Abstract

This paper deals with the statistical analysis of the nine years of model simulation described in Part I. Here the focus is on different applications of EOF analysis in the time domain, pointing out the spatial–temporal scales of the Mediterranean general circulation variability. The analysis is carried out either in 2D or 3D space and is based on the singular value decomposition technique. Seasonal and interannual variability of the Mediterranean Basin occur on the subbasin gyre spatial scales. Two major events of interannual variability occurring during the winters of 1981 and 1986 are identified through the analysis of the barotropic/baroclinic circulation. Barotropic streamfunction EOF analysis shows that after strong winter wind anomaly events, which enhance cyclonic circulation, the basin relaxes to opposite sign vorticity regimes. The analysis confirms that the largest barotropic anomalies are locked to the winter season.

The temperature 3D EOF analysis highlights that three vertical amplitude modes can represent all the variance in the upper 400 m of the water column. The second mode, which has basin-wide scales with meridional gradients, is surface intensified and contains the seasonal cycle anomalies. The surface ocean signal propagates into the mixed layer within a period of three months.

The dynamic height field interannual variability is pronounced at the subbasin-scale gyres and it shows anomalies in summer of 1981 and 1986, six months after the winter forcing anomaly events.

The study of the correlation between surface heat fluxes and SST variability (extended EOF) shows that winter anomalous cooling events (1981, 1987) can generate different effects on the SST response involving both large-scale advection/diffusion processes and subduction events.

Abstract

In this study a general circulation model is used in order to investigate the interannual response of the Mediterranean Basin to low-frequency interannual variability in atmospheric forcing for the period 1980–88. The model incorporates a realistic scheme for the air–sea interaction physics, has 31 levels in the vertical, and a quarter of a degree horizontal resolution.

The simulations show the strong seasonal and interannual signal of the upper thermocline Mediterranean general circulation. Interannual variability of the basin has an eventlike character (anomalous winter wind curl for 1981 and 1986, heat flux winter anomalies in 1981 and 1987) and it is mainly forced by wintertime anomalies;for example, it is locked to the seasonal cycle. The Ionian and the eastern Levantine areas are found to be more prone to interannual changes. The Gibraltar mass transport undergoes small seasonal changes around an average value of 0.95 Sverdrup (Sv) while the Sicily Strait transport is characterized by much stronger seasonal and interannual fluctuations around an average of 1.5 Sv. Sensitivity experiments to atmospheric forcing show that large anomalies in winter wind events can shift the timing of occurrence of the seasonal cycle. Energy cycles involve exchanges between barotropic, baroclinic kinetic energy, and available potential energy reservoirs. The analysis presented here shows that the direction of conversion between these reservoirs depends on the wind strength. During winter, energy is stored in the available potential energy pool and the kinetic energy is directly forced by winds. During summer kinetic energy grows at the expense of available potential energy following the well-known baroclinic instability conversion process.

Abstract

In this article the impact of multisatellite altimeter observations assimilation in a high-resolution Mediterranean model are analyzed. Four different altimeter missions [Jason-1, Envisat, Ocean Topography Experiment (TOPEX)/Poseidon interleaved and Geosat Follow-On] are used over a 7-month period (from September 2004 to March 2005) to study the impact of the assimilation of one to four satellites on the analyses quality. The study highlights three important results. First, it shows the positive impact of the altimeter data on the analyses. The corrected fields capture missing structures of the circulation, and eddies are modified in shape, position, and intensity with respect to the model simulation. Second, the study demonstrates the improvement in the analyses induced by each satellite. The impact of the addition of a second satellite is almost equivalent to the improvement given by the introduction of the first satellite: the second satellite’s data bring a 12% reduction of the root-mean-square of the differences between the analyses and observations for the sea level anomaly (SLA). The third and fourth satellites also improve the rms, with a more than 3% reduction for each of them. Finally, it is shown that Envisat and Geosat Follow-On additions to Jason-1 impact the analyses more than the addition of TOPEX/Poseidon, suggesting that the across-track spatial resolution is still one of the important aspects of a multimission satellite observing system. This result could support the concept of multimission altimetric monitoring done by complementary horizontal resolution satellite orbits.

Abstract

The Aegean water masses and circulation structure are studied via two large-scale surveys performed during the late winters of 1988 and 1990 by the R/V Yakov Gakkel of the former Soviet Union. The analysis of these data sheds light on the mechanisms of water mass formation in the Aegean Sea that triggered the outflow of Cretan Deep Water (CDW) from the Cretan Sea into the abyssal basins of the eastern Mediterranean Sea (the so-called Eastern Mediterranean Transient). It is found that the central Aegean Basin is the site of the formation of Aegean Intermediate Water, which slides southward and, depending on their density, renews either the intermediate or the deep water of the Cretan Sea. During the winter of 1988, the Cretan Sea waters were renewed mainly at intermediate levels, while during the winter of 1990 it was mainly the volume of CDW that increased. This Aegean water mass redistribution and formation process in 1990 differed from that in 1988 in two major aspects: (i) during the winter of 1990 the position of the front between the Black Sea Water and the Levantine Surface Water was displaced farther north than during the winter of 1988 and (ii) heavier waters were formed in 1990 as a result of enhanced lateral advection of salty Levantine Surface Water that enriched the intermediate waters with salt. In 1990 the 29.2 isopycnal rose to the surface of the central basin and a large volume of CDW filled the Cretan Basin. It is found that, already in 1988, the 29.2 isopycnal surface, which we assume is the lowest density of the CDW, was shallower than the Kassos Strait sill and thus CDW egressed into the Eastern Mediterranean.

Abstract

A formalism to obtain a mean sea level equation (MSLE) is constructed for any limited ocean region and/or the global ocean by considering the mass conservation equation with compressible effects and a linear equation of state. The MSLE contains buoyancy fluxes terms representing the steric effects and the mass flux is represented by surface water fluxes and volume transport terms. The MSLE is studied for the Mediterranean Sea case using a simulation experiment for the decade 1999–2008. It is found that the Mediterranean MSL tendency is made of a steric contribution that is almost periodic in time superimposed on a stochastic-like signal due to the mass balance, dominating the MSL tendency. The MSL tendency stochastic-like term is a result of the imbalance between the volume flux at Gibraltar and the area average surface water flux.

Abstract

In order to perform real-time dynamical forecasts and hindcasts, three high-resolution hydrographic surveys were made of a (150 km)² domain off northern California, providing two sets of initialization and verification fields. The data was objectively analyzed and regularly gridded for model compatibility. These maps initially show an anticyclonic eddy segment in the northeast and part of another in the northwest. Two weeks later only the northwest anticyclonic eddy remained, with the domain center dominated by a 0.6 m s⁻¹ jet. Two weeks after that only a larger northwest eddy with fairly weak velocities remained. Numerical forecasts with persistent boundary conditions and forecast experiments with boundary conditions linearly interpolated between surveys were performed. The real-time forecast successfully predicted the formation of the zonal jet prior to its observation. Dynamical interpolation shows unambiguously that the two anticyclonic eddies have merged and formed a single eddy. Even the forecast with incorrect boundary conditions demonstrates the internal dynamical processes involved in the merger event.

Two examples are given of four-dimensional data assimilation: direct insertion and a backward-forward combination technique. These results justify the use of the dynamical forecasts as synoptic time series. Parameter sensitivity experiments were performed to determine the sensitivity of the model to physical parameters such as stratification, to explore the dynamical balance, and to choose a reference level. The dynamics were found to be controlled by horizontal nonlinear interactions. A reference level of 1550 m was chosen. A set of energy and vorticity equations, consistent with quasi-geostrophic dynamics, were evaluated term by term for the forecast experiments. The evolutions of the streamfunction and vorticity fields are shown to be a three-phase (merging, expanding, and relaxation) process. Available gravitational energy increases due to buoyancy work; the merger event is interpreted as a finite amplitude barotropic instability process.

Abstract

In the second part of the paper dedicated to the Adriatic Sea general circulation, the horizontal structure of the hydrographic parameters and dissolved oxygen fields is described on a seasonal timescale.

Maps of temperature and salinity climatological fields reveal the enhanced seasonal variability of the Adriatic Sea, which at the surface is associated with the major dilution effects of river runoff.

The density and derived dynamic height fields show for the first time the baroclinic geostrophic structure of the general circulation. Winter is dominated by compensation effects between temperature and salinity fronts along the western coastline. The resulting baroclinic circulation is weak and suggests the presence of barotropic current components not accessible by the dataset. Spring and summer seasons have the smallest spatial scales in the temperature and salinity fields and stronger subbasin-scale gyres and current systems, which have been classified in a schematic representation of the circulation. The Adriatic Sea general circulation comprises boundary currents and jets that strengthen and change spatial scales in different seasons. Two separate cyclonic gyres clearly exist in the middle and southern Adriatic except during winter.

The rates of formation of the northern Adriatic deep waters and southern Adriatic deep waters are estimated to be 0.07 and 0.36 Sv (Sv ≡ 10⁶ m³ s⁻¹), respectively. Likely driving mechanisms of the circulation are discussed.

Abstract

A comprehensive historical hydrographic dataset for the overall Adriatic Sea basin is analyzed in order to define the open ocean seasonal climatology of the basin. The authors also define the regional climatological seasons computing the average monthly values of heat fluxes and heat storage from a variety of atmospheric datasets. The long term mean surface heat balance corresponds to a heat loss of 19–22 W m⁻². Thus, in steady state, the Adriatic should import about the same amount of heat from the northern Ionian Sea through the Otranto Channel. The freshwater balance of the Adriatic Sea is defined by computing the average monthly values of evaporation, precipitation, and river runoff, obtaining an annual average gain of 1.14 m. The distribution of heat marks the difference between eastern and western Adriatic areas, showing the winter heat losses in different parts of the basin.

Climatological water masses are defined for three regions of the Adriatic: (i) the northern Adriatic where seasonal variations in temperature penetrate to the bottom; deep water (NAdDW) with σ _t > 29.2 kg m⁻³ is produced and salinity is greatly affected by river discharges; (ii) the middle Adriatic where a pool of modified NAdDW is stored during the summer season after being renewed in winter and modified Levantine Intermediate Water (MLIW) intrudes from the southern regions between spring and autumn; and (iii) the southern Adriatic where homogeneous water properties are found below 150 m (the local maximum depth of the seasonal thermocline) and a different deep water mass (SAdDW) is found with σ _t > 29.1 kg m⁻³, T ≈ 13.5°C, and S ≈ 38.6 psu. Due to river runoff waters, the surface layers of all three regions are freshened during the spring–summer seasons. The vertical distributions of dissolved oxygen vary quantitatively in the three regions showing a spring–summer subsurface maximum due to the balance between phytoplankton growth in the euphotic zone and low vertical mixing in the water column. This behavior can be reconciled with open ocean conditions except for the northernmost part of the Adriatic where well-mixed oxygen conditions prevail throughout the year.

Large interannual anomalies of both temperature and salinity are found at the geographical center of the basin in surface and deep waters (100 m).

In response to the joint European Commission and European Space Agency initiative to establish by 2008 a system for Global Monitoring for Environment and Security (GMES), the Marine Environment and Security for the European Area (MERSEA) Strand-1 Project was executed to assess and demonstrate the capacity of present monitoring and forecasting systems. The study area covered the North Atlantic, with its northwest European shelf seas, and the Mediterranean. By integrating of existing satellite observations with data from in situ measurement networks and ocean models, daily mean products and forecasts from four core data assimilation systems (~1 0 km resolution) were compared and distributed through an Open-source Project for a Network Data Access Protoco (OPeNDAP) server from1 June 2003 to 31 May 2004.Moreover, downscaling to high-resolution (1–5 km) models was used for specific applications to harmful algal bloom, eutrophication, and oil spill monitoring in the Baltic, North Sea, Irish Sea, Iberian coastal shelf seas, and the Aegean Sea. The lessons learned from this project are reported here.