1,721,226 research outputs found
The halting effect of baroclinicity in vortex merging
No full textStudies the quasigeostrophic merging dynamics of axisymmetric baroclinic vortices to understand how baroclinicity affects merging rates and the development of the nonlinear cascade of enstrophy. The initial vortices are taken to simulate closely the horizontal and vertical structure of Gulf Stream rings. A quasigeostrophic model is set with a horizontal resolution of 9 km and 6 vertical levels to resolve the mean stratification of the Gulf Stream region. The results show that the baroclinic merging is slower than the purely barotropic process. -from Author
DYNAMICS OF DEEP THERMOCLINE JETS IN THE POLYMODE REGION
No full textThe dynamics of deep thermocline jets is investigate din the Polymore are
Mesoscale data assimilation studies in the Middle Adriatic Sea
No full textIn this paper we describe a mesoscale data assimilation experiment in the Middle Adriatic Sea. In order to perform dynamical forecasts we provide a quasigeostrophic numerical model with a set of initial fields regularly gridded via an objective analysis technique. Maps of this initial condition show a surface intensified jet meandering around a cyclonic eddy at the thermocline and deep levels. We dynamically forecasted the flow evolution for 30 days after initialization. The time scale of the variability is of the order of a few weeks and the cyclonic vortex seems to be locked to the topography. A set of numerical experiments with different initial bottom boundary conditions, with and without topography, are made to explore the influence of the topographic constraint on the mesoscale flow evolution. As expected the influence of the topography on the dynamical evolution of the flow is very strong and it confines the jet-cyclone along the bathymetric contours, strengthening the flow. On the other hand the flow is not sensitive to changes in the density bottom initial condition. Finally we analyze the energy and vorticity fields of the 1-month dynamical forecast experiment. It is concluded that a local frontogenetic process consisting of jet strengthening and cyclone development occurs and that available gravitational energy is converted into kinetic energy during the process. © 1994
The Adriatic Sea modelling system: A nested approach
No full textA modelling system for the Adriatic Sea has been built within the framework of the Mediterranean Forecasting System Pilot Project. The modelling system consists of a hierarchy of three numerical models (whole Mediterranean Sea, whole Adriatic Sea, Northern Adriatic Basin) coupled among each other by simple one-way, off-line nesting techniques, to downscale the larger scale flow field to highly resolved coastal scale fields. Numerical simulations have been carried out under climatological surface forcing. Simulations were aimed to assess the effectiveness of the nesting techniques and the skill of the system to reproduce known features of the Adriatic Sea circulation phenomenology (main circulation features, dense water formation, flow at the Otranto Strait and coastal circulation characteristics over the northern Adriatic shelf), in view of the pre-operational use of the modelling system. This paper describes the modelling system setup, and discusses the simulation results for the whole Adriatic Sea and its northern basin, comparing the simulations with the observed climatological circulation characteristics. Results obtained with the northern Adriatic model are also compared with the corresponding simulations obtained with the coarser resolution Adriatic model
Variability of the large scale general circulation of the Mediterranean Sea from observations and modelling: A review
No full textIn this paper we present an overview of the most recent findings about the Mediterranean Sea present-day circulation structure. Both historical observations and numerical model simulations are presented, outlining the differences and agreement. The model simulations are presented for both an eddy resolving and a coarse resolution numerical model and the results are intercompared. The importance of the mesoscales in modifying the large scale flow field is elucidated. The critical point is the discovery of the large amplitude interannual variability of the circulation and water mass structure associated with the anomalies of atmospheric forcing over the basin. The seasonal variability can be strictly related to changes in heat and momentum fluxes, while the interannual variability has a component which is related to the mesoscale field. The latter is very intense in the 2Algerian Current region and Levantine basin but different in structure in the two regions. Results are shown, which confirm the importance of wind driving in establishing the kinetic energy of the flow field, by comparing the current transport at the Strait of Corsica with observations. In conclusion, the essential characteristics of the present-day circulation are associated with the atmospheric forcing and the basin topographic structure. Results from palaeoceanographic simulations for the last 20000 years show that changing the atmospheric forcing can cause large changes in the circulation structure which may have affected sapropel formation. (C) 2000 Elsevier Science B.V
Abrupt cooling of the mediterranean levantine intermediate water at the beginning of the 1980s: Observational evidence and model simulations
No full textThe Levantine Intermediate Water (LIW) is an important water mass for the overall hydrology of the Mediterranean Sea and there are open questions connected with the possible long-term variability of its physical characteristics. This paper is dedicated to the analysis and the interpretation of the LIW long-term variations over the last 50 years. It is based on data analysis and model simulations. On the one hand, new temperature and salinity gridded data of interannual and decadal anomalies have been produced from existing historical datasets. On the other hand, a long-term primitive equation model simulation has been generated, to be compared to the observational reconstructions. Results indicate that the major feature of both datasets (observations and model) is an intense cooling of the LIW (0.24°-0.28°C at 200-m depth) at the beginning of the 1980s (winters 1981 and 1983). Around the Aegean Sea and the Cretan Arc, the amplitude of the cooling is as large as 0.4°C. The model simulations, forced by the Comprehensive Ocean-Atmosphere Data Set atmospheric fluxes, reproduce the cooling event quite faithfully. The possible processes at the origin of these interannual/decadal variations are discussed. Hypotheses are proposed and tested against observations. In particular it is shown that over the period of interest, the major part of the LIW interannual/decadal variability has been directly forced by anomalies in the surface heat budget of the Eastern Mediterranean
A seasonal model of the Mediterranean Sea general circulation
No full textThis paper describes the seasonal characteristics of the Mediterranean Sea general circulation as simulated by a primitive equation general circulation model. The general circulation is composed of subbasin gyres, and cyclonic motion dominates the northern and anticyclonic motion the southern part of the basin. The Atlantic stream is a coherent structure at the surface. At depth it appears as current segments and jets around a vigorous gyre system. The seasonal variability is manifested by a change in amplitude and location of the gyres and by the appearance of seasonally recurrent gyres in different parts of the basin. For the first time a Mersa-Matruh Gyre is successfully simulated due to the introduction of our heat fluxes at the air-sea interface. The seasonal thermocline is formed each summer, and a deep winter mixed layer is produced in the region of Levantine intermediate water formation. -from Author
A model study of air-sea interactions in the Mediterranean Sea
No full textThe surface heat balance is analyzed in all its components to assess the most commonly used heat flux bulk formulas applied to the Mediterranean basin and to provide simulations of better air-sea interaction boundary conditions for Mediterranean OGCMs (Ocean General Circulation Models). A 9-year data set (1980-1988) from 12 hourly NMC atmospheric analysis combined with Reynolds SST and COADS cloud coverage is used. The surface heat balance of the Mediterranean, including the ocean heat advection through the Strait of Gibraltar and the water budget of the basin, is computed using the NMC data at monthly and 12-hourly frequency. The 12-hourly NMC data yield the correct heat loss in the basin. This analysis identifies a set of heat flux bulk formulas ('the calibrated set') which represent best the heat balance of the Mediterranean Basin with our available data sets. The calibrated set of bulk formulas, used as heat flux boundary condition for the OGCM, is capable to improve the representation of the water masses in the Mediterranean basin and to reduce the model climatic drift
Teleconnections between Indian monsoon and Sahel rainfall and the Mediterranean
No full textThe teleconnections with Indian monsoon and Sahel rainfall indices are investigated here on an interannual time scale in terms of meteorological and marine dynamics over the Mediterranean area. Sea-level pressure from gridded data sets and from individual stations, together with sea-level data from stations all around the Mediterranean coastlines, are used. In summer (July-August-September, JAS) the sea-level pressure field over the eastern Mediterranean anticorrelates with the Indian monsoon index (correlation coefficient C = -0.5 on average). A Mediterranean pressure index (MPI), defined as the standardized difference between sea-level atmospheric pressure at Mersa Matruh (southeastern Mediterranean) and Marseille (northwestern Mediterranean) stations, anticorrelates with Indian monsoon index even more (C = -0.68). The MPI is proportional to the mean geostrophic surface flow field across an imaginary line joining the two stations and turns out to be significantly correlated with the meridional wind component over the eastern Mediterranean, known as the low-level Etesian wind regime. This wind regime represents the inflow surface field into the African inter-tropical convergence zone and, therefore, has an association with the Indian monsoon regime. The ocean response, evident by sea-level anomalies at coastal stations, shows a maximum anticorrelation with Indian monsoon index in late summer and autumn (September-October-November, SON). The Sahel index anticorrelates with sea-level pressure, with the maximum absolute value in June-July-August. This may be interpreted as a tendency of the Mediterranean sea-level pressure anomalies to precede those of Sahel precipitation, which is characterized by maximum rainfall in July-September. The MPI anticorrelates with Sahel index during and before JAS, indicating that the Etesian wind regime intensity is connected to Sahel rainfall. The sea level again anticorrelates with the Sahel index, with the maximum absolute value in SON, as for the sea-level-Indian monsoon correlation. © 2003 Royal Meteorological Society
Characteristics of the summer 1987 flow field in the Ionian Sea
No full textA four water mass structure of the basin is found to be represented by a first internal baroclinic Rossby radius of deformation of 11.8 km. The horizontal correlation scales decrease with depth, and the subsurface flow is dominated by anticyclonic gyres. Large-scale circulation trends of the temperature and salinity covariance matrices are compensated below 200 m, and only the gyre scales (~100 km) persist at intermediate and deep levels. The empirical orthogonal functions of the data set show that an horizontal scale separation exists between the first and higher vertical modes of the dynamic height field. -Autho
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