Title: Multiplatform analysis of the Algerian Basin from satellite observations and high resolution glider data

Transcription

1 Scientific Report for a Short Term Scientific Mission (STSM) Reference code: COST-STSM-ECOST-STSM-ES Name of scientist: Dr. Giuseppe Aulicino Affiliation and address: Università Politecnica delle Marche Via Brecce Bianche, Ancona (Italy) g.aulicino@staff.univpm.it Country of hosting institution: Spain Hosting institution: IMEDEA (CSIC-UIB) Contact scientist: Dr Ananda Pascual (ananda.pascual@imedea.uib-csic.es) Dates: 22 June 2 July 2016 Title: Multiplatform analysis of the Algerian Basin from satellite observations and high resolution glider data Purpose: The Algerian Basin is a wide basin crossed by Atlantic water entering the Mediterranean Sea and dominated by the presence of very energetic mesoscale structures. In recent years the use of new generation remotely sensed data, i.e. satellite altimetry and the ocean-color dataset, offered the opportunity to study some of the characteristics of this region, even though the lack of high resolution in situ data limited the validation of this information and their use in the numerical simulations. In the framework of the E-COST Ocean Synthesis Evaluation objectives, this STSM aims at comparing MODIS and SARAL-AltiKa products with the high resolution (2 and 4 Km) data previously collected by two glider missions performed in the Algerian Basin during Autumn 2014 and In particular, glider data have been collected between Mallorca and the Algerian Coast along the SARAL-AltiKa groundtrack in order to optimize data comparison and integration. Merging the glider sampling capabilities with satellite information, we expect also to advance our knowledge on mesoscale features in this area, to evaluate how including them in the oceanographic models and, generally, to get insights into strengths and weaknesses of ocean syntheses. The final goal is to confirm the value of the multiplatform ocean monitoring strategies for the Algerian Basin. Finally, this STSM will improve the collaboration between home and host institutions in order to optimize each one s skills and facilities, being the starting point for new joint studies on biochemical and dynamics of the Algerian Basin and its links with the Mediterranean circulation.

2 Description of work done: During the STSM, several steps have been completed toward achieving the goals of merging SARAL-AltiKa satellite observations and high resolution glider data and of confirming the added value of a multiplatform strategy for the ocean monitoring of the Algerian Basin. After the kick-off meeting on June the 23 rd, an intense series of activities has been carried on in close collaboration with several IMEDEA scientists. Firstly glider, satellite and numerical simulations products previously processed at IMEDEA and UNIVPM have been shared, analyzed separately and carefully compared. Glider 3D high-resolution measurements, collected in the framework of the ABACUS and ABACUS-2 projects along two SARAL-AltiKa groundtracks between Mallorca and Algeria, have been used to describe the physical and biogeochemical features of the water column and to estimate the glider surface dynamic height. AVISO altimetry products and MODIS imagery have been used to characterize the largescale characteristics of the surface circulation in the study area and to give information about the evolution of the main mesoscale structures. Taking advantage of the techniques developed by IMEDEA scientists, SARAL-AltiKa observations have been processed to derive absolute dynamic topography during the glider mission and precisely along the glider track. Then, an intense data analysis focusing on the comparison and merging of glider and SARAL-AltiKa co-located information has been carried on. The opportunity to meet at IMEDEA also several scientists and engineers from SOCIB (Balearic Islands Coastal Observing and forecasting System) represented a fruitful opportunity to discuss further both oceanographic features and technical aspects. Finally, the achieved results have been summarized, the possibility to publish the main results has been evaluated (and confirmed) and the basis for future joint activities in the Algerian Basin have been set. Description of main results: During the STSM, glider in situ measurements and SARAL-AltiKa along-track observations have been analyzed for describing the main features of the Algerian Basin during autumn 2014 and A preliminary qualitative comparison of the achieved results to several numerical simulation products has been also realized and a cross-validation between altimetric and glider current velocities has been started. A preliminary analysis of the performance/accuracy of the state-of-theart mean dynamic topography (MDT) for the Western Mediterranean Sea [Rio et al., 2014] has been also developed through the comparison of SARAL-AltiKa products and glider derived information. In particular, data collected by three Slocum deep glider missions carried out during September- December 2014 and October-November 2015 in the framework of the ABACUS (Algerian BAsin

3 Circulation Unmanned Survey, supported by JERICO-TNA) and ABACUS-2 projects have been analyzed. Data have been collected along four transects, between the Island of Mallorca and the northern border of the Algerian Current, designed in order to follow the SARAL-AltiKa ground tracks #773 and #229. The presence of typical Mediterranean water masses has been identified, i.e. Atlantic Water entering from the Gibraltar Strait, Levantine Intermediate Water and Western Mediterranean Deep Water. Some seasonal variability between September and December glider missions emerged from the data too (Figure 1). Other scientists were in charge of analyzing in detail these oceanographic features. Figure 1: Examples of potential temperature versus Salinity diagrams for the glider missions of November 2014 (left) and October 2015 (right). Colours indicate the depth of the samples. Presence and distribution of the Atlantic Water are strictly linked to the variability of the Algerian Current and of the meanders that usually detach from it in the Algerian Basin. A large-scale analysis of the region during the three glider missions at sea has been carried on thanks to the use of AVISO multi-satellite products (see an example in Figure 2) and the MODIS imagery (Figure 3). The presence and the dynamics of several mesoscale structures, as well as the variability of the Algerian Current northern boundary, have been clearly identified by these satellite data and allowed to reconstruct the story of the surface circulation before, during and after the glider surveys. The analysis of the glider data vertical sections (not shown) helped to monitor the vertical structure of the water column and to confirm the effect of the mesoscale dynamics on the ocean surface and intermediate to deep layers. Numerical simulations derived from the Mediterranean Forecasting System model (MFS) at 1/16 resolution have been then used for a preliminary qualitative comparison with AVISO altimetry in the study area. MFS temperature and salinity horizontal maps (Figure 4), as well as surface sea level anomaly (SLA) maps (Figure 5) show a very good qualitative agreement with satellite observations and therefore will be further investigated (quantitative and statistical comparison) during the ongoing collaboration with the IMEDEA scientists.

4 Figure 2: Mean Absolute dynamic topography from AVISO multisatellite products during the September 2014 glider mission. Algerian Current, its meanders in the Algerian Basin and an anticyclonic eddy south of Mallorca are visible. Black line represents the glider track. Figure 3: Sea surface temperature (left) and chlorophyll concentration (right) imagery of the Algerian Basin from MODIS L2 data during September Both maps show the presence of the Algerian Current and of some interesting meanders detaching from its main flow.

5 Figure 4: Temperature (right) and Salinity (left) surface distributions from MFS numerical simulations during September Black line represents the glider track. Figure 5: SLA distribution from MFS numerical simulations during September Then, temperature and salinity fields from glider data have been used to estimate the glider surface dynamic height (DH) with respect to an arbitrary reference level set at the maximum common depth of the glider measurements (900 m). This assumption implies that geostrophic velocities at this reference level are negligible; this is not always the case as dynamical features may have deeper extension, but previous studies focusing on this region reported velocities close to level of no motion at m (about 1 2 cm/s in Millot [2005]); sensitivity tests carried on several reference levels (Figure 6) confirmed these results, showing that glider DH is not influenced

6 significantly from the chosen reference level. This was confirmed also through the analysis of the glider profiles which showed how the water column is quite homogenous under the 400 m depth. Figure 6: Glider dynamic height sensitivity to the arbitrary reference level chosen for its computation. Example derived from the December 2014 glider mission in the Algerian Basin. Dashed lines represent a second passage of the glider along the same path. Finally, SARAL-AltiKa alongtrack data have been used to derive the Absolute Dynamic Topography (ADT) along the glider path (Figure 7). The comparison between glider DH and SARAL-AltiKa ADT showed a good correlation (R 0.7) even if some differences can be noticed at about 37.4 N N and at about 38.7 N. These differences could be due to the uncertainties in the MDT that is used for the ADT calculation. In this study, the improved MDT for the Mediterranean Sea recently suggested by Rio et al., (2014) has been used. In the framework of the ongoing collaboration we are evaluating the possibility to further improve the MDT in the study area taking advantage of our glider observations (Figure 8). This corrected MDT will be then applied for the comparison with an independent set of glider DHs collected in the Algerian Basin. Next steps will include also the DHs derived by the MSF numerical simulations along the glider path in this DH-ADT crossvalidation and will focus on the multiplatform evaluation of the derived geostrophic current velocities.

7 Figure 7: Comparison between glider Dynamic Height (black line) and SARAL-AltiKa Absolute Dynamic Topography (green line) during September (top), November (middle) and December (bottom) Dashed lines represent SARAL ADT data before the Loess filtering.

8 Figure 8: Comparison between the Mean Dynamic Topography by Rio et al (red line) and the Mean Dynamic Topography values estimated after the correction derived by the use of glider data collected during September 2014 (green line), November 2014 (blue line) and both September and November 2014 (black line). Future collaboration with the host institution: The results of the ongoing analyses on the glider high-resolution observations and the SARAL- AltiKa products will continue in order to provide detailed material for improving the knowledge of the study area. In particular, we will focus on the analysis of the correlation between sea surface heights observed by glider and altimetry and those estimated by the main ocean models. To this aim, additional in situ data are needed to complete the scarcity of information about the seasonal and interannual variability of the ocean features in the study area. New joint glider experiments between the two institutions are possible and will be scheduled in the next months. These activities are also expected to improve the use of glider and satellite merged data in the numerical simulations in the framework of the ocean synthesis purposes. Foreseen publications: The results of these activities, and those expected from the ongoing collaborations (with the host institution, the SOCIB and the other scientists met during the STSM) will be submitted to a refereed journal in the next few months. The study will focus on the combined monitoring of the Algerian Basin from glider high-resolution three-dimensional observations and co-located SARAL- AltiKa altimetry products along the ABACUS projects chokepoint. A comparison with the numerical simulations, provided by several oceanographic models, will complete our analyses. Other interesting findings that will emerge from ongoing (and future) collaborations with IMEDEA scientists are also expected to be published in additional papers if worthy of publication.

9 Acknowledgements All these results have been achieved thanks to the joint efforts of the scientists from IMEDEA, SOCIB and University of Naples Parthenope that I had the opportunity to meet during this STSM. In particular, I would like to mention: Dr. Simon Ruiz for the comparison between glider data and MFS numerical simulations; Dr. Antonio Sanchez Roman for the comparison between glider DH and SARAL-AltiKa alongtrack ADT; Dr. Ananda Pascual for the analysis of the SARAL-AltiKa altimetric products and the evaluation of the MDT products; Eng. Marc Tomas Torner for his important support in the evaluation of the glider technical characteristics. Moreover, I have to thank Dr. Joaquín Tintoré Subirana, Dr. Emma Heslop, Dr. Charles Troupin and Dr. Yuri Cotroneo for all the fruitful discussions on the Algerian Basin and its mesoscale dynamics. Part of this work has been carried out as part of the Copernicus Marine Environment Monitoring Service (CMEMS) MedSUB project and the CMEMS Sea Level Thematic Assembly Center (SL-TAC). CMEMS is implemented by Mercator Ocean in the framework of a delegation agreement with the European Union. Confirmation by host of successful STSM: Host: Dr. Ananda Pascual Institution: IMEDEA (CSIC-UIB) Instituto Mediterráneo de Estudios Avanzados I confirm that Dr. Giuseppe Aulicino spent a successful STSM at IMEDEA in compliance with the presented project and the expected program. His scientific report is consistent with the research activities carried on during his short visit to IMEDEA. Esporles, 14th July 2016 Dr. Ananda Pascual, PhD Ancona, 14th July 2016 Dr. Giuseppe Aulicino, PhD