibilities, including higher along track data rates and multi-altimetry scenarios that need to be explored. There are also difficulties of accessing and manipulating data from multiple sources, many of which undergo regular revision and enhancement. In response to these needs, the ALTICORE (ALTImetry for COastal REgions - www.alticore.eu) project started in December 2006, funded for two-years by the European INTAS scheme (www.intas.be). The overall aim of ALTICORE is to build up capacity for provision of altimeter-based information in support of coastal ocean studies in some European Seas (Mediterranean, Black, Caspian, White and Barents). ALTICORE will also contribute to improved cooperation between Europe and Eastern countries and enhance networking of capacity in the area of satellite altimetry. This paper discusses the approach, summarizes the planned work and shows how the coastal community should eventually benefit from better access to improved altimeter-derived information.

Satellite radar altimetry from open ocean to coasts: challenges and perspectives

The history of satellite radar altimetry stems from the need to capture a global view of the surface topography of the oceans. As altimeters are specifically designed for global observations, they encounter major problems in coastal regions, such as relatively poor sampling and inaccuracy of the corrections, so measurements are generally discarded. Nevertheless, a global archive of 15 years of raw data from a series of missions is presently available. The huge amount of unused data in coastal regions can be re-analyzed, improved and more intelligently exploited, possibly promoting coastal altimetry to the rank of operational service. This paper outlines the obstacles limiting the use of the data, discusses some areas of improvement, shows the lessons learned from a case-study in the Mediterranean Sea, and shows that the improved coastal altimetry concept can be extended to other regions, e.g. along the coasts of India. This paper also explores the implications of adopting the emerging vision of the Internet infrastructure in the coastal altimetry context: a collection of unstructured information becomes a network of linked data and software, necessary to perform the specialized on-the-fly processing of the raw data to provide ready-to-use geophysical parameters such as sea level and significant wave height.

Inter-annual variations of surface currents and transports in the Sicily Channel derived from coastal altimetry

Twenty-year coastal altimetry data set (X-TRACK) is used, for the first time, to gain insight into the long-term inter-annual variations of the surface circulation in the Sicily Channel. Firstly, a spectral along with a time/space diagram analysis are applied to the monthly means of the X-TRACK geostrophic currents over the period 1993-2013. They reveal a regionally coherent current patterns from track to track with a marked inter-annual variability that is unequally shared between the Atlantic Tunisian Current and Atlantic Ionian Stream inflows in the Sicily Channel and the Bifurcation Tyrrhenian Current outflow northeast of Sicily. Secondly, an empirical altimetry-based transport-like technique is proposed to quantify volume budgets inside the closed boxes formed by the crossing of the altimetry tracks and coastlines over the study area. A set of hydrographic measurements is used to validate the method. The inferred altimetry transports give a well-balanced mean eastward Atlantic Waters baroclinic flow of 0.4 Sv and standard deviations of 0.2 Sv on a yearly basis throughout the Sicily Channel and toward the Ionian Sea, which is fairly coherent with those found in the literature. Furthermore, the analysis allows to quantify the intrusions of Atlantic Waters over the Tunisian Shelf (0.12 +/- 0.1 Sv) and highlights two main modes of variability of the main surface waters path over the Sicily Channel through the Bifurcation Atlantic Tunisian Current and Atlantic Ionian Stream systems. Some physical mechanisms are finally discussed with regards to changes in the observed currents and transports.

Reprocessing Altimeter Data Records along European Coasts: Lessons Learned from the Alticore Project

A coastal-oriented processing strategy has been developed in the Northwestern part of the Mediterranean Sea and has showed that improved altimetry in the coastal ocean is feasible and could be extended to other regions. In this work, we will provide an overview of current capabilities and challenges of existing altimetry products in Black, Caspian, White and Barents seas, in the prospect of increasing the quantity and quality of data in these regions. With respect to the work done in the project called ALTICORE (ALTImetry for COastal REgions - www.alticore.eu), the obstacles limiting the use of the data and the possible areas of improvement are highlighted and discussed.