Recent improvements in mesoscale characterization of the western Mediterranean Sea: synergy between satellite altimetry and other observational approaches (original) (raw)
Related papers
Scientia Marina, 2013
Satellite altimetry is a key component of the global observing system and plays a major role in the study of the mesoscale processes that drive most of the ocean circulation variability at middle and high latitudes. However, satellite altimetry alone provides only surface information at a limited spatio-temporal resolution. To address these limitations and to better describe the mesoscale three-dimensional variability, it is necessary to complement altimetry data with additional remote and in situ measurements. This study provides an update of the recent advances in the study of the mesoscale variability using a combination of altimetry and other independent observations, with an emphasis on the results obtained for the western Mediterranean Sea. The circulation in this area is complex because of the presence of multiple interacting scales, including basin-scale, sub-basin-scale and mesoscale structures. Thus, characterizing these processes requires highresolution observations and multi-sensor approaches. Accordingly, multi-platform experiments and analyses have been designed and undertaken in the different sub-basins of the western Mediterranean Sea. These studies have demonstrated the advantages of synergetic approaches that use a combination of observation techniques and are able to resolve different spatiotemporal scales with the aim of better understanding mesoscale dynamics.
2012
In this study we review recent advances on mesoscale variability as a result of the synergy between satellite altimetry and other observational platforms. We focus on the Western Mediterranean Sea, where the circulation is rather complex due to the presence of multiple interacting scales, including basin, sub-basin scale and mesoscale structures. The challenges of characterizing these processes imply therefore precise and high-resolution samplings in addition to multisensor approaches. Accordingly, intensive experiments have been designed and carried out in the different subbasins of the Western Mediterranean. These experiments, which have combined different observing systems at a wide range of spatial/temporal scales, have contributed to enhance our understanding of mesoscale variability.
Ocean Mesoscale Variability: A Case Study on the Mediterranean Sea From a Re-Analysis Perspective
Frontiers in Earth Science, 2021
The mesoscale variability in the Mediterranean Sea is investigated through eddy detection techniques. The analysis is performed over 24 years (1993–2016) considering the three-dimensional (3D) fields from an ocean re-analysis of the Mediterranean Sea (MED-REA). The objective is to achieve a fit-for-purpose assessment of the 3D mesoscale eddy field. In particular, we focus on the contribution of eddy-driven anomalies to ocean dynamics and thermodynamics. The accuracy of the method used to disclose the 3D eddy contributions is assessed against pointwise in-situ measurements and observation-based data sets. Eddy lifetimes ≥ 2 weeks are representative of the 3D mesoscale field in the basin, showing a high probability (> 60%) of occurrence in the areas of the main quasi-stationary mesoscale features. The results show a dependence of the eddy size and thickness on polarity and lifetime: anticyclonic eddies (ACE) are significantly deeper than cyclonic eddies (CE), and their size tends t...
2008
The spatial and temporal resolution of satellite altimetry is usually sufficient for monitoring the changes of sea surface topography in the open ocean. However, coastal ocean dynamics are much more complex, being characterized by smaller spatial and temporal scales of variability. The quality and availability of satellite-derived products along the coasts have to be improved, with a strategy optimized for coastal targets. Therefore a coastal multi-satellite altimetry dataset (TOPEX/Poseidon, Jason-1; Envisat; GFO) at a 10-20 Hz sampling rate has been derived from routine geophysical data products using a new processing software dedicated to coastal zone applications. Improved along-track sea level variations with fine space scales are available in the Northwestern Mediterranean Sea from 2001 to 2003, and are compared with high-resolution numerical model elevations from the eddy-resolving model SYMPHONIE. This preparatory work emphasizes the potential of improved multi-satellite altimetry for validating coastal hydro-dynamical models and could contribute in the future to a better tuning of the boundary conditions of the simulations.
Ocean Dynamics, 2013
The Northern current is the main circulation feature of the North-Western Mediterranean Sea. While the large-scale to mesoscale variability of the northern current (NC) is well known and widely documented for the Ligurian region, off Nice or along the Gulf of Lions shelf, few is known about the current instabilities and its associated mesoscale dynamics in the intermediate area, off Toulon. Here, we took advantage of an oceanographic cruise of opportunity, the start of a HF radar monitoring programme in the Toulon area and the availability of regular satellite sea surface temperature and chlorophyll a data, to evaluate the realism of a NEMO-based regional high-resolution model and the added value brought by HF radar. The combined analysis of a 1/64 • configuration, named GLAZUR64, and of all data sets revealed the occurrence of an anticyclonic coastal trapped eddy, generated inside a NC meander and passing the Toulon area during the field campaign. We show that this anticyclonic eddy is advected downstream along the French Riviera up to the study region and disturbs the Northern current flow. This study aims to show the importance of combining observations and modelling when dealing with mesoscale processes, as well as the importance of high-resolution modelling.
2011
We study the interannual and seasonal variability in the Mediterranean Sea over the period 1958-2004 by comparing a numerical simulation (the 1/4º ORCA-R025 G70 model run, 'ORCA' hereafter) with altimetry and the ME-DAR temperature and salinity database. The model is forced by the ERA40 atmospheric forcing and has a salinity restoring term applied at surface. Comparing temperature between ORCA and MEDAR shows good interannual variability agreement (correlations of ~0.8 in the western Mediterranean and ~0.5 in the eastern Mediterranean) at surface layers (0-150 m), but slightly higher mean values in the model (0.08-0.16°C). The salinity analysis shows that the surface salinity restoring term has obliterated most of the interannual variability. Mean surface salinities are slightly lower in the model (~0.3), replicated in deeper layers to a lesser degree, and could mean that the restoring term applies insufficient evaporation to compensate for a weak atmospheric forcing (ERA40) water loss flux. The sea level analysis comparing sea surface height (SSH) and steric height from ORCA and sea level anomalies from altimetry (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004) shows good correlations (~0.8) in the interannual variability and annual cycle. However, the model's SSH overestimates (~15 mm/yr) observed positive altimetric trends (~3-4 mm/yr). In an attempt to identify the source of this overestimation, a water budget calculation was performed between the horizontal and vertical water fluxes in the Mediterranean Sea. Horizontal transport through the main straits shows appropriate values when compared to observations. Thus, the cause of the exaggerated SSH trend is probably a water flux imbalance. By improving surface salinity restoring and atmospheric forcing, the ORCA simulations can provide very promising tools for studies of interannual variability in the Mediterranean Sea. RESUMEN: Evaluación de un retroanálisis de 44 años para el mar Mediterráneo: comparación con altimetría y observaciones in situ. -Estudiamos la variabilidad estacional e interanual en el mar Mediterráneo durante el periodo 1958-2004, comparando una simulación numérica (la simulación de 1/4º ORCA-R025 G70, 'ORCA' de ahora en adelante) con datos de altimetría, y temperatura y salinidad (MEDAR). El modelo utiliza el forzamiento atmosférico ERA40 y tiene aplicado un término de relajación a la salinidad en superficie. La comparación de temperatura entre ORCA y MEDAR muestra un buen acuerdo de la variabilidad interanual (correlación ~0.8 en el Mediterráneo Occidental (WMED), ~0.5 en el Mediterráneo Oriental (EMED)) en las capas superficiales (0-150 m), pero con valores medios ligeramente superiores en el modelo (0.08-0.16ºC). El análisis de salinidad muestra que la mayor parte de la variabilidad en superficie ha sido destruida por el término de relajación. Las salinidades medias en superficie son ligeramente inferiores en el modelo (~0.3), lo cual se repite en capas más profundas pero en menor grado. Esto podría significar que el término de relajación no aplica suficiente evaporación para compensar un débil flujo de pérdida de agua en el forzamiento atmosférico (ERA 40). El análisis de altura de nivel del mar (SSH) y altura estérica (SH) del modelo ORCA y anomalía del nivel del mar proveniente de la altimetría (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004) muestra buenas correlaciones (~0.8) en la variabilidad interanual y ciclo estacional. Sin embargo la SSH del modelo sobreestima (~15 mm/año) la tendencia positiva observada por la altimetría (~3-4 mm/año). En un intento de identificar el origen de esta sobreestimación, se hizo un cálculo de balance de masas entre los flujos horizontales y verticales (E-P-R) que entran al mar Mediterráneo. Los flujos horizontales a través de los principales estrechos muestran valores adecuados cuando se comparan con observaciones. Por lo tanto, la exagerada tendencia en SSH del modelo es probablemente debido a un desequilibrio entre la E-P-R (evaporación, precipitación y aporte fluvial). Mejorando el término de relajación de salinidad y el forzamiento atmosférico, las simulaciones ORCA pueden proporcionar unas herramientas muy prometedoras para estudios de variabilidad interanual en el mar Mediterráneo.
Ocean Dynamics, 2019
In numerical ocean modeling, dynamical downscaling is the approach consisting in generating high-resolution regional simulations exploiting the information from coarser resolution models for initial and boundary conditions. Here we evaluate the impacts of downscaling the 1/16 o (~6-7 km) CMEMS Mediterranean reanalysis model solution into a high-resolution 2-km free-run simulation over the Western Mediterranean basin, focusing on the surface circulation and mesoscale activity. Multi-platform observations from satellite-borne altimeters, high-frequency radar, fixed moorings, and gliders are used for this evaluation, providing insights into the variability from basin to coastal scales. Results show that the downscaling leads to an improvement of the time-averaged surface circulation, especially in the topographically complex area of the Balearic Sea. In particular, the path of the Balearic current is improved in the high-resolution model, also positively affecting transports through the Ibiza Channel. While the high-resolution model produces a similar number of large eddies as CMEMS Med Rea and altimetry, it generates a much larger number of small-scale eddies. Looking into the variability, in the absence of data assimilation, the high-resolution model is not able to properly reproduce the observed phases of mesoscale structures, especially in the southern part of the domain. This negatively affects the representation of the variability of the surface currents interacting with these eddies, highlighting the importance of data assimilation in the high-resolution ocean model in this region to constrain the evolution of these mesoscale structures.
Sea level variability and surface eddy statistics in the Mediterranean Sea from TOPEX/POSEIDON data
Journal of Geophysical Research, 1998
Two years of TOPEX/POSEIDON altimeter data have successfully been used to study the mesoscale field in the Mediterranean and to investigate the seasonal and year-toyear variability of the sea level and eddy statistics in this basin. The mesoscale field described by TOPEX/POSEIDON revealed a strong, but subbasin dependent, seasonal signal. Year-toyear variations are also evident in terms of both intensity and position of the main mesoscale features. Crossover analysis indicated the existence of a meridional transport of eddy momentum away from the Algerian current due to the northward migration of mesoscale eddies. A comparison between mesoscale features detected by the altimeter and contemporaneous features observed using advanced very high resolution radiometer (AVHRR) sea surface temperature has been made for the following Mediterranean subbasins: the Algerian basin, the Levantine basin, and the Tyrrhenian Sea. The results definitively prove the direct relation between sea level anomalies and the Mediterranean eddy field.
Journal of Geophysical Research: Oceans, 2016
This work is the first study exploiting along track altimetry data to observe and monitor coastal ocean features over the transition area between the western and eastern Mediterranean Basins. The relative performances of both the AVISO and the X‐TRACK research regional altimetric data sets are compared using in situ observations. Both products are cross validated with tide gauge records. The altimeter‐derived geostrophic velocities are also compared with observations from a moored Acoustic Doppler Current Profiler. Results indicate the good potential of satellite altimetry to retrieve dynamic features over the area. However, X‐TRACK shows a more homogenous data coverage than AVISO, with longer time series in the 50 km coastal band. The seasonal evolution of the surface circulation is therefore analyzed by conjointly using X‐TRACK data and remotely sensed sea surface temperature observations. This combined data set clearly depicts different current regimes and bifurcations, which all...