Volfango Rupolo

Lagrangian Velocity Spectra at 700 m in the Western North Atlantic

Abstract Pending an appropriate scaling of each trajectory by its Lagrangian integral timescale TL, there exists a generic shape of the Lagrangian frequency spectrum for the trajectories of the 700-m dataset in western North Atlantic, which are stationary on the timescale of 200 days. The generic spectral shape contains a plateau at the lowest frequencies extending up to ν0∼(30TL)−1, a power-law behavior with an intermediate spectral slope α = 0.25 between ν0 and ν1∼(3∼4TL)−1, and a steeper slope n ≥ 3 at larger frequencies. Such a steep slope at large frequencies implies that most of Lagrangian dispersion can be ascribed to low and intermediate frequency motions. The variance of the Lagrangian acceleration computed from such a spectrum is finite, indicating continuous particle accelerations and supporting a truly Lagrangian behavior of the 700-m floats. The existence of an intermediate power-law behavior in the spectrum can be linked with the trapping of some trajectories by persistent energetic structur...

Distribution and ecology of Pseudo-nitzschia species (Bacillariophyceae) in surface waters of the Weddell Sea (Antarctica)

The distribution of six Pseudo-nitzschia species and their relationship with environmental conditions were studied for the first time in a vast zone of the Weddell Sea (∼61–77°S, Antarctica). Both qualitative and quantitative phytoplankton samples, collected during summer 2004, were examined using light and scanning electron microscopy. Phytoplankton abundance and composition showed great variability along our study area. Diatoms were the most conspicuous phytoplankton group in the northern area while small flagellates were generally dominant in the southern stations. The genus Pseudo-nitzschia was broadly distributed and significantly contributed to total diatom densities. A marked contrast in Pseudo-nitzschia species distribution was observed in three main zones divided by the Weddell Front (WF) and the Antarctic Slope Front (ASF). P. subcurvata and P. turgiduloides were the most abundant species in the neritic Weddell Sea zone, south of the ASF, mainly near the ice-edge in shallower waters and in conditions of long photoperiod. In contrast, P. prolongatoides and P. lineola dominated north of the ASF; the first was associated with deeper and nutrient-rich waters whereas the latter showed a weak relation with environmental variables examined. Finally, P. turgidula and P. heimii were mostly observed in the Weddell–Scotia Confluence Zone in the warmest and far from ice covered waters, north of the WF. A brief morphological Pseudo-nitzschia species description is given in the Appendix, including morphometrics and pictures.

Role of surface fluxes in ocean general circulation models using satellite sea surface temperature: Validation of and sensitivity to the forcing frequency of the Mediterranean thermohaline circulation

[1] In this article we study the effect of high-frequency surface momentum and heat fluxes in the numerical simulation of some key ocean processes ofthe Mediterranean thermohaline circulation. The lack of synoptic and reliable heat and freshwater flux data sets is bypassed using the relaxation approach both for the salinity and temperature surface fields. We propose a parameterization of the heat fluxes in which the temperature-restoring coefficient depends on wind intensity and regime and in which the use of simuoultaneous satellite daily sea surface temperature (SST) estimates as a restoring field is required. The consistency of the proposed parameterization and of its numerical implementation with the previous oceanic boundary layer studies has been verified trough the analysis of the Saunders’ proportionality constant. This parameterization coupling simultaneous surface heat fluxes and wind trough the skin-bulk temperature difference, recovers the high variability of the air-sea exchanges of the extreme events in the Mediterranean Sea. The effect of highfrequency surface momentum and heat fluxes is studied comparing results from two differentexperimentsforcedwithmonthlyanddailysurfacewindandsatelliteSSTdatasets. Thesecomparisonsshowtherelevanceofhigh-frequencyforcingintherepresentationofthe dynamical processes relative to the intermediate water mass transformation and horizontal advection as well as in the deep water formation in the northwestern Mediterranean Sea. INDEX TERMS: 4504 Oceanography: Physical: Air/sea interactions (0312); 4255 Oceanography: General: Numerical modeling; 4243 Oceanography: General: Marginal and semienclosed seas; KEYWORDS: air/ interaction, numerical modeling, Mediterranean, satellite, circulation

Sensitivity of numerical tracer trajectories to uncertainties in OGCM velocity fields

Abstract Three different data sets of numerical drifters are obtained with degrading the time sampling (1 day, 1 month and 1 year) of the Eulerian velocity field computed from a Mediterranean general circulation model. The Finite-Scale Lyapunov Exponent (FSLE) technique is used to characterize, for each of the three data sets, Lagrangian dispersion properties in relation to the time resolution of the field. In particular, we are interested in measuring the unpredictability of trajectories due to the uncertainty in the knowledge of the velocity field. Our data analysis indicates that surface relative dispersion of the Mediterranean Sea has two regimes: exponential spreading due to chaotic advection at small scales (∼mesoscale) and super-diffusion at larger scales (up to ∼sub-basin scales). In this scenario, it is shown that trajectory evolution is most sensitive to the time sampling of the field at small spatial scales, while, at scales larger than ∼100 km, it is essentially independent from the details of the models. Also, FSLE is employed to visualize the geographical regions characterized by high Lagrangian unpredictability. The relation of FSLE with common oceanographic observables (e.g., local shear, velocity variance) is discussed.

A Lagrangian-Based Approach for Determining Trajectories Taxonomy and Turbulence Regimes

Abstract The use of the ratio between the acceleration and velocity time scales y = Ta/Tυ to separate Lagrangian trajectories in homogeneous classes is proposed. In fact, when analyzing subsurface floats data in the Atlantic Ocean and surface drifters data in the world’s ocean basins, it is observed that trajectories having different values of y are characterized by different shapes, correlation, and dispersal properties. In particular, trajectories having similar values of the acceleration and velocity time scales clearly show the influence of eddies and are characterized by an oscillating velocity correlation function. It is shown here that this trajectory screening is a useful procedure to rationalize the analysis of real Lagrangian trajectories and to avoid a mixture of different regimes, when averaging quantities. The mean statistical quantities computed averaging on quasi-homogeneous datasets put in evidence the role of the coherent structures in the dispersion properties, both in time and in the ma...

Chapter 5 The Atlantic and Mediterranean Sea as connected systems

Publisher Summary This chapter discusses that the North Atlantic plus the Mediterranean Sea is viewed as a unique system whose internal dynamics, regulated by the exchanges at the Strait of Gibraltar, is still rather unknown. It discusses numerical modeling results as a complement to the data analysis. Three issues are discusses in the chapter related to the dynamics of such a system. It focuses on the variability of water mass transformation processes inside the Mediterranean Sea with special attention to the time necessary for water masses formed within the Mediterranean Sea to spread into the North Atlantic. The chapter also discusses the physics of the interface between the two sub-systems, which is the exchange at the Strait of Gibraltar. Using results from a hierarchy of numerical ocean model, the chapter reviews the spreading of Mediterranean Outflow Water (MOW) in the North Atlantic and its possible relation to the variability of the meridional overturning circulation of the global ocean.

Satellite investigations of a cyclonic gyre in the central Tyrrhenian Sea (western Mediterranean Sea)

Thermal satellite images related to surface temperature of the Tyrrhenian Sea (western Mediterranean Sea) show an elliptic cold patch, suggesting the presence of a wind-induced cyclonic gyre with well-defined meanders of comparatively constant wavelength. In order to gain insight into the dynamics of the gyre these data are compared with results of two classical linear models, that of Crepon and Richez on wind-induced upwellings and that of Gill on currents due to a buoyancy discontinuity in rotating channels. The large-scale gyre meanders are analyzed using theoretical information on baroclinic instability models. It results that the gyre is due to winds blowing eastward from the Mouths of Bonifacio and to the buoyancy due to a thermal front along 41°E. With this general procedure, from satellite images an estimate of the deep hydrologic quantities is obtained; this technique is of interest also for other similar systems.

Assimilation of satellite AVHRR SST in an OGCM of the Mediterranean Sea: data processing, new parametrizations, and physical results

OGCMs numerical experiments for climatic studies can be performed only after testing their capability to well represent the today ocean circulation. In order to improve the model results we assimilated satellite SST (from NOAA-AVHRR sensors) by means of a nudging method. Daily 1987 - 1992 NASA Pathfinder SST have been interpolated in time and space by means of an objective analysis algorithm to fill gaps in the time series. These data were analyzed to investigate the main variability of the Mediterranean SST field. The 1988 data was selected to be assimilated in the OGCM. To relate more closely the assimilation of the SST data to the physics of the air-sea interaction, here we propose a new parameterization of the surface relaxation time. Actually the use of realistic surface forcing strongly contribute to reproduce the correct hydrological values of the Mediterranean water masses, the right energetic basin budgets and the mesoscale features of the Mediterranean circulation.