Bruno Buongiorno Nardelli

Altimetric signal and three‐dimensional structure of the sea in the Channel of Sicily

The 1996 Altimeter/Synoptic Mesoscale Plancton Experiment (ALT/SYMPLEX) was specifically designed to perform in situ measurements simultaneous with the passage of TOPEX/POSEIDON (T/P) and ERS 2 over selected tracks in the central and eastern Sicily Channel. This experiment made it possible to have, for the first time, a validation of altimetry with in situ data over the Mediterranean, where weak dynamics results in a modest sea elevation, rarely exceeding 10 cm. Historical infrared and altimetric satellite data were first analyzed in order to study the variability of the circulation in the area. The comparative and integrative analysis of simultaneous satellite data and in situ measurements permitted investigation of the relation between the altimeter-derived surface topography and the three-dimensional structure of the sea. The Pearson correlation coefficients between altimeter data and dynamic heights along track resulted to be 0.72-0.89 (T/P) and 0.88 (ERS 2) when using conventional repeat track analysis. For T/P, a correlation value of 0.87 was found for time differences computed basing on a collinear analysis technique. This analysis also led to the identification of a strong barotropic component of the velocity field located near the Sicilian continental shelf, where it is responsible for approximately 60% of the signal.

Small Mesoscale Features at a Meandering Upper-Ocean Front in the Western Ionian Sea (Mediterranean Sea): Vertical Motion and Potential Vorticity Analysis

Abstract Transient mesoscale and submesoscale processes, such as small eddies and filaments, could play a fundamental role in the marine ecosystem, especially in oligotrophic seas like the Mediterranean. However, very little is known about the biological and physical dynamics characterizing such structures. In this work, the 8-km horizontal resolution data collected during the SYMPLEX 1998 survey are analyzed to describe the physical dynamics of small mesoscale features along the Atlantic–Ionian stream (western Ionian Sea). The data were optimally interpolated over a regular grid and used to compute the 3D ageostrophic circulation by solution of the Q-vector formulation of the omega equation. The relative importance of stratification, relative vorticity, and twisting terms in the Rossby–Ertel potential vorticity is thus examined along selected isopycnals, together with the associated vertical motions and vortex stretching. A surface cyclonic eddy ∼15 km of radius was observed near a meander of the Atlanti...

The role of hydrodynamic processes on anchovy eggs and larvae distribution in the sicily channel (mediterranean sea): a case study for the 2004 data set.

Knowledge of the link between ocean hydrodynamics and distribution of small pelagic fish species is fundamental for the sustainable management of fishery resources. Both commercial and scientific communities are indeed seeking to provide services that could “connect the dots” among in situ and remote observations, numerical ocean modelling, and fisheries. In the Mediterranean Sea and, in particular, in the Sicily Channel the reproductive strategy of the European Anchovy (Engraulis encrasicolus) is strongly influenced by the oceanographic patterns, which are often visible in sea surface temperature satellite data. Based on these experimental evidences, we propose here a more general approach where the role of ocean currents, wind effects, and mesoscale activity are tied together. To investigate how these features affect anchovy larvae distribution, we pair ichthyoplankton observations to a wide remote sensing data set, and to Lagrangian numerical simulations for larval transport. Our analysis shows that while the wind-induced coastal current is able to transport anchovy larvae from spawning areas to the recruiting area off the Sicilian south-eastern tip, significant cross-shore transport due to the combination of strong northwesterly mistral winds and topographic effects delivers larvae away from the coastal conveyor belt. We then use a potential vorticity approach to describe the occurrence of larvae cross-shore transport. We conclude that monitoring and quantifying the upwelling on the southern Sicilian coast during the spawning season allows to estimate the cross-shore transport of larvae and the consequent decrease of individuals within the recruiting area.

A Novel Approach for the High-Resolution Interpolation of In Situ Sea Surface Salinity

AbstractA novel technique for the high-resolution interpolation of in situ sea surface salinity (SSS) observations is developed and tested. The method is based on an optimal interpolation (OI) algorithm that includes satellite sea surface temperature (SST) in the covariance estimation. The covariance function parameters (i.e., spatial, temporal, and thermal decorrelation scales) and the noise-to-signal ratio are determined empirically, by minimizing the root-mean-square error and mean error with respect to fully independent validation datasets. Both in situ observations and simulated data extracted from a numerical model output are used to run these tests. Different filters are applied to sea surface temperature data in order to remove the large-scale variability associated with air–sea interaction, because a high correlation between SST and SSS is expected only at small scales. In the tests performed on in situ observations, the lowest errors are obtained by selecting covariance decorrelation scales of 4...

Net primary production in the Gulf Stream sustained by quasi‐geostrophic vertical exchanges

We analyze 12 years of mesoscale vertical motion derived from an observation-based product in the top 1000 m of the North West Atlantic Ocean. Vertical velocities (O(10 m d−1)) associated with Gulf Stream instabilities consist of alternating cells of upwelling and downwelling. Here we show that the magnitude of the vertical motions decays exponentially southward with an e-folding length scale that is informative on the dynamics of the system. We further investigate the impact of the vertical supply of nutrients about phytoplankton growth with a conceptual model incorporating the mean effect of nutrient distribution, quasi-geostrophic dynamics, and Ekman suction/pumping. Results confirm that the mean effect of mesoscale vertical velocity variability alone can sustain observed levels of net primary production in the immediate vicinity of the Gulf Stream, while other mechanisms, including horizontal advection and submesoscale dynamics, need to be considered when moving toward the subtropical gyre.

Methods for the Reconstruction of Vertical Profiles from Surface Data: Multivariate Analyses, Residual GEM, and Variable Temporal Signals in the North Pacific Ocean

Abstract Different methods for the extrapolation of vertical profiles from sea surface measurements have been tested on 14 yr of conductivity–temperature–depth (CTD) data collected within the Hawaii Ocean Time-series (HOT) program at A Long-Term Oligotrophic Habitat Assessment (ALOHA) station in the North Pacific Ocean. A new technique, called multivariate EOF reconstruction (mEOF-R), has been proposed. The mEOF-R technique is similar to the previously developed coupled pattern reconstruction (CPR) technique and relies on the availability of surface measurements and historical profiles of salinity, temperature, and steric heights. The method is based on the multivariate EOF analysis of the vertical profiles of the three parameters and on the assumption that only a few modes are needed to explain most of the variance/covariance of the fields. The performances of CPR, single EOF reconstruction (sEOF-R), and mEOF-R have been compared with the results of residual GEM techniques and with ad hoc climatologies, ...

Reconstructing Synthetic Profiles from Surface Data

Abstract A method for the extrapolation of vertical profiles of temperature (and/or steric heights) from measurements of sea surface elevation and sea surface temperature has been developed and is described here. The technique, called coupled pattern reconstruction (CPR), is based on a multivariate analysis of the coupled variability of vertical profiles from historical hydrographic data and on the hypothesis that only few modes are needed to explain most of the covariance of the fields. Through a linear regression between the amplitudes of the coupled modes it is possible to reconstruct the first two modes by solving a simple linear system written for the surface values, which are supposed to be known. The CPR method has been applied and tested on 9 yr of conductivity– temperature–depth (CTD) measurements collected in the northern Mediterranean Sea during the Dynamiques des Flux de Matiere en Mediterranee (DYFAMED) program (1994–2002). The first 6 yr were used as a training dataset, while the last 3 were...

Detection by altimetry and AVHRR images of an intense anticyclonic eddy in the Balearic Sea (Western Mediterranean)

Satellite data have revealed the presence of an intens4e mesoscale anticyclonic eddy at the northern boundary of the Balearic Sea, which appeared in September 1998 and remained almost stationary until March 1999. Sea Surface Temperature (SST) images have allowed to investigate the formation of the eddy indicating an entrance of Atlantic Water through the Ibiza and Mallorca channels much stronger than normal conditions and following an anomalous path. Additional data form an oceanographic cruise on February 1999 confirmed the signature of Atlantic Water in the region. This structure produced a relevant inversion in the usual cyclonic circulation of the basis. The observed strong anticyclonic eddy induced anomalous northeastwards geostrophic velocities near the Catalan Coast reaching the maximums of about 50 cm/s, by contrast of the typical 20 cm/s in opposite sense. The three data-sets examined in this work are in good agreement in terms of the dimensions and intensity of the eddy. This suggests that remote sensing would be able to detect such kind of events in the future constituting a powerful tool providing a real-time monitoring of the Basin.

SYMPLEX experiment: first results on oceanic mesoscale dynamics and related primary production from AVHRR and SeaWIFS satellite data and field experiments

Upper ocean dynamics is characterized by a strong variability, at different scales, both in direction and structure of the flow. Mesoscale variability, which is ubiquitous in the world ocean, is often the dominant component in the variance spectrum of velocity with relevant implications on water mass mixing and transformation and on the carbon transfer in the marine food web. Mesoscale activity is manifested through the formation of instabilities, meanders and eddies. Eddies generate either a doming of isopycnals (cyclones) or a central depression (anticyclones). This in turn modifies, among the others, nutrient and organism distributions in the photic zone eventually enhancing or depressing photosynthetic activity and other connected biological responses. The mechanism is similar to what has been thoroughly studied for the warm and cold core rings but at different spatial and temporal scales. The enhancement of phytoplankton growth and the modification of photosynthetic parameters has been shown to occur in situ by means of a modulated fluorescence probe. More recently, an attempt to estimate the magnitude of this specific forcing on nutrient fluxes and primary production has also been conducted at different scales by modeling exercises, though with contrasting estimates the relative importance concerns. Because phytoplankton growth takes place when light, nutrients and cells are found at the same place, the increase in primary production favored by mesoscale eddies cannot be easily predicted. The incident light, the seasonality, the life-time of the structure, its intensity etc. can all influence the final yield. In addition, it has still to be determined which component of the community reacts faster and takes advantage of the new nutrients and how efficiently the new carbon is channeled in the food web. For what remote sensing is concerned, the detectability form the space of such structures is certainly dependent on the depth at which the upward distortion of isopycnals takes places. It can be supposed that a change in bio-optical signature of the whole structure could occur because of the 3-D dynamics of the eddy. If this holds true, then color remote sensing coupled with sea level topography and sea surface temperature should be a powerful tool to track such transient structures. The ALT-SYMPLEX program has been designed to better understand the relationship between short living eddies and carbon transfer in the food web. This is based on several experiments aimed to integrate remote sensing data (ocean color and surface topography) and in situ data in order to evaluate the relationship between surface and sub-surface physical dynamics and its relations on chemical and biological aspects in presence of mesoscale features.

A New Global Sea Surface Salinity and Density Dataset From Multivariate Observations (1993–2016)

Monitoring sea surface salinity and density variations is crucial to investigate the global water cycle and the ocean dynamics, and to analyse how they are impacted by climate change. Historically, ocean salinity and density have suffered a poor observational coverage, which hindered an accurate assessment of their surface patterns, as well as of associated space and time variability and trends. Different approaches have thus been proposed to extend the information obtained from sparse in situ measurements and provide gap-free fields at regular spatial and temporal resolution, based on the combination of in situ and satellite data. In the framework of the Copernicus Marine Environment Monitoring Service, a daily (weekly sampled) global reprocessed dataset at ¼°x¼° resolution has been produced by modifying a multivariate optimal interpolation technique originally developed within MyOcean project. The algorithm has been applied to in situ salinity/density measurements covering the period from 1993 to 2016, using satellite sea surface temperature differences to constrain the surface patterns. This improved algorithm and the new dataset are described and validated here with holdout approach and independent data.