UTMEA Energy and Environmental Modeling

Mediterranean thermohaline circulation

This work has been carried out by volfango rupolo who died in 2010.

Ocean General Circulation Models of relatively low resolution were and are used to study the Mediterranena Thermohaline Circulation (MTHC), the main path of circulation and its variability.

Historically we began in the nineties using as numerical tool for this tudy the GFDL-MOM implemented in the Mediterranean at 1/4° x 1/4° of resolution with 19 levels in the vertical. This, rigid lid, model was open at Gibraltar, extending up in the Atlantic up to 13° W, where sponge boundary condition were applied. Model drif were eliminated introducing new heat fluxes and diffusivity parameterization and centennial run O (500 years) were performed in order to study the internal variability of he MTHC.

Finally Lagrangian diagnostics was implemented in the to study:

• the Eastern Mediterranen transient
• the inter basin water mass transport and transformation, dispersion properties
• relation between Eulerian and Lagrangian quantities

Figure. Upper branch of the MTHC, transport are expressed in Sv

Figure. Upper branch of the MTHC, transport are expressed in Sv

References:

• V. Artale, D. Iudicone, R. Santoleri, V. Rupolo,S. Marullo, F. D'Ortenzio 2002: 'The role of surface fluxes in A OGCM using satellite SST. Validation and sensitivity to forcing frequency of the Mediterraneanthermohaline circulation' J. of Geophys. Res. Vol 107 C8
• Iudicone D., G. Lacorata, V. Rupolo, R. Santoleri, A. Vulpiani; 2002: 'Sensitivity of numericaltracer trajectories to uncertainities in OGCM velocity fields' Ocean Modelling 4 (2002) pp 313-325.
• Rupolo, V., S. Marullo,D. Iudicone,2003: ' The Eastern Mediterranean transient studied with Lagrangian diagnostics applied to a Mediterranean OGCM forced by satellite S Nuovo Cimento,26 C 4 ,387-415
• Artale, V. , S. Calmanti, G. Pisacane and V. Rupolo , 2006: The Atlantic and Mediterranean Sea as connected systems. In: P. Lionello, P., Malanotte-Rizzoli & R. Boscolo (Eds), Mediterranean Climate Variability, Amsterdam: Elsevier, pp. 283?323.
• Pisacane G., Artale, V. , S. Calmanti, V. Rupolo , 2006: Decadal oscillations in the Mediterranean Sea: a result of the overturning circulation variability in the Eastern Basin ? Clim. Res. ,31 ,257-271.

Hydraulic of the Strait of Gibraltar

The hydraulic state of the exchange circulation through the Strait of Gibraltar is defined using a recently developed critical condition that accounts for cross-channel variations in layer thickness and velocity, applied to the output of a high-resolution three-dimensional numerical model simulating the tidal exchange. The numerical model uses a coastal-following curvilinear orthogonal grid, which includes, besides the Strait of Gibraltar, the Gulf of Cadiz and the Alboran Sea. The model is forced at the open boundaries through the specification of the surface tidal elevation that is characterized by the two principal semidiurnal and two diurnal harmonics: M2, S2, O1, and K1. The simulation covers an entire tropical month.

The hydraulic analysis is carried out approximating the continuous vertical stratification first as a two-layer system and then as a three-layer system. In the latter, the transition zone, generated by entrainment and mixing between the Atlantic and Mediterranean flows, is considered as an active layer in the hydraulic model. As result of these vertical approximations, two different hydraulic states have been found; however, the simulated behavior of the flow only supports the hydraulic state predicted by the three-layer case. Thus, analyzing the results obtained by means of the three-layer hydraulic model, the authors have found that the flow in the strait reaches maximal exchange about 76% of the tropical month-long period.

Figure. Horizontal model grid for the region of the strait.

Figure. Bars indicating the presence of provisional supercritical flow with respect to one mode (black) and with respect to both modes (gray) in the three main regions of the strait: Espartel Sill, Camarinal Sill, and Tarifa Narrow. (bottom) Tidal elevation at Tarifa.

References:

• G. Sannino, L. Pratt and A. Carillo (2009), Hydraulic criticality of the exchange flow through the Strait of Gibraltar, J. of Physical Oceanography, Vol 39, 11, 2779-2799. DOI: 10.1175/2009JPO4075.1

Evaluation of the transport through the Strait of Gibraltar

Three-yearlong time series of Acoustic Doppler Current Profiler (ADCP) observations at a single station in Espartel Sill (Strait of Gibraltar) were used to compute an outflow of Q2 = 0.82 Sv through the main channel. The cross-strait structure of the velocity field or the outflow through a secondary channel north of the submarine ridge of Majuan in Espartel section is not captured by observations so that an improved version of a numerical model (CEPOM) has been used to fill the observational gap.

Previously, the model performance has been checked against historical data sets by comparing harmonic constants of the main diurnal and semidiurnal constituents from observed and modeled data at different sites of the strait. Considering the great complexity of tidal dynamics in the area, the comparison is quite satisfactory and validates the model to infer the exchange at longer timescales. Using a ''climatological'' April in the simulation, extracting a ''single station'' from the model at the same position as the monitoring station and processing the data similarly, the model gives an outflow through the southern channel 13% higher than observations.

The inclusion of the cross-strait structure of velocity reduces the computed outflow through the southern channel, whereas the contribution of the northern channel brings the total outflow close to that computed using a single station (5% smaller). If the same correction is applied to observations, the total outflow would reduce to Q2 = 0.78 Sv. The paper also assesses the importance of eddy fluxes to the total outflow, their contribution being negligible (<5%).

Figure. (a-f) Along-strait velocity across ES (m/s) simulated by the model during spring tide. Solid lines indicate negative velocities (toward the Atlantic), and dashed lines indicate positive velocities (toward the Mediterranean). The thick line indicates zero velocity. Different snapshots are 2 hours apart. The tidal time, referred to the surface tide in Tarifa, is indicated at the bottom of each panel.

References:

• Sánchez Román, G. Sannino, Garcia-Lafuente, A. Carillo, F. Criado-Aldeanueva (2009), Transport estimates at the western section of the Strait of Gibraltar: a combined experimental and numerical modelling study, Journal of Geophysical Research, Vol 114, C06002, DOI: 10.1029/2008JC005023.
• Garrido, J.C. S., J. Garcia-Lafuente, F. Criado-Aldeanueva, A. Baquerizo, and G. Sannino (2008), Time-spatial variability observed in velocity of propagation of the internal bore in the Strait of Gibraltar, Journal of Geophysical Research,Vol 113, C07034, doi:10.1029/2007JC004624.
• Garcia-Lafuente, J., A. Sánchez Román, G. Díaz del Río, G. Sannino, and J. C. Sánchez Garrido (2007), Recent observations of seasonal variability of the Mediterranean outflow in the Strait of Gibraltar, Journal of Geophysical Research,Vol 112, C10005, doi:10.1029/2006JC003992.
• Sannino G., A. Carillo, V. Artale (2007), Three-layer view of transports and hydraulics in the Strait of Gibraltar: A three-dimensional model study, Journal of Geophysical Research, 112, C03010, doi:10.1029/2006JC003717, 2007.
• Sannino G., A. Carillo, V. Artale, V. Ruggiero and P. Lanucara (2005), Flow regimes study within the Strait of Gibraltar using an high performance numerical model, Il Nuovo Cimento, Vol. 28, Issue 02, DOI:10.1393/ncc/i2005-10177-2.
• Sannino G., A. Bargagli and V. Artale (2004), Numerical modeling of the semidiurnal tidal exchange through the Strait of Gibraltar, Journal of Geophysical Research, Vol. 109, C05011, doi:10.1029/2003JC002057.
• Sannino G., A. Bargagli and V. Artale (2002), Numerical modeling of the mean exchange through the Strait of Gibraltar, Journal of Geophysical Research, Vol. 107, NO. C8, 3094, 10.1029/2001JC000929.

Grid refinement

An eddy-permitting model of 1/8°x1/8° resolution has been implemented covering the whole Mediterranean Sea. Within this grid a 1/24°x1/24° resolution model of the Strait of Gibraltar is embedded. The two grids belong to different models that are coupled through an external parallel driver. The robustness of the adopted grid refinement procedure is tested on a multi-decadal integration simulating the present climate. The good agreement found between the model circulation and most of the available observations confirms both the robustness and effectiveness of the two-way grid refinement technique. The effects produced on the Mediterranean circulation by the grid refinement has been investigated through the comparison of two simulations differing only in the presence of the grid refinement.

Even though the main characteristics of the thermohaline circulation appear similar in the two simulations, some quantitative and qualitative differences are observed: the main differences found in the Strait of Gibraltar propagate into the whole basin, have an impact on the water column stratification, and consequently on the convection events.

Figure. Maps of the 40-year average of: (a and b) the kinetic energy averaged over the whole water column (m2/s2); (c and d) the RMS of the kinetic energy averaged over the whole water column (m2/s2); (e and f) the depth of the 1029 kg/m3 density isosurface (m); (g and h) the salinity averaged over the whole water column (psu); (i and l) the yearly maximum MLD (m); (m and n) the total columnar buoyancy BC(z = bottom). Left panels are for GR experiment and right panels show the difference between GR and NOGR experiment.

References:

• G. Sannino, Herrmann M., Carillo A., Rupolo V., Ruggiero V., Artale V. Heimbach P. (2009), An eddy-permitting model of the Mediterranean Sea with a two-way grid refinement at the Strait of Gibraltar. Ocean Modelling, Vol 30, 1, 56-72. DOI: 10.1016/j.ocemod.2009.06.002

Tyrrhenian sea

Prompted by the development of an operational model of the Tyrrhenian Sea (TYS) circulation, a line of research has started in recent years, dedicated to the understanding of the dynamics of this basin.

Despite a long observational history, starting with the extensive Russian campaigns made in the sixties and in the seventies, a detailed description of the TYS circulation is still lacking. While some features, such as the large-scale patterns through the basin openings (Sardinia and Corsica channels, and Sicily Strait) and some large gyres (the cyclone-anticyclone couple off the Bonifacio Strait, and the wide cyclonic recirculation between Sardinia and Sicily) are reasonably well-established, the dynamics in the central and eastern parts of the basin is still poorly understood.

In fact, recent observational and numerical works suggest that the presence of a complex mesoscale structure in the area, that needs further investigation. The main purpose of this line of research is to shed light on this structure, through a combined use of satellite and in-situ observations, and of dedicated numerical simulations.

Figure. Spring 2004 surface circulation (75 m of depth) as reconstructed from an inverse box model (panel a) and from a dedicated numerical experiment (panel b) (Vetrano et al., 2010)

References:

• Vetrano A., E. Napolitano, R. Iacono. K. Schroeder, G. P. Gasparini (2010), The Tyrrhenian sea Circulation and water mass fluxes in spring 2004: observations and model results, J. of Geophysical Research, in press

Effects of topography on the circulation around planetary island and ridges

This line of activity has started in 2007 as a collaboration with Joe Pedlosky, of WHOI. The main objective is to understand the effects of a topographic skirt on the large-scale circulation around a planetary island or ridge, in the context of simplified models of the dynamics, and idealized, but realistic, geometrical settings and forcings.

The activity started with a combined theoretical and numerical analysis of the barotropic problem, using the shallow water model, that has subsequently been completed by beautiful laboratory experiments in a rotating tank (lid-driven sliced cylinder analogue of wind-driven circulation on a beta-plane) carried out by Karl Helfrich. We are currently investigating the same problem in a simple baroclinic context.

Figure. On the left is the equilibrium stream function pattern from a numerical shallow water simulation, while on the righ is a picture from a laboratory experiment. Both clearly show a wide meander on the western side of the island, that is one of the distinctive effects of the presence of a topographic skirt (the white area in the picture).

References:

• Pedlosky, J., R. Iacono, E. Napolitano, K. Helfrich (2009), The skirted island: the effect of topography on the flow around planetary scale islands, J. of Mar. Research, in press

postprocessing of the MFS eulerian archive

This work has been carried out by volfango rupolo and Claudia Pizzigalli. Volfango died in 2010 and Claudia is not any more at ENEA.

Seasonal probability dispersion maps from numerical Lagrangian trajectories as well as a user-friendly web site containing the results were produced using the Eulaerain archiveof the Mediterranean Forecasting System project

The main aim is to provide useful tools for the description of dispersion properties at the Mediterranean Sea surface. Trajectories were obtained integrating with an off-line Lagrangian algorithm the Eulerian velocity fields, from January 2000 to December 2004, provided by the ocean general circulation model Modular Ocean Model used in the MFSt.

The statistics of dispersion properties over the entire surface of the Mediterranean Sea is calculated from the time evolution of ''clusters'' of numerical particles released in nonoverlapping grid boxes. Some examples of seasonal probability dispersion maps taken from the web site are shown and discussed.

The results are checked against drifters of the Mediterranean Surface Drifter Database (1986 ? 1999) by comparing the statistics of numerical and observed trajectories. The validation shows both deficiencies and skills of the methodology proposed for estimating Lagrangian dispersion variability. It also gives a starting point for discussion about future developments and improvements.

Figure. Rectangles indicate the standard deviation, computed in winter, of the center of mass of the single realizations around the seasonal center of mass trajectory for particles released in the Corsica Channel. The four panel shows dispersion statistics after 7,14,21 and 28 days.

The Eulerian archibe of MFS was used also to simulate ARGO floats movements in orfir to optimize tge experimental strategy, to study interannual variability of the inter basin Lagrangian transport of tracer with imposed 'mortality' in the Western Mediterranean basin and to study the possible present day dispersal pattern of Posidonia.

References:

• Pizzigalli C. and V. Rupolo ; 2007: Simulations of ARGO profilers and of surface floating objects: applications in MFSTEP. Oc. Sci. 3 ,205-222.
• Pizzigalli , V. V. Rupolo , E. Lombardi and B. Blanke 2007: Seasonal Probability Dispersion Maps in the Mediterranean Sea obtained from the MFS Eulerian velocity field. J. of Geophys. Res., 112, C05012, doi:10.1029/2006JC003870.
• I.A. Serra, A.M. Innocenti, G. Dimaida, S.Calvo, M.Migliaccio, E. Zambianchi, C. Pizzigalli, S. Arnaud-haond, C.M. Duarte, E.A. Serrao, G. Procaccini, 2010: Genetic structure in the Mediterranean seagrass Posidonia oceanica: disentangling past vicariance events from contemporary patterns of gene flow. Molecular Ecology (2010) 19, 557?568 doi: 10.1111/j.1365-294X.2009.04462.x