Drifter observations reveal intense vertical velocity in a surface ocean front

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Tarry, D., Ruiz, S., Johnston, T., Poulain, P., Özgökmen, T., Centurioni, L., Berta, M., Esposito, G., Farrar, J., Mahadevan, A., & Pascual, A. Drifter observations reveal intense vertical velocity in a surface ocean front. Geophysical Research Letters, 49(18), (2022): e2022GL098969, https://doi.org/10.1029/2022gl098969.Measuring vertical motions represent a challenge as they are typically 3–4 orders of magnitude smaller than the horizontal velocities. Here, we show that surface vertical velocities are intensified at submesoscales and are dominated by high frequency variability. We use drifter observations to calculate divergence and vertical velocities in the upper 15 m of the water column at two different horizontal scales. The drifters, deployed at the edge of a mesoscale eddy in the Alboran Sea, show an area of strong convergence (urn:x-wiley:00948276:media:grl64766:grl64766-math-0001(f)) associated with vertical velocities of −100 m day−1. This study shows that a multilayered-drifter array can be an effective tool for estimating vertical velocity near the ocean surface.This research was supported by the Office of Naval Research (ONR) Departmental Research Initiative CALYPSO under program officers Terri Paluszkiewicz and Scott Harper. The authors' ONR Grant No. are as follows: DT, SR, AM, and AP N000141613130, TMSJ N000146101612470, PP N000141812418, TO N000141812138, LRC N000141712517, and N00014191269, MB and GE N000141812782 and N000141812039, and JTF N000141812431

Inertial Oscillations and Frontal Processes in an Alboran Sea Jet: Effects on Divergence and Vertical Transport

Vertical transport pathways in the ocean are still only partially understood despite their importance for biogeochemical, pollutant, and climate applications. Detailed measurements of a submesoscale frontal jet in the Alboran Sea (Mediterranean Sea) during a period of highly variable winds were made using cross-frontal velocity, density sections and dense arrays of surface drifters deployed across the front. The measurements show divergences as large as ±f implying vertical velocities of order 100 m/day for a ≈ 20 m thick surface layer. Over the 20 hr of measurement, the divergences made nearly one complete oscillation, suggesting an important role for near-inertial oscillations. A wind-forced slab model modified by the observed background frontal structure and with initial conditions matched to the data produces divergence oscillations and pattern compatible with that observed. Significant differences, though, are found in terms of mean divergence, with the data showing a prevalence of negative, convergent values. Despite the limitations in data sampling and model uncertainties, this suggests the contribution of other dynamical processes. Turbulent boundary layer processes are discussed, as a contributor to enhance the observed convergent phase. Water mass properties suggest that symmetric instabilities might also be present but do not play a crucial role, while downward stirring along displaced isopycnals is observed.This work has been supported and co-financed by the CALYPSO project, within the Office of Naval Research Departmental Research Initiative, under the following grants: N00014-18-1-2782 and N00014-22-1-2039 (GE,SD,MB,AG), N00014-18-1-2139 (AYS,EAD), N00014-18-1-2138 (TO), N00014-18-1-2418 and N00014-20-1-2754 (PMP), N00014-19-1-2692 and N00014-19-1-2380 (LC and part of the drifter data collection/analysis), N00014-18-1-2431 (JTF), N00014-18-1-2416 (TMSJ), N00014-16-1-3130 (AP,DRT,SR), N00014-21-1-2702 (AM). MF was supported by the Scripps Institutional Postdoctoral Fellowship (MAF). Investigation of front dynamics in the Mediterranean Sea from multiplatform observations is supported as well by the European Union's JERICO-S3 project through Grant 871153. Open Access Funding provided by Consiglio Nazionale delle Ricerche within the CRUI-CARE Agreement.Peer reviewe

Coastal high-frequency radars in the Mediterranean ??? Part 2: Applications in support of science priorities and societal needs

International audienceThe Mediterranean Sea is a prominent climate-change hot spot, with many socioeconomically vital coastal areas being the most vulnerable targets for maritime safety, diverse met-ocean hazards and marine pollution. Providing an unprecedented spatial and temporal resolution at wide coastal areas, high-frequency radars (HFRs) have been steadily gaining recognition as an effective land-based remote sensing technology for continuous monitoring of the surface circulation, increasingly waves and occasionally winds. HFR measurements have boosted the thorough scientific knowledge of coastal processes, also fostering a broad range of applications, which has promoted their integration in coastal ocean observing systems worldwide, with more than half of the European sites located in the Mediterranean coastal areas. In this work, we present a review of existing HFR data multidisciplinary science-based applications in the Mediterranean Sea, primarily focused on meeting end-user and science-driven requirements, addressing regional challenges in three main topics: (i) maritime safety, (ii) extreme hazards and (iii) environmental transport process. Additionally, the HFR observing and monitoring regional capabilities in the Mediterranean coastal areas required to underpin the underlying science and the further development of applications are also analyzed. The outcome of this assessment has allowed us to provide a set of recommendations for future improvement prospects to maximize the contribution to extending science-based HFR products into societally relevant downstream services to support blue growth in the Mediterranean coastal areas, helping to meet the UN's Decade of Ocean Science for Sustainable Development and the EU's Green Deal goals

Coastal high-frequency radars in the Mediterranean ??? Part 1: Status of operations and a framework for future development

Due to the semi-enclosed nature of the Mediterranean Sea, natural disasters and anthropogenic activities impose stronger pressures on its coastal ecosystems than in any other sea of the world.With the aim of responding adequately to science priorities and societal challenges, littoral waters must be effectively monitored with high-frequency radar (HFR) systems. This land-based remote sensing technology can provide, in near-real time, fine-resolution maps of the surface circulation over broad coastal areas, along with reliable directional wave and wind information. The main goal of this work is to showcase the current status of the Mediterranean HFR network and the future roadmap for orchestrated actions. Ongoing collaborative efforts and recent progress of this regional alliance are not only described but also connected with other European initiatives and global frameworks, highlighting the advantages of this cost-effective instrument for the multi-parameter monitoring of the sea state. Coordinated endeavors between HFR operators from different multi-disciplinary institutions are mandatory to reach a mature stage at both national and regional levels, striving to do the following: (i) harmonize deployment and maintenance practices; (ii) standardize data, metadata, and quality control procedures; (iii) centralize data management, visualization, and access platforms; and (iv) develop practical applications of societal benefit that can be used for strategic planning and informed decision-making in the Mediterranean marine environment. Such fit-for-purpose applications can serve for search and rescue operations, safe vessel navigation, tracking of marine pollutants, the monitoring of extreme events, the investigation of transport processes, and the connectivity between offshore waters and coastal ecosystems. Finally, future prospects within the Mediterranean framework are discussed along with a wealth of socioeconomic, technical, and scientific challenges to be faced during the implementatio

Surface Ocean Dispersion Observations From the Ship-Tethered Aerostat Remote Sensing System

Oil slicks and sheens reside at the air-sea interface, a region of the ocean that is notoriously difficult to measure. Little is known about the velocity field at the sea surface in general, making predictions of oil dispersal difficult. The Ship-Tethered Aerostat Remote Sensing System (STARSS) was developed to measure Lagrangian velocities at the air-sea interface by tracking the transport and dispersion of bamboo dinner plates in the field of view of a high-resolution aerial imaging system. The camera had a field of view of approximately 300 × 200 m and images were obtained every 15 s over periods of up to 3 h. A series of experiments were conducted in the northern Gulf of Mexico in January-February 2016. STARSS was equipped with a GPS and inertial navigation system (INS) that was used to directly georectify the aerial images. A relative rectification technique was developed that translates and rotates the plates to minimize their total movement from one frame to the next. Rectified plate positions were used to quantify scale-dependent dispersion by computing relative dispersion, relative diffusivity, and velocity structure functions. STARSS was part of a nested observational framework, which included deployments of large numbers of GPS-tracked surface drifters from two ships, in situ ocean measurements, X-band radar observations of surface currents, and synoptic maps of sea surface temperature from a manned aircraft. Here we describe the STARSS system and image analysis techniques, and present results from an experiment that was conducted on a density front that was approximately 130 km offshore. These observations are the first of their kind and the methodology presented here can be adopted into existing and planned oceanographic campaigns to improve our understanding of small-scale and high-frequency variability at the air-sea interface and to provide much-needed benchmarks for numerical simulations

Utilizzo dei Finite-Size Lyapunov Exponents per la valutazione del trasporto superficiale nel Mar Adriatico nord-orientale

2010/2011The surface transport in the Northeastern Adriatic Sea has been investigated by evaluating, for the first time, the Finite-Size Lyapunov Exponent (FSLE) on the current field detected by the High Frequency (HF) coastal radar network active in the period August 2007 - August 2008. A similar analysis has been carried out on the MITgcm (Massachusetts Institute of Technology general circulation model) current field in order to have a perspective on the various results given by the application of the same FSLE evaluation procedure to different velocity fields. This work includes also the development, testing and calibration of the FSLE evaluation algorithm for the specific Adriatic area considered. The aim of this thesis is to study the surface dynamics of the Northeastern Adriatic current field associated with the typical wind regimes, namely Bora and Sirocco. The strongest and most persistent wind events, coinciding with the radar network activity, have been identified in the first instance from ALADIN (Aire Limitée Adaptation dynamique Développement InterNational) model meteorological data and then confirmed by the in situ meteo-mareographic time series in Trieste and Venice. For comparison purposes, the transport structures developing throughout the longest calm wind periods have also been investigated. In this thesis, the combination of the FSLE resulting from model and radar current fields contributes to interpret the surface transport dynamics in the studied area. In fact, it allows: i) to determine the strongest dynamical features, ii) to extend the transport information beyond the radar coverage and iii) to bring to the light the possible ambiguity of some structures originated from model currents. The FSLE analysis evidenced along the northern Adriatic margin an attractive transport structure with a filament-shaped conformation. The dynamics of this structure is driven by the water exchange between the Gulf of Trieste (GoT) and the North Adriatic Sea. The spatial location of this filament and the advective direction associated with it, vary according to the wind patterns. In fact, in calm wind periods this attractive filament is found right in front of the gulf entrance indicating the pattern of the GoT outflow (east-west direction). During Bora episodes this attractive filament is advected westward and it elongates following the northern Adriatic coast. Differently, Sirocco wind pushes this filament closer to the northeastern coastline reversing the transport direction along it (from west to east). Previous studies evidenced that the current signal in the southern part of the domain is less correlated to the wind pattern with respect to what observed in the northern area of the current field. Therefore also from the FSLE analysis no recurrent transport dynamics is observed in the southern area except for the Bora cases, when a repulsive structure originates from the Istrian coast and it looses strength while it is advected northwestward.Il trasporto superficiale nella parte nord-orientale del Mar Adriatico è stato studiato calcolando, per la prima volta, i Finite-Size Lyapunov Exponent (FSLE) sui campi di corrente misurati dalla rete di radar ad alta frequenza attiva durante il periodo Agosto 2007 - Agosto 2008. Uno studio analogo è stato effettuato sui campi di corrente del modello MITgcm (Massachusetts Institute of Technology general circulation model), per avere una panoramica sulla varietà dei risultati prodotti dalla valutazione degli FSLE su diversi campi di velocità. Il lavoro include anche lo sviluppo, i test e la calibrazione dell'algoritmo di calcolo degli FSLE per la specifica area dell'Adriatico considerata. L'obiettivo è analizzare la dinamica superficiale del campo di corrente associata ai regimi di vento tipici dell'area nord adriatica, ovvero Bora e Scirocco. Gli episodi di vento più intensi e persistenti, verificatisi durante il periodo di attività dei radar, sono stati identificati in primo luogo dai dati meteorologici prodotti dal modello ALADIN (Aire Limitée Adaptation dynamique Développement InterNational) e successivamente sono stati confermati dalle serie temporali dei dati meteo-mareografici delle stazioni di Trieste e Venezia. A scopo comparativo, sono state analizzate anche le strutture di trasporto sviluppatesi durante i prolungati periodi di calma di vento. In questa tesi, la combinazione degli FSLE risultanti dai campi di corrente dei radar e del modello contribuisce ad una più completa interpretazione della dinamica del trasporto superficiale nell'area di interesse. I risultati ottenuti permettono: i) di determinare le strutture dinamiche più intense, ii) di estendere l'informazione sul trasporto al di fuori della copertura della rete dei radar, nonché iii) di portare alla luce eventuali ambiguità di alcune strutture originatesi dalle correnti del modello. L'analisi degli FSLE ha evidenziato una struttura di trasporto attrattiva che assume l'aspetto di un filamento lungo il margine settentrionale del Mar Adriatico. La dinamica di questa struttura è regolata dagli scambi di acqua tra il Golfo di Trieste ed il Nord Adriatico. La posizione del filamento e la direzione avvettiva associata ad esso dipendono dal regime di vento. Infatti, nei periodi di calma di vento il filamento attrattivo si trova di fronte all'entrata del golfo, ad indicare la direzione del flusso d'acqua in uscita dal golfo stesso (da est ad ovest). Durante gli episodi di Bora il filamento attrattivo è trasportato verso ovest e si allunga seguendo la costa nord adriatica. Invece, lo Scirocco spinge questo filamento verso la costa nord-orientale invertendo la direzione del trasporto associata ad esso (da ovest ad est). Precedenti lavori hanno evidenziato che le correnti nella parte meridionale del dominio sono meno correlate al segnale di vento, rispetto a quanto osservato nell'area settentrionale del campo delle correnti. Quindi anche dall'analisi degli FSLE non si osserva una dinamica di trasporto ricorrente ad eccezione dei casi di Bora, durante i quali si sviluppa una struttura repulsiva lungo la costa Istriana, la cui intensità diminuisce mentre viene trasportata verso nordovest.XXIV Ciclo198

Vertical Structure of Ocean Surface Currents Under High Winds from Massive Arrays of Drifters

Abstract. Very near surface ocean currents are dominated by wind and wave forcing and have large impacts on the transport of buoyant materials in the ocean, but have proved difficult to measure with many modern instrumentations. Here, observations of ocean currents at two depths within the first meter of the surface are made utilizing trajectory data from both drogued and undrogued CARTHE drifters, which have draft depths of 60 cm and 5 cm, respectively. Trajectory data of dense, co-located drogued and undrogued drifters, were collected during the LAgrangian Submesoscale ExpeRiment (LASER) that took place from January to March of 2016 in the Northern Gulf of Mexico. Examination of the drifter velocities reveals that the surface currents become strongly wind- and wave-driven during periods of high wind, with the pre-existing regional circulation having a smaller, but non-negligible, influence on the total surface velocity. During these high wind events, we deconstruct the full surface current velocities captured by each drifter type into their wind- and wave-driven components after subtracting an estimate for the regional circulation which pre-exists each wind event. In order to capture the regional circulation in the absence of strong wind and wave forcing, a Lagrangian variational method is used to create hourly velocity fields for both drifter types separately, during the hours preceding each high wind event. Synoptic wind and wave output data from the Unified Wave INterface-Coupled Model (UWIN-CM), a fully coupled atmosphere, wave and ocean circulation model, are used for analysis. The wind-driven component of the surface current exhibits a rotation to the right with depth between the two surface layers measured. We find that the averaged wind-driven surface current from 0-5 cm (0-60 cm) travels at ~ 3.4-6.0 % (~ 2.3-4.1 %) of the wind speed, and is deflected ~ 5°-55° (~ 30-85°) to the right of the wind, reaching higher deflection angles at higher wind speeds. Results provide new insight to the vertical shear present in wind-driven surface currents under high winds, which have vital implications for any surface transport problem