Charles Troupin

Implications of refined altimetry on estimates of mesoscale activity and eddy-driven offshore transport in the Eastern Boundary Upwelling Systems

We investigate the extent to which the recently upgraded version of the Ssalto/Duacs sea level anomaly product affects the description of mesoscale activity in the Eastern Boundary Upwelling Systems (EBUS). Drifter observations confirm that the new data set released by Archiving, Validation and Inter- pretation of Satellite Oceanographic data (AVISO) in April 2014 (DT14) offers an enhanced description of mesoscale activity for the four EBUS. DT14 returns significantly higher eddy kinetic energy levels (+80%) within a 300 km coastal band, where mesoscale structures are known to induce important lateral phys- ical and biogeochemical fluxes. When applied to DT14, an automatic eddy detection algorithm detects more eddies in the EBUS (+37%), and lower eddy radius estimates, in comparison with results using the for- mer altimetry product (DT10). We show that despite higher eddy densities, the smaller eddy radii result in westward eddy transport estimates that are smaller than those obtained from DT10 (−12%).

Assessing SARAL/AltiKa Data in the Coastal Zone: Comparisons with HF Radar Observations

We present an initial assessment of SARAL/AltiKa data in the coastal band. The study focuses on the Ibiza Channel where the north-south water exchanges play a key role in controlling the circulation variability in the western Mediterranean. In this area, the track 16 of SARAL/AltiKa intercepts the domain covered by a coastal high-frequency (HF) radar system, which provides surface currents with a range up to 60 km. We evaluate the performance of the SARAL/AltiKa Ssalto/Duacs delayed-time along-track products compared to the HF radar surface velocity fields. SARAL/AltiKa data are retrieved at a distance of only 7 km from the coast, putting in evidence the emerging capabilities of the new altimeter. The derived velocities resolved the general features of the seasonal mesoscale variability with reasonable agreement with HF radar fields (significant correlations of 0.54). However, some discrepancies appear, which might be caused by instrumental hardware radar errors, ageostrophic velocities as well as inaccurate corrections and editing in the altimeter data. Root mean square (rms) differences between the estimated SARAL/AltiKa and the HF radar velocities are about 13 cm/s. These results are consistent with recent studies in other parts of the ocean applying similar approaches to Topex/Poseidon and Jason-1 missions and using coastal altimeter corrections.

A Multiplatform Experiment to Unravel Meso- and Submesoscale Processes in an Intense Front (AlborEx)

The challenges associated with meso- and submesoscale variability (between 1-100 km) require high-resolution observations and integrated approaches. Here we describe a major oceanographic experiment designed to capture the intense but transient vertical motions associated with mesoscale and submesoscale features in an area characterized by strong fronts. Finescale processes were studied in the eastern Alboran Sea (Western Mediterranean) about 400 km east of the Strait of Gibraltar, a relatively sparsely sampled area. In-situ systems were coordinated with satellite data to provide a full description of the physical and biogeochemical variability. Hydrographic data confirmed the presence of an intense salinity front formed by the confluence of Atlantic Waters, entering from Gibraltar, with the local Mediterranean waters. The drifters coherently followed the northeastern limb of an anticyclonic gyre. Near real time data from acoustic current meter data profiler showed consistent patterns with currents of up to 1m/s in the southern part of the sampled domain. High-resolution glider data revealed submesoscale structures with tongues of chlorophyll-a and oxygen associated with the frontal zone. Numerical results show large vertical excursions of tracers that could explain the subducted tongues and filaments captured by ocean gliders. A unique aspect of AlborEx is the combination of high-resolution synoptic measurements of vessel-based measurements, autonomous sampling, remote sensing and modeling, enabling the evaluation of the underlying mechanisms responsible for the observed distributions and biogeochemical patchiness. The main findings point to the importance of fine-scale processes enhancing the vertical exchanges between the upper ocean and the ocean interior.

Untangling spatial and temporal trends in the variability of the Black Sea Cold Intermediate Layer and mixed Layer Depth using the DIVA detrending procedure

Current spatial interpolation products may be biased by uneven distribution of measurements in time. This manuscript presents a detrending method that recognizes and eliminates this bias. The method estimates temporal trend components in addition to the spatial structure and has been implemented within the Data Interpolating Variational Analysis (DIVA) analysis tool. The assets of this new detrending method are illustrated by producing monthly and annual climatologies of two vertical properties of the Black Sea while recognizing their seasonal and interannual variabilities : the mixed layer depth and the cold content of its cold intermediate layer (CIL). The temporal trends, given as by-products of the method, are used to analyze the seasonal and interannual variability of these variables over the past decades (1955–2011). In particular, the CIL interannual variability is related to the cumulated winter air temperature anomalies, explaining 88 % of its variation.

Approximate and Efficient Methods to Assess Error Fields in Spatial Gridding with Data Interpolating Variational Analysis (DIVA)

AbstractThis paper presents new approximate methods to provide error fields for the spatial analysis tool Data Interpolating Variational Analysis (DIVA). The first method shows how to replace the costly analysis of a large number of covariance functions with a single analysis for quick error computations. Then another method is presented where the error is only calculated in a small number of locations, and from there the spatial error field itself is interpolated by the analysis tool. The efficiency of the methods is illustrated on simple schematic test cases and a real application in the Mediterranean Sea. These examples show that with these methods, one has the possibility for quick masking of regions void of sufficient data and the production of “exact” error fields at reasonable cost. The error-calculation methods can also be generalized for use with other analysis methods such as three-dimensional variational data assimilation (3DVAR) and are therefore potentially interesting for other implementations.

The Cape Ghir filament system in August 2009 (NW Africa)

In the framework of the Canaries-Iberian marine ecosystem Exchanges (CAIBEX) experiment, an interdisciplinary high-resolution survey was conducted in the NW African region of Cape Ghir (30°38′N) during August 2009. The anatomy of a major filament is investigated on scales down to the submesoscale using in situ and remotely sensed data. The filament may be viewed as a system composed of three intimately connected structures: a small, shallow, and cold filament embedded within a larger, deeper, and cool filament and an intrathermocline anticyclonic eddy (ITE). The cold filament, which stretches 110 km offshore, is a shallow feature 60 m deep and 25 km wide, identified by minimal surface temperatures and rich in chlorophyll a. This structure comprises two asymmetrical submesoscale (∼18 km) fronts with jets flowing in opposite directions. The cold filament is embedded near the equatorward boundary of a much broader region of approximately 120 km width and 150 m depth that forms the cool filament and stretches at least 200 km offshore. This cool region, partly resulting from the influence of cold filament, is limited by two asymmetrical mesoscale (∼50 km) frontal boundaries. At the ITE, located north of the cold filament, we observe evidence of downwelling as indicated by a relatively high concentration of particles extending from the surface to more than 200 m depth. We hypothesize that this ITE may act as a sink of carbon and thus the filament system may serve dual roles of offshore carbon export and carbon sink.

The 48th Liege colloquium: submesoscale processes: mechanisms, implications, and new frontiers

The 48th International Liège Colloquium onOcean Dynamics took place from the 23 to the 27May, 2016, in the Academic Room of the University of Liège (Belgium). Every year, the Colloquium addresses a cutting-edge topic in Ocean Science with the goal to foster discussions and collaborations among scientists from all around the world. This edition aimed to advance our collective understanding of submesoscale processes, their mechanistic functioning, relevance, and implications across a range of oceanic disciplines. Discussions included observational, modeling, and theoretical approaches for elucidating submesoscale phenomena. This 48th edition was a success with 199 participants (Fig. 1) from 25 countries, 72 oral presentations, 8 keynote talks, and 143 posters, distributed over 7 topical sessions. All sessions are represented in the present topical collection of Ocean Dynamics.

Mediterranean Sea climatic indices: monitoring long term variability and climate changes

We present a new product composed of a set of thermohaline climatic indices from 1950 to 2015 for the Mediterranean Sea such as decadal temperature and salinity anomalies, their mean values over selected depths, decadal ocean heat and salt content anomalies at selected depth layers as well as their long time series. It is produced from a new high-resolution climatology of temperature and salinity on a 1/8 regular grid based on historical high-quality in situ observations. Ocean heat and salt content differences between 1980–2015 and 1950–1979 are compared for evaluation of the climate shift in the Mediterranean Sea. The two successive periods are chosen according to the standard WMO climate normals. The spatial patterns of heat and salt content shifts demonstrate that the climate changes differently in the several regions of the basin. Long time series of heat and salt content for the period 1950 to 2015 are also provided which indicate that in the Mediterranean Sea there is a net mean volume warming and salinification since 1950 that has accelerated during the last two decades. The time series also show that the ocean heat content seems to fluctuate on a cycle of about 40 years and seems to follow the Atlantic Multidecadal Oscillation climate cycle, indicating that the natural large-scale atmospheric variability could be superimposed onto the warming trend. This product is an observation-based estimation of the Mediterranean climatic indices. It relies solely on spatially interpolated data produced from in situ observations averaged over decades in order to smooth the decadal variability and reveal the long-term trends. It can provide a valuable contribution to the modellers’ community, next to the satellite-based products, and serve as a baseline for the evaluation of climate-change model simulations, thus contributing to a better understanding of the complex response of the Mediterranean Sea to the ongoing global climate change. The product is available in netCDF at the following sources: annual and seasonal T/S anomalies (https://doi.org/10.5281/zenodo.1408832), annual and seasonal T/S vertical averaged anomalies (https://doi.org/10.5281/zenodo.1408929), annual and seasonal areal density of OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408877), annual and seasonal linear trends of T/S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408917), annual and seasonal time series of T/S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1411398), and differences of two 30-year averages of annual and seasonal T/S, OHC/OSC anomalies (https://doi.org/10.5281/zenodo.1408903). Published by Copernicus Publications. 1830 A. Iona et al.: Mediterranean Sea climatic indices

Observational and numerical evidence for ocean frontogenesis inducing submesoscale processes and impacting biochemistry

Trabajo presentado en la Ocean Sciences Meeting, celebrada en Nueva Orleans, Estados Unidos, del 21 al 26 de febrero de 2016

Data Interpolating Variational Analysis for the Generation of Atmospheric Pollution Maps at Various Scales

Ordinary kriging is a widely used method to estimate the spatial distribution of atmospheric pollutants at all scales. However, more sophisticated strategies exist. For local mapping, where one often focuses on pollutants with a high spatio-temporal variability, such as nitrogen dioxide or black carbon, land use regression models are commonly used. In epidemiological research, several model reviews have already been published on this topic Hoek et al. (A review of land-use regression models to assess spatial variation of outdoor air pollution. Atmos Environ 42:7561–7578, 2008); Gaines et al. (A review of intraurban variations in particulate air pollution: Implications for epidemiological research. Atmos Environ 39:6444–6462, 2005). For regional mapping, de- and retreading procedures also make use of ancillary variables, such as the population density or the land use, to take into account the local characteristics of the sampling sites before and after the actual interpolation. Due to their low computational cost, these techniques can be implemented operationally Janssen et al. (Spatial interpolation of air pollution measurements using CORINE land cover data. Atmos Environ 42:4884–4903, 2008). In this study we introduce DIVA, a variational inverse method, originally designed for oceanographic applications, that allows one to take into account some new constraints. As it is based on a finite-element approach, physical boundaries such as buildings are naturally taken into account since they actually define the domain of interest. Another useful feature is the possibility to consider an advection field and hence propagate the information in the preferred direction. Finally, this technique also allows one to attribute a different weight to each available measurement, according to the quality of the data, so that heterogeneous data sources, consisting for example of monitoring network, passive sampler and mobile device values, can be used simultaneously and consistently. The model will be tested for two situations: the mapping of NO2 in the Walloon Region and the air pollution assessment of year 2012 in Antwerp. Results will be qualitatively compared with those of operational models: an ordinary kriging method run at AwAC by Bonvalet et al. (Validation of a geostatistical interpolation model using measurement of particulate matter concentration, Matinee des chercheurs a l’Universite de Mons 2013) and a detrended kriging run at ISSeP and originally implemented by Merbitz (Untersuchung und Modellierung der raumzeitlichen Variabilitat urbaner und regionaler Feinstaubkonzentrationen. Ph.D. thesis 2013) for the first case, and the RIO-IFDM-OSPM modelling system for the second case as implemented by Maiheu et al. (Luchtkwaliteitsmodellering Ringland, Studie uitgevoerd in opdracht van Stramien cvba en Ring genootschap vzw 2015/RMA/R/13 2015).